Meiosis: Wikis

  
  
  

Did you know ...


More interesting facts on Meiosis

Include this on your site/blog:

Encyclopedia

From Wikipedia, the free encyclopedia

Events involving meiosis, showing chromosomal crossover
.In biology, meiosis (pronounced en-us-meiosis.ogg maɪˈoʊsɨs ) is a process of reductional division in which the number of chromosomes per cell is cut in half.^ A direct consequence to the cell is a reduction by one of the number of chromosomes in the cell.
  • robertsonian translocation - talk.origins | Google Groups 11 January 2010 10:35 UTC groups.google.com [Source type: FILTERED WITH BAYES]

^ Meiosis is a type of cell reproduction that reduces the number of chromosomes in half in egg and sperm cells.
  • Overview of Genetic Concepts. 12 October 2009 14:26 UTC members.shaw.ca [Source type: Academic]

^ Meiosis I is thus called a reductional division, because the number of chromosomes is reduced in half.
  • Why Meiosis is my Favorite Biological Problem 11 January 2010 10:35 UTC www.ocf.berkeley.edu [Source type: Academic]

.In animals, meiosis always results in the formation of gametes, while in other organisms it can give rise to spores.^ Sporophytes give rise to haploid spores which go on to found the haploid organism .

^ A spore produced as a result of meiosis .
  • Glossary for Plant Morphology 12 October 2009 14:26 UTC www.csupomona.edu [Source type: Reference]

^ Sporophyte -- The generation or part of the life cycle which begins with fertilization and ends with the formation of spores at meiosis (See gametophyte).
  • Iris Glossary 12 October 2009 14:26 UTC www.worldiris.com [Source type: FILTERED WITH BAYES]

.As with mitosis, before meiosis begins, the DNA in the original cell is replicated during S-phase of the cell cycle.^ The doubling of chromosomes occurs during what phase of the cell cycle?
  • TE Cell Division and Reproduction - CK12 - Flexbooks 12 October 2009 14:26 UTC authors.ck12.org [Source type: Academic]

^ It is the first three phases of the cell cycle.
  • TE Cell Division and Reproduction - CK12 - Flexbooks 12 October 2009 14:26 UTC authors.ck12.org [Source type: Academic]

^ During interphase the cell performs its routine functions, replicates its DNA and prepares for mitosis.
  • TE Cell Division and Reproduction - CK12 - Flexbooks 12 October 2009 14:26 UTC authors.ck12.org [Source type: Academic]

.Two cell divisions separate the replicated chromosomes into four haploid gametes or spores.^ Spores are haploid reproductive cells.
  • TE Cell Division and Reproduction - CK12 - Flexbooks 12 October 2009 14:26 UTC authors.ck12.org [Source type: Academic]

^ Meiosis -- Two specialized cell divisions from which are derived spores and gametes with the half number of chromosomes.
  • Iris Glossary 12 October 2009 14:26 UTC www.worldiris.com [Source type: FILTERED WITH BAYES]

^ Thats where cells divide into two .
  • Student poems - mitosis 11 January 2010 10:35 UTC www.starsandseas.com [Source type: FILTERED WITH BAYES]

.Meiosis is essential for sexual reproduction and therefore occurs in all eukaryotes (including single-celled organisms) that reproduce sexually.^ Meiosis is a key stage in the life cycle of all sexually reproducing eukaryotes.
  • Dissecting plant meiosis using Arabidopsis thaliana mutants -- Caryl et al. 54 (380): 25 -- Journal of Experimental Botany 12 October 2009 14:26 UTC jxb.oxfordjournals.org [Source type: Academic]

^ Gamete -- The male or female germ cell of sexually reproducing organisms.
  • Iris Glossary 12 October 2009 14:26 UTC www.worldiris.com [Source type: FILTERED WITH BAYES]

^ An organism’s reproductive cells are called ________.
  • TE Cell Division and Reproduction - CK12 - Flexbooks 12 October 2009 14:26 UTC authors.ck12.org [Source type: Academic]

.A few eukaryotes, notably the Bdelloid rotifers, have lost the ability to carry out meiosis and have acquired the ability to reproduce by parthenogenesis.^ Yes, the bdelloid rotifers don't do meiosis.
  • Basics: How can chromosome numbers change? : Pharyngula 12 October 2009 14:26 UTC scienceblogs.com [Source type: General]

^ In the life cycle of sexually reproducing eukaryotes, meiosis halves the DNA content from diploidy in the germline cells to haploidy in the gametes.
  • Live observation of fission yeast meiosis in recombination-deficient mutants: a study on achiasmate chromosome segregation -- Molnar et al. 114 (15): 2843 -- Journal of Cell Science 11 January 2010 10:35 UTC jcs.biologists.org [Source type: Academic]

^ Meiosis is a process of importance for sexually reproducing eukaryotic organisms.
  • Activation of the pheromone-responsive MAP kinase drives haploid cells to undergo ectopic meiosis with normal telomere clustering and sister chromatid segregation in fission yeast -- Yamamoto et al. 117 (17): 3875 -- Journal of Cell Science 11 January 2010 10:35 UTC jcs.biologists.org [Source type: Academic]

.Meiosis does not occur in archaea or bacteria, which reproduce via asexual processes such as binary fission.^ Meiosis is a process of importance for sexually reproducing eukaryotic organisms.
  • Activation of the pheromone-responsive MAP kinase drives haploid cells to undergo ectopic meiosis with normal telomere clustering and sister chromatid segregation in fission yeast -- Yamamoto et al. 117 (17): 3875 -- Journal of Cell Science 11 January 2010 10:35 UTC jcs.biologists.org [Source type: Academic]

^ Note that the diploid to haploid transition in sexual life cycles occurs via meiosis .

^ Chromosome breakage and reunion are known to occur spontaneously (and can be induced) outside meiosis, but whether or not such a process occurs in crossing-over and chiasma formation has not been entirely proven.
  • The Cell Nucleus | NZETC 12 October 2009 14:26 UTC www.nzetc.org [Source type: FILTERED WITH BAYES]

.During meiosis, the genome of a diploid germ cell, which is composed of long segments of DNA packaged into chromosomes, undergoes DNA replication followed by two rounds of division, resulting in four haploid cells.^ Meiosis thus consists of a single phase of DNA replication followed by two cell divisions.
  • Germ Cells and Fertilization -- Molecular Biology of the Cell -- NCBI Bookshelf 11 January 2010 10:35 UTC www.ncbi.nlm.nih.gov [Source type: Academic]

^ Four haploid cells are therefore produced from each cell that enters meiosis.
  • Germ Cells and Fertilization -- Molecular Biology of the Cell -- NCBI Bookshelf 11 January 2010 10:35 UTC www.ncbi.nlm.nih.gov [Source type: Academic]

^ Are fertilized eggs haploid cells or diploid cells?

.Each of these cells contains one complete set of chromosomes, or half of the genetic content of the original cell.^ This means that the cell contains two sets of chromosomes , i.e., two haploid sets .

^ Nucleus -the region of the cell that contains the chromosomes.
  • Technical Glossary of Breast Cancer Terms 12 October 2009 14:26 UTC www.cancerlynx.com [Source type: Academic]

^ Half the sperm cells receive an X chromosome, and half receive a Y chromosome.
  • Chapter 15 - The Chromosomal Basis of Inheritance 12 October 2009 14:26 UTC homepage.mac.com [Source type: Academic]

.If meiosis produces gametes, these cells must fuse during fertilization to create a new diploid cell, or zygote before any new growth can occur.^ New mutations can occur in the genes of these cells at any stage during this process.
  • Overview of Genetic Concepts. 12 October 2009 14:26 UTC members.shaw.ca [Source type: Academic]

^ Any diploid cell that undergoes meiosis .
  • Glossary for Plant Morphology 12 October 2009 14:26 UTC www.csupomona.edu [Source type: Reference]

^ These different products of meiosis are manifest in a diploid organism after fertilization.
  • The Cell Nucleus | NZETC 12 October 2009 14:26 UTC www.nzetc.org [Source type: FILTERED WITH BAYES]

.Thus, the division mechanism of meiosis is a reciprocal process to the joining of two genomes that occurs at fertilization.^ This process, known as meiosis or chromosome reduction, occurs in two steps.
  • An Evolutionary Manifesto 12 October 2009 14:26 UTC www.uvm.edu [Source type: FILTERED WITH BAYES]

^ Two meiotic divisions occur: meiosis I and meiosis 11.
  • Technical Glossary of Breast Cancer Terms 12 October 2009 14:26 UTC www.cancerlynx.com [Source type: Academic]

^ Meiosis comprises two meiotic divisions.
  • Meiosis arrest in oocytes in vitro - Patent application - The method for in vitro synchronisation of nuclear and cytoplasmatic maturation of GV oocytes from domestic animals 11 January 2010 10:35 UTC www.faqs.org [Source type: Academic]

.Because the chromosomes of each parent undergo homologous recombination during meiosis, each gamete, and thus each zygote, will have a unique genetic blueprint encoded in its DNA. Together, meiosis and fertilization constitute sexuality in the eukaryotes, and generate genetically distinct individuals in populations.^ Segregation The separation of homologous chromosomes into different gametes during meiosis.

^ Homologous chromosomes Chromosomes that synapse during meiosis.

^ The genotype is the genetic constitution of an individual.
  • PROCEDURES FOR USE OF GENIC MALE STERILITY IN PRODUCTION OF COMMERCIAL HYBRID MAIZE - Patent 3861079 12 October 2009 14:26 UTC www.freepatentsonline.com [Source type: Academic]

.In all plants, and in many protists, meiosis results in the formation of haploid cells that can divide vegetatively without undergoing fertilization, referred to as spores.^ Spores are haploid reproductive cells.
  • TE Cell Division and Reproduction - CK12 - Flexbooks 12 October 2009 14:26 UTC authors.ck12.org [Source type: Academic]

^ Are fertilized eggs haploid cells or diploid cells?

^ Meiosis in the MMC results in the formation of four haploid megaspores.
  • Dissecting plant meiosis using Arabidopsis thaliana mutants -- Caryl et al. 54 (380): 25 -- Journal of Experimental Botany 12 October 2009 14:26 UTC jxb.oxfordjournals.org [Source type: Academic]

.In these groups, gametes are produced by mitosis.^ A haploid organism arising from meiospores ; produces gametes by mitosis .
  • Glossary for Plant Morphology 12 October 2009 14:26 UTC www.csupomona.edu [Source type: Reference]

^ What might happen if gametes were produced by mitosis and were diploid?

^ Some simple animals and plants have haploid somatic cells, produce gametes by mitosis, and meiosis immediately follows fertilization.
  • The Cell Nucleus | NZETC 12 October 2009 14:26 UTC www.nzetc.org [Source type: FILTERED WITH BAYES]

.Meiosis uses many of the same biochemical mechanisms employed during mitosis to accomplish the redistribution of chromosomes.^ Mechanically, meiosis II is essentially the same as somatic mitosis.
  • The Cell Nucleus | NZETC 12 October 2009 14:26 UTC www.nzetc.org [Source type: FILTERED WITH BAYES]

^ Suggested Reading : Grewal SI, Klar AJ (1996) Chromosomal inheritance of epigenetic states in fission yeast during mitosis and meiosis.

^ We then examined the subcellular localization of Spo20 T170I during proliferation and meiosis/sporulation by using wild-type cells expressing a Spo20 T170I -HA fusion protein.
  • The Sec14 family glycerophospholipid-transfer protein is required for structural integrity of the spindle pole body during meiosis in fission yeast -- Nakase et al. 9 (12): 1275 -- Genes to Cells 11 January 2010 10:35 UTC www.genestocellsonline.org [Source type: Academic]

.There are several features unique to meiosis, most importantly the pairing and recombination between homologous chromosomes.^ Crossing over between homologous chromosomes occurs .

^ Homologous chromosomes -chromosomes that pair during meiosis.
  • Technical Glossary of Breast Cancer Terms 12 October 2009 14:26 UTC www.cancerlynx.com [Source type: Academic]

^ P airing and crossing-over of segments occurs between homologous chromosome pairs.

Meiosis comes from the root -meio, meaning less.

Contents

History

.Meiosis was discovered and described for the first time in sea urchin eggs in 1876, by noted German biologist Oscar Hertwig (1849–1922).^ Trisomy 21 Causes Down Syndrome A century after Down syndrome was first described, scientists discovered that the root cause of this disorder is a condition called trisomy 21.
  • Stages of Meiosis and Sexual Reproduction | Learn Science at Scitable 11 January 2010 10:35 UTC www.nature.com [Source type: Academic]

^ The egg at this time has often produced the first polar body and is arrested in metaphase of the second meiotic division.
  • Evolution as a Self-Limiting Process 12 October 2009 14:26 UTC www.uvm.edu [Source type: FILTERED WITH BAYES]

^ The egg at this time has often produced the first polar body and is arrested in metaphase of the second meiotic division and so is still diploid as it has completed only the first meiotic division.
  • An Evolutionary Manifesto 12 October 2009 14:26 UTC www.uvm.edu [Source type: FILTERED WITH BAYES]

.It was described again in 1883, at the level of chromosomes, by Belgian zoologist Edouard Van Beneden (1846–1910), in Ascaris worms' eggs.^ Ascaris which arise by a fusion of eggs, the number of chromosomes increases proportionately with the number of nuclei which unite.
  • Dissertation of Michael F. Guyer on Spermatogenesis in Pigeons 12 October 2009 14:26 UTC qspace.library.queensu.ca [Source type: FILTERED WITH BAYES]

^ In 1883, it was discovered that, whereas the fertilized egg of a roundworm contains four chromosomes, the nucleus of the egg and that of the sperm each contain only two chromosomes.
  • Germ Cells and Fertilization -- Molecular Biology of the Cell -- NCBI Bookshelf 11 January 2010 10:35 UTC www.ncbi.nlm.nih.gov [Source type: Academic]

.The significance of meiosis for reproduction and inheritance, however, was described only in 1890 by German biologist August Weismann (1834–1914), who noted that two cell divisions were necessary to transform one diploid cell into four haploid cells if the number of chromosomes had to be maintained.^ Note that each transforming group of chromosomes consists, not of a diploid number of single chromosomes (cf.
  • The Cell Nucleus | NZETC 12 October 2009 14:26 UTC www.nzetc.org [Source type: FILTERED WITH BAYES]

^ Four haploid cells are therefore produced from each cell that enters meiosis.
  • Germ Cells and Fertilization -- Molecular Biology of the Cell -- NCBI Bookshelf 11 January 2010 10:35 UTC www.ncbi.nlm.nih.gov [Source type: Academic]

^ The gametes are created by meiosis, a specialized pair of cell divisions that partition a diploid genome into two equivalent haploid complements.
  • Why Meiosis is my Favorite Biological Problem 11 January 2010 10:35 UTC www.ocf.berkeley.edu [Source type: Academic]

In 1911 the American geneticist Thomas Hunt Morgan (1866–1945) observed crossover in Drosophila melanogaster meiosis and provided the first genetic evidence that genes are transmitted on chromosomes,

Evolution

Meiosis is thought to have appeared 1.4 billion years ago. .The only supergroup of eukaryotes which does not have meiosis in all organisms is excavata.^ Meiosis is a process of importance for sexually reproducing eukaryotic organisms.
  • Activation of the pheromone-responsive MAP kinase drives haploid cells to undergo ectopic meiosis with normal telomere clustering and sister chromatid segregation in fission yeast -- Yamamoto et al. 117 (17): 3875 -- Journal of Cell Science 11 January 2010 10:35 UTC jcs.biologists.org [Source type: Academic]

^ This method requires analysis of all four products of individual meiosis, and it has not been applied previously to multicellular eukaryotes because their meiotic products typically are dissociated.
  • Plant artificial chromosome compositions and methods - Patent 6900012 12 October 2009 14:26 UTC www.freepatentsonline.com [Source type: Academic]

^ It is well documented that centromere function is crucial for stable chromosomal inheritance in almost all eukaryotic organisms (reviewed in Nicklas 1988).
  • Plant artificial chromosome compositions and methods - Patent 6900012 12 October 2009 14:26 UTC www.freepatentsonline.com [Source type: Academic]

.The other five major supergroups, opisthokonts, amoebozoa, rhizaria, archaeplastida and chromalveolates all seem to have genes for meiosis universally present, even if not always functional.^ The results indicated an overall expression pattern for all five genes that was very similar ( Figures 2 and 3 ).
  • The Arabidopsis-mei2-Like Genes Play a Role in Meiosis and Vegetative Growth in Arabidopsis -- Kaur et al. 18 (3): 545 -- THE PLANT CELL 11 January 2010 10:35 UTC www.plantcell.org [Source type: Academic]

^ All five AML genes were expressed in both vegetative and reproductive tissues.
  • The Arabidopsis-mei2-Like Genes Play a Role in Meiosis and Vegetative Growth in Arabidopsis -- Kaur et al. 18 (3): 545 -- THE PLANT CELL 11 January 2010 10:35 UTC www.plantcell.org [Source type: Academic]

^ For AML1 , the gene-specific primer is downstream of the site of insertion, whereas for all others, the gene-specific primer is upstream of the site of insertion.
  • The Arabidopsis-mei2-Like Genes Play a Role in Meiosis and Vegetative Growth in Arabidopsis -- Kaur et al. 18 (3): 545 -- THE PLANT CELL 11 January 2010 10:35 UTC www.plantcell.org [Source type: Academic]

.Some excavata species do have meiosis which is consistent with the hypothesis that this group is an ancient, paraphyletic grade.^ Some of these groups are involved in large-scale genome sequencing and function search, specially in the case of the Myb family of transcriptional factors.
  • Multinational Coordinated Arabidopsis thaliana Genome Research Project 12 October 2009 14:26 UTC arabidopsis.info [Source type: Academic]

^ The male gametangium , consisting of a single specialized cell or group of cells in which sperm are produced.
  • Glossary for Plant Morphology 12 October 2009 14:26 UTC www.csupomona.edu [Source type: Reference]

^ Furthermore, while mitosis is universal in eukaryotic species, meiosis is merely ubiquitous, consistent with its loss in some eukaryotic lineages.
  • The Evolution of Meiosis From Mitosis -- Wilkins and Holliday 181 (1): 3 -- Genetics 11 January 2010 10:35 UTC www.genetics.org [Source type: Academic]

.An example of a eukaryotic organism in which meiosis does not exist is euglenoid.^ Meiosis is a process of importance for sexually reproducing eukaryotic organisms.
  • Activation of the pheromone-responsive MAP kinase drives haploid cells to undergo ectopic meiosis with normal telomere clustering and sister chromatid segregation in fission yeast -- Yamamoto et al. 117 (17): 3875 -- Journal of Cell Science 11 January 2010 10:35 UTC jcs.biologists.org [Source type: Academic]

^ Meiosis, a process of general importance for sexually reproducing eukaryotic organisms, generates inheritable haploid gametes from a diploid cell.
  • Activation of the pheromone-responsive MAP kinase drives haploid cells to undergo ectopic meiosis with normal telomere clustering and sister chromatid segregation in fission yeast -- Yamamoto et al. 117 (17): 3875 -- Journal of Cell Science 11 January 2010 10:35 UTC jcs.biologists.org [Source type: Academic]

^ Sexually reproducing eukaryotic organisms undergo meiosis, a special type of cell division, to generate inheritable haploid gametes from diploid parental cells.
  • Mcp6, a meiosis-specific coiled-coil protein of Schizosaccharomyces pombe, localizes to the spindle pole body and is required for horsetail movement and recombination -- Saito et al. 118 (2): 447 -- Journal of Cell Science 12 October 2009 14:26 UTC jcs.biologists.org [Source type: Academic]

Occurrence of meiosis in eukaryotic life cycles

Gametic life cycle.
Zygotic life cycle.
Sporic life cycle.
.Meiosis occurs in eukaryotic life cycles involving sexual reproduction, comprising of the constant cyclical process of meiosis and fertilization.^ The sexual life cycle depends on when meiosis occurs and the type of cell that undergoes meiosis.
  • TE Cell Division and Reproduction - CK12 - Flexbooks 12 October 2009 14:26 UTC authors.ck12.org [Source type: Academic]

^ Meiosis is a key stage in the life cycle of all sexually reproducing eukaryotes.
  • Dissecting plant meiosis using Arabidopsis thaliana mutants -- Caryl et al. 54 (380): 25 -- Journal of Experimental Botany 12 October 2009 14:26 UTC jxb.oxfordjournals.org [Source type: Academic]

^ The sexual reproductive cycle Figure 20-2 .
  • Germ Cells and Fertilization -- Molecular Biology of the Cell -- NCBI Bookshelf 11 January 2010 10:35 UTC www.ncbi.nlm.nih.gov [Source type: Academic]

.This takes place alongside normal mitotic cell division.^ Cells proliferate by mitotic division.
  • Germ Cells and Fertilization -- Molecular Biology of the Cell -- NCBI Bookshelf 11 January 2010 10:35 UTC www.ncbi.nlm.nih.gov [Source type: Academic]

^ Cell division Cells proliferate by mitotic division.
  • http://www.med.yale.edu/genetics/ashley/text/meiosis.html 11 January 2010 10:35 UTC www.med.yale.edu [Source type: Academic]

^ Then two divisions take place.
  • An Evolutionary Manifesto 12 October 2009 14:26 UTC www.uvm.edu [Source type: FILTERED WITH BAYES]

.In multicellular organisms, there is an intermediary step between the diploid and haploid transition where the organism grows.^ The organism phase of the life cycle can occur between the haploid to diploid transition or the diploid to haploid transition.
  • Meiosis - Hwiki 11 January 2010 10:35 UTC hwiki.fzk.de [Source type: Academic]

^ In organisms with a haploid life cycle, there is no diploid phase.
  • TE Cell Division and Reproduction - CK12 - Flexbooks 12 October 2009 14:26 UTC authors.ck12.org [Source type: Academic]

^ Diploid and haploid designate an organisms karyotype .

.The organism will then produce the germ cells that continue in the life cycle.^ In the life cycle of sexually reproducing eukaryotes, meiosis halves the DNA content from diploidy in the germline cells to haploidy in the gametes.
  • Live observation of fission yeast meiosis in recombination-deficient mutants: a study on achiasmate chromosome segregation -- Molnar et al. 114 (15): 2843 -- Journal of Cell Science 11 January 2010 10:35 UTC jcs.biologists.org [Source type: Academic]

^ The portion of an organisms life cycle devoted to asexual reproduction .
  • Glossary for Plant Morphology 12 October 2009 14:26 UTC www.csupomona.edu [Source type: Reference]

^ The Sertoli cells in turn produce the signal molecules that promote the development of male characteristics, suppress the development of female characteristics, and induce the primordial germ cells to commit to sperm development.
  • Germ Cells and Fertilization -- Molecular Biology of the Cell -- NCBI Bookshelf 11 January 2010 10:35 UTC www.ncbi.nlm.nih.gov [Source type: Academic]

.The rest of the cells, called somatic cells, function within the organism and will die with it.^ An organism’s reproductive cells are called ________.
  • TE Cell Division and Reproduction - CK12 - Flexbooks 12 October 2009 14:26 UTC authors.ck12.org [Source type: Academic]

^ The body cells of organisms are termed somatic cells and in most organisms such somatic cells typically have a given, specific number of chromosomes in their nuclei.
  • PROCEDURES FOR USE OF GENIC MALE STERILITY IN PRODUCTION OF COMMERCIAL HYBRID MAIZE - Patent 3861079 12 October 2009 14:26 UTC www.freepatentsonline.com [Source type: Academic]

^ The chromosomes of many sperm have dispensed with the histones of somatic cells and are packed instead with simple, highly positively charged proteins called protamines.
  • Germ Cells and Fertilization -- Molecular Biology of the Cell -- NCBI Bookshelf 11 January 2010 10:35 UTC www.ncbi.nlm.nih.gov [Source type: Academic]

.Cycling meiosis and fertilization events produces a series of transitions back and forth between alternating haploid and diploid states.^ Sexually reproducing organisms have a specialized developmental pathway for gametogenesis in which diploid cells undergo meiosis to produce haploid germ cells.
  • Mnd1/Hop2 Facilitates Dmc1-Dependent Interhomolog Crossover Formation in Meiosis of Budding Yeast -- Henry et al. 26 (8): 2913 -- Molecular and Cellular Biology 11 January 2010 10:35 UTC mcb.asm.org [Source type: Academic]

^ The sexual reproductive cycle involves an alternation of diploid and haploid states: diploid cells divide by meiosis to form haploid cells, and the haploid cells from two individuals fuse in pairs at fertilization to form new diploid cells.
  • Germ Cells and Fertilization -- Molecular Biology of the Cell -- NCBI Bookshelf 11 January 2010 10:35 UTC www.ncbi.nlm.nih.gov [Source type: Academic]

^ Four haploid cells are therefore produced from each cell that enters meiosis.
  • Germ Cells and Fertilization -- Molecular Biology of the Cell -- NCBI Bookshelf 11 January 2010 10:35 UTC www.ncbi.nlm.nih.gov [Source type: Academic]

.The organism phase of the life cycle can occur either during the diploid state (gametic or diploid life cycle), during the haploid state (zygotic or haploid life cycle), or both (sporic or haplodiploid life cycle, in which there two distinct organism phases, one during the haploid state and the other during the diploid state).^ The doubling of chromosomes occurs during what phase of the cell cycle?
  • TE Cell Division and Reproduction - CK12 - Flexbooks 12 October 2009 14:26 UTC authors.ck12.org [Source type: Academic]

^ In most organisms that reproduce sexually, this proliferation occurs during the diploid phase.
  • Germ Cells and Fertilization -- Molecular Biology of the Cell -- NCBI Bookshelf 11 January 2010 10:35 UTC www.ncbi.nlm.nih.gov [Source type: Academic]

^ This question has two aspects, one, what is life?
  • Overview of Genetic Concepts. 12 October 2009 14:26 UTC members.shaw.ca [Source type: Academic]

.In this sense, there are three types of life cycles that utilize sexual reproduction, differentiated by the location of the organisms phase(s).^ Name the three different sexual life cycles.
  • TE Cell Division and Reproduction - CK12 - Flexbooks 12 October 2009 14:26 UTC authors.ck12.org [Source type: Academic]

^ The portion of an organisms life cycle devoted to asexual reproduction .
  • Glossary for Plant Morphology 12 October 2009 14:26 UTC www.csupomona.edu [Source type: Reference]

^ In organisms with a haploid life cycle, there is no haploid phase.
  • TE Cell Division and Reproduction - CK12 - Flexbooks 12 October 2009 14:26 UTC authors.ck12.org [Source type: Academic]

.In the gametic life cycle, of which humans are a part, the species is diploid, grown from a diploid cell called the zygote.^ Haploid and diploid cells in the life cycle of higher (more...
  • Germ Cells and Fertilization -- Molecular Biology of the Cell -- NCBI Bookshelf 11 January 2010 10:35 UTC www.ncbi.nlm.nih.gov [Source type: Academic]

^ Haploid cells that are specialized for sexual fusion are called gametes .
  • Germ Cells and Fertilization -- Molecular Biology of the Cell -- NCBI Bookshelf 11 January 2010 10:35 UTC www.ncbi.nlm.nih.gov [Source type: Academic]

^ This is called a diploid cell.
  • Overview of Genetic Concepts. 12 October 2009 14:26 UTC members.shaw.ca [Source type: Academic]

.The organism's diploid germ-line stem cells undergo meiosis to create haploid gametes (the spermatozoa for males and ova for females), which fertilize to form the zygote.^ A cell that undergoes meiosis to form microspores .
  • Glossary for Plant Morphology 12 October 2009 14:26 UTC www.csupomona.edu [Source type: Reference]

^ Chapter 20: Germ Cells and Fertilization .
  • Germ Cells and Fertilization -- Molecular Biology of the Cell -- NCBI Bookshelf 11 January 2010 10:35 UTC www.ncbi.nlm.nih.gov [Source type: Academic]

^ Germ line -the cell line from which gametes are derived.
  • Technical Glossary of Breast Cancer Terms 12 October 2009 14:26 UTC www.cancerlynx.com [Source type: Academic]

.The diploid zygote undergoes repeated cellular division by mitosis to grow into the organism.^ The zygote undergoes repeated mitosis and differentiation to become the diploid organism again.
  • Meiosis - Hwiki 11 January 2010 10:35 UTC hwiki.fzk.de [Source type: Academic]

^ Mitosis then brings about the development of the diploid cell into an organism.
  • Biology: Meiosis - CliffsNotes 12 October 2009 14:26 UTC www.cliffsnotes.com [Source type: Academic]

^ The gametes proliferate by mitosis, growing into a haploid organism.
  • Meiosis - Hwiki 11 January 2010 10:35 UTC hwiki.fzk.de [Source type: Academic]

.Mitosis is a related process to meiosis that creates two cells that are genetically identical to the parent cell.^ Mitosis is cell division that starts with one cell and ends with two identical cells.
  • Sex Cells 11 January 2010 10:35 UTC faculty.stcc.edu [Source type: FILTERED WITH BAYES]

^ Genetically identical daughter cells .
  • Meiosis and Sexual Life Cycles 11 January 2010 10:35 UTC wc.pima.edu [Source type: Academic]

^ The cell cycle, mitosis and meiosis Virtual Genetics Education Centre .
  • The cell cycle, mitosis and meiosis 11 January 2010 10:35 UTC www.le.ac.uk [Source type: Academic]
  • The cell cycle, mitosis and meiosis 11 January 2010 10:35 UTC www.le.ac.uk [Source type: Academic]

.The general principle is that mitosis creates somatic cells and meiosis creates germ cells.^ Mitosis, Cell cycle control, Meiosis .
  • Mitosis, Cell cycle control, meiosis 11 January 2010 10:35 UTC www.prism.gatech.edu [Source type: Academic]

^ Meiosis occurs only in the germ cells i.e.

^ CELL DIVISION see cell ; mitosis ; meiosis .
  • Meiosis: Free Encyclopedia Articles at Questia.com Online Library 11 January 2010 10:35 UTC www.questia.com [Source type: Academic]

.In the zygotic life cycle the species is haploid instead, spawned by the proliferation and differentiation of a single haploid cell called the gamete.^ Gametes are haploid cells .

^ Haploid cells that are specialized for sexual fusion are called gametes .
  • Germ Cells and Fertilization -- Molecular Biology of the Cell -- NCBI Bookshelf 11 January 2010 10:35 UTC www.ncbi.nlm.nih.gov [Source type: Academic]

^ The development of haploid cells into mature gametes is called ________.
  • TE Cell Division and Reproduction - CK12 - Flexbooks 12 October 2009 14:26 UTC authors.ck12.org [Source type: Academic]

.Two organisms of opposing gender contribute their haploid germ cells to form a diploid zygote.^ Two organisms of opposing gender contribute their haploid germ cells to form a diploid zygote.
  • Meiosis - Hwiki 11 January 2010 10:35 UTC hwiki.fzk.de [Source type: Academic]

^ The organism's diploid germ-line stem cells undergo meiosis to create haploid gametes, which fertilize to form the zygote.
  • Meiosis - Hwiki 11 January 2010 10:35 UTC hwiki.fzk.de [Source type: Academic]

^ The chromatids then separate and form two cells.
  • About Gender: Genetics - Meiosis and Fertilisation 11 January 2010 10:35 UTC www.gender.org.uk [Source type: FILTERED WITH BAYES]

.The zygote undergoes meiosis immediately, creating four haploid cells.^ Four haploid cells are therefore produced from each cell that enters meiosis.
  • Germ Cells and Fertilization -- Molecular Biology of the Cell -- NCBI Bookshelf 11 January 2010 10:35 UTC www.ncbi.nlm.nih.gov [Source type: Academic]

^ A cell that undergoes meiosis to form microspores .
  • Glossary for Plant Morphology 12 October 2009 14:26 UTC www.csupomona.edu [Source type: Reference]

^ Any diploid cell that undergoes meiosis .
  • Glossary for Plant Morphology 12 October 2009 14:26 UTC www.csupomona.edu [Source type: Reference]

.These cells undergo mitosis to create the organism.^ These cells undergo mitosis to create the organism.
  • Meiosis - Hwiki 11 January 2010 10:35 UTC hwiki.fzk.de [Source type: Academic]

^ These resultant haploid cells can fuse with other haploid cells of the opposite gender or mating type during fertilization to create a new diploid cell, or zygote.
  • When does meiosis occur and how many times does it happen? - Yahoo! UK & Ireland Answers 11 January 2010 10:35 UTC uk.answers.yahoo.com [Source type: General]

^ Mitosis occurs from the zygote stage onward through the life of the organism: meiosis occurs in specialized cells.  Text : 31 - 57 (mitosis); 102 - 109 (meiosis) .
  • Untitled Document 12 October 2009 14:26 UTC barleyworld.org [Source type: Academic]

Many fungi and many protozoa are members of the zygotic life cycle.
.Finally, in the sporic life cycle, the living organism alternates between haploid and diploid states.^ The organism phase of the life cycle can occur between the haploid to diploid transition or the diploid to haploid transition.
  • Meiosis - Hwiki 11 January 2010 10:35 UTC hwiki.fzk.de [Source type: Academic]

^ D. All 3 different kinds of life cycles have a diploid stage and a haploid stage.
  • Mitosis and the Cell Cycle 11 January 2010 10:35 UTC jade.ccccd.edu [Source type: Academic]

^ Sexual cycle Alternating haploid-diploid: In the sexual cycle haploidy and diploidy alternate.

.Consequently, this cycle is also known as the alternation of generations.^ In the sexual reproductive cycle, haploid generations of cells, each carrying a single set of chromosomes, alternate with diploid generations of cells, each carrying a double set of chromosomes ( Figure 20-2 ).
  • Germ Cells and Fertilization -- Molecular Biology of the Cell -- NCBI Bookshelf 11 January 2010 10:35 UTC www.ncbi.nlm.nih.gov [Source type: Academic]

^ The sexual reproductive cycle It involves an alternation of haploid and diploid generations of cells.
  • Germ Cells and Fertilization -- Molecular Biology of the Cell -- NCBI Bookshelf 11 January 2010 10:35 UTC www.ncbi.nlm.nih.gov [Source type: Academic]

.The diploid organism's germ-line cells undergo meiosis to produce gametes.^ Can a haploid cell undergo meiosis?

^ Meiosis Meiosis is the type of cell division by which germ cells (eggs and sperm) are produced.

^ Most of the cells in your body are diploid, germ line diploid cells will undergo meiosis to produce gametes, with fertilization closely following meiosis.

.The gametes proliferate by mitosis, growing into a haploid organism.^ A haploid organism arising from meiospores ; produces gametes by mitosis .
  • Glossary for Plant Morphology 12 October 2009 14:26 UTC www.csupomona.edu [Source type: Reference]

^ The gametes proliferate by mitosis, growing into a haploid organism.
  • Meiosis - Hwiki 11 January 2010 10:35 UTC hwiki.fzk.de [Source type: Academic]

^ The haploid adult produces gametes by mitosis.
  • CHAPTER 13 MEIOSIS AND SEXUAL LIFE CYCLES 11 January 2010 10:35 UTC jade.ccccd.edu [Source type: Academic]
  • Chapter 13 - Meiosis and Sexual Life Cycles 11 January 2010 10:35 UTC web.me.com [Source type: Academic]

.The haploid organism's germ cells then combine with another haploid organism's cells, creating the zygote.^ The haploid organism's germ cells then combine with another haploid organism's cells, creating the zygote.
  • Meiosis - Hwiki 11 January 2010 10:35 UTC hwiki.fzk.de [Source type: Academic]

^ The zygote undergoes meiosis immediately, creating four haploid cells.
  • Meiosis - Hwiki 11 January 2010 10:35 UTC hwiki.fzk.de [Source type: Academic]

^ A haploid reproductive cell that fuses with another gamete to produce a zygote .
  • Glossary for Plant Morphology 12 October 2009 14:26 UTC www.csupomona.edu [Source type: Reference]

.The zygote undergoes repeated mitosis and differentiation to become the diploid organism again.^ The zygote undergoes repeated mitosis and differentiation to become the diploid organism again.
  • Meiosis - Hwiki 11 January 2010 10:35 UTC hwiki.fzk.de [Source type: Academic]

^ The diploid zygote undergoes repeated cellular division by mitosis to grow into the organism.
  • Meiosis - Hwiki 11 January 2010 10:35 UTC hwiki.fzk.de [Source type: Academic]

^ Most animals and plants are normally diploid, and use meiosis to produce sexual gametes , which fuse to form zygotes that develop into new organisms.
  • Meiosis - Biocrawler, the free encyclopedia 11 January 2010 10:35 UTC www.biocrawler.com [Source type: FILTERED WITH BAYES]

.The sporic life cycle can be considered a fusion of the gametic and zygotic life cycles.^ In the life cycle of sexually reproducing eukaryotes, meiosis halves the DNA content from diploidy in the germline cells to haploidy in the gametes.
  • Live observation of fission yeast meiosis in recombination-deficient mutants: a study on achiasmate chromosome segregation -- Molnar et al. 114 (15): 2843 -- Journal of Cell Science 11 January 2010 10:35 UTC jcs.biologists.org [Source type: Academic]

^ Zygote The diploid cell ( anew individual) formed by fusion of two haploid gametes.

Process

.Because meiosis is a "one-way" process, it cannot be said to engage in a cell cycle as mitosis does.^ In the life cycle of sexually reproducing eukaryotes, meiosis halves the DNA content from diploidy in the germline cells to haploidy in the gametes.
  • Live observation of fission yeast meiosis in recombination-deficient mutants: a study on achiasmate chromosome segregation -- Molnar et al. 114 (15): 2843 -- Journal of Cell Science 11 January 2010 10:35 UTC jcs.biologists.org [Source type: Academic]

^ Each cell that begins meiosis gives rise to four mature gametes rather than one.
  • Germ Cells and Fertilization -- Molecular Biology of the Cell -- NCBI Bookshelf 11 January 2010 10:35 UTC www.ncbi.nlm.nih.gov [Source type: Academic]

^ Here, we have argued that the origins of meiosis from mitosis initially involved only one new step, namely homolog synapsis.
  • The Evolution of Meiosis From Mitosis -- Wilkins and Holliday 181 (1): 3 -- Genetics 11 January 2010 10:35 UTC www.genetics.org [Source type: Academic]

.However, the preparatory steps that lead up to meiosis are identical in pattern and name to the interphase of the mitotic cell cycle.^ Cell cycle Control of the Cell Cycle, Steps in the Cell cycle, Meiosis and the Cell Cycle .
  • Cell diffusion and osmosis, Cell biology, anatomy of the cell 11 January 2010 10:35 UTC www.educypedia.be [Source type: Academic]

^ Mitosis, Cell cycle control, Meiosis .
  • Mitosis, Cell cycle control, meiosis 11 January 2010 10:35 UTC www.prism.gatech.edu [Source type: Academic]

^ Describe the key steps in the cell cycle.
  • TE Cell Division and Reproduction - CK12 - Flexbooks 12 October 2009 14:26 UTC authors.ck12.org [Source type: Academic]

Interphase is divided into three phases:
.
  • Growth 1 (G1) phase: This is a very active period, where the cell synthesizes its vast array of proteins, including the enzymes and structural proteins it will need for growth.^ Going down to the specifics, the information in the database could be implemented with information on pathways and networks, protein interaction maps, protein structures, subcellular organelles, cell structure, etc.
    • Multinational Coordinated Arabidopsis thaliana Genome Research Project 12 October 2009 14:26 UTC arabidopsis.info [Source type: Academic]

    ^ M phase, in which nuclear and cytoplasmic division occurs; (2) a gap or growth G1 phase, which is the period between mitosis and the start of DNA synthesis; (3) the synthesis S phase , in which DNA synthesis occurs; (4) a second gap or growth phase G2 , which is the period between the completion of DNA synthesis and mitosis.
    • Technical Glossary of Breast Cancer Terms 12 October 2009 14:26 UTC www.cancerlynx.com [Source type: Academic]

    ^ There are two possible explanations for the asynchronous M-phase, if we assume that the temperature sensitivity of the tam allele allows a partially active TAM protein at the permissive temperature.
    • The Arabidopsis Gene Tardy Asynchronous Meiosis Is Required for the Normal Pace and Synchrony of Cell Division during Male Meiosis -- Magnard et al. 127 (3): 1157 -- PLANT PHYSIOLOGY 11 January 2010 10:35 UTC www.plantphysiol.org [Source type: Academic]

    .In G1 stage each of the chromosomes consists of a single (very long) molecule of DNA. In humans, at this point cells are 46 chromosomes, 2N, identical to somatic cells.
  • Synthesis (S) phase: The genetic material is replicated: each of its chromosomes duplicates, producing 46 chromosomes each made up of two sister chromatids.^ Daughter cells are genetically identical .

    ^ Meiosis consists of two rounds of cell division, without DNA replication in between.
    • TE Cell Division and Reproduction - CK12 - Flexbooks 12 October 2009 14:26 UTC authors.ck12.org [Source type: Academic]

    ^ The DNA has replicated, so the chromosomes are composed of two identical, or sister, chromatids held together by the centromere.
    • TE Cell Division and Reproduction - CK12 - Flexbooks 12 October 2009 14:26 UTC authors.ck12.org [Source type: Academic]

    .The cell is still considered diploid because it still contains the same number of centromeres.^ The two daughter cells therefore contain a haploid number of chromosomes but a diploid amount of DNA. They differ from normal diploid cells in two ways.
    • Germ Cells and Fertilization -- Molecular Biology of the Cell -- NCBI Bookshelf 11 January 2010 10:35 UTC www.ncbi.nlm.nih.gov [Source type: Academic]

    ^ Mitosis Mitosis a process Which the nucleus of the Cell divides into two Nuclei with the same Number of Chromosomes.
    • Student poems - mitosis 11 January 2010 10:35 UTC www.starsandseas.com [Source type: FILTERED WITH BAYES]

    ^ Union of two cells containing characteristics of the same species would occasion a reversion to that species.

    .The identical sister chromatids have not yet condensed into the densely packaged chromosomes visible with the light microscope.^ For simplicity, the division of each chromosome into its component sister chromatids is not shown.
    • PROCEDURES FOR USE OF GENIC MALE STERILITY IN PRODUCTION OF COMMERCIAL HYBRID MAIZE - Patent 3861079 12 October 2009 14:26 UTC www.freepatentsonline.com [Source type: Academic]

    ^ Pds5p is an essential chromosomal protein required for both sister chromatid cohesion and condensation in Saccharomyces cerevisiae.
    • Fission yeast Pds5 is required for accurate chromosome segregation and for survival after DNA damage or metaphase arrest -- Wang et al. 115 (3): 587 -- Journal of Cell Science 12 October 2009 14:26 UTC jcs.biologists.org [Source type: Academic]

    ^ Light Microscopy Chromosome spreads, DAPI staining, and the microscope and digital camera settings used were as described (Y. Wang et al., 2004 ).
    • Progression through Meiosis I and Meiosis II in Arabidopsis Anthers Is Regulated by an A-Type Cyclin Predominately Expressed in Prophase I -- Wang et al. 136 (4): 4127 -- PLANT PHYSIOLOGY 11 January 2010 10:35 UTC www.plantphysiol.org [Source type: Academic]

    This will take place during prophase I in meiosis.
  • Growth 2 (G2) phase: G2 phase is absent in Meiosis
.Interphase is followed by meiosis I and then meiosis II. Meiosis I consists of separating the pairs of homologous chromosome, each made up of two sister chromatids, into two cells.^ The chromatid pairs pair up with their homologous pair, forming a tetrad.
  • Overview of Genetic Concepts. 12 October 2009 14:26 UTC members.shaw.ca [Source type: Academic]

^ Segregation The separation of homologous chromosomes into different gametes during meiosis.

^ The two sister chromatid copies in each pair are held together by a link.
  • Overview of Genetic Concepts. 12 October 2009 14:26 UTC members.shaw.ca [Source type: Academic]

One entire haploid content of chromosomes is contained in each of the resulting daughter cells; the first meiotic division therefore reduces the ploidy of the original cell by a factor of 2.
.Meiosis II consists of decoupling each chromosome's sister strands (chromatids), and segregating the individual chromatids into haploid daughter cells.^ At this stage in meiosis, each of the paired homologous chromosomes consists of two strands (sister chromatids).
  • PROCEDURES FOR USE OF GENIC MALE STERILITY IN PRODUCTION OF COMMERCIAL HYBRID MAIZE - Patent 3861079 12 October 2009 14:26 UTC www.freepatentsonline.com [Source type: Academic]

^ Sister chromatid cohesion and recombination in meiosis.
  • Dissecting plant meiosis using Arabidopsis thaliana mutants -- Caryl et al. 54 (380): 25 -- Journal of Experimental Botany 12 October 2009 14:26 UTC jxb.oxfordjournals.org [Source type: Academic]

^ Eukaryotic cells divide the chromosomes Making it into daughter cells.
  • Student poems - mitosis 11 January 2010 10:35 UTC www.starsandseas.com [Source type: FILTERED WITH BAYES]

.The two cells resulting from meiosis I divide during meiosis II, creating 4 haploid daughter cells.^ The result is that the two daughter cells receive identical genetic complements.
  • Evolution as a Self-Limiting Process 12 October 2009 14:26 UTC www.uvm.edu [Source type: FILTERED WITH BAYES]

^ Then eukaryotic cell comes by, and divides them into two daughter cells.
  • Student poems - mitosis 11 January 2010 10:35 UTC www.starsandseas.com [Source type: FILTERED WITH BAYES]

^ It is two new daughter cells What is it?
  • Student poems - mitosis 11 January 2010 10:35 UTC www.starsandseas.com [Source type: FILTERED WITH BAYES]

.Meiosis I and II are each divided into prophase, metaphase, anaphase, and telophase stages, similar in purpose to their analogous subphases in the mitotic cell cycle.^ Mitosis, Mitosis divided into four stages.
  • Student poems - mitosis 11 January 2010 10:35 UTC www.starsandseas.com [Source type: FILTERED WITH BAYES]

^ Thats where cells divide into two .
  • Student poems - mitosis 11 January 2010 10:35 UTC www.starsandseas.com [Source type: FILTERED WITH BAYES]

^ Prophase, Metaphase, Anaphase and Telophase Are some you should learn.
  • Student poems - mitosis 11 January 2010 10:35 UTC www.starsandseas.com [Source type: FILTERED WITH BAYES]

.Therefore, meiosis includes the stages of meiosis I (prophase I, metaphase I, anaphase I, telophase I), and meiosis II (prophase II, metaphase II, anaphase II, telophase II).^ Diakinesis The final stage of prophase I in meiosis, just prior to metaphase I. .

^ Meiosis II can be divided into prophase, metaphase, anaphase and telophase II and these stages are very similar to those found in somatic mitosis.
  • The Cell Nucleus | NZETC 12 October 2009 14:26 UTC www.nzetc.org [Source type: FILTERED WITH BAYES]

^ Divisions and phases: First Division (reduction division) -Prophase I -Metaphase I -Anaphase I -Telophase I Second Division -Prophase II -Metaphase II -Anaphase II -Telophase II Overview of Meiosis I -Prior to meiosis I, DNA replication occurs and each chromosome has two sister chromatids produced.
  • Overview of Genetic Concepts. 12 October 2009 14:26 UTC members.shaw.ca [Source type: Academic]

.Meiosis generates genetic diversity in two ways: (1) independent alignment and subsequent separation of homologous chromosome pairs during the first meiotic division allows a random and independent selection of each chromosome segregates into each gamete; and (2) physical exchange of homologous chromosomal regions by homologous recombination during prophase I results in new combinations of DNA within chromosomes.^ Each homologous pair of chromosomes aligns and orients independently of the other pairs, thus, the first meiotic division results in an independent assortment of maternal and paternal chromosomes.
  • Overview of Genetic Concepts. 12 October 2009 14:26 UTC members.shaw.ca [Source type: Academic]

^ First DNA condenses to create chromosomes.
  • Student poems - mitosis 11 January 2010 10:35 UTC www.starsandseas.com [Source type: FILTERED WITH BAYES]

^ Defects in pairing lead to defective recombination, which in turn impairs the faithful segregation of homologous chromosomes.
  • A novel meiosis-specific protein of fission yeast, Meu13p, promotes homologous pairing independently of homologous recombination : Article : The EMBO Journal 12 October 2009 14:26 UTC www.nature.com [Source type: Academic]

A diagram of the meiotic phases

Phases Of Meiosis

Meiosis I

.Meiosis I separates homologous chromosomes, producing two haploid cells (N chromosomes, 23 in humans), so meiosis I is referred to as a reductional division.^ Four haploid cells are therefore produced from each cell that enters meiosis.
  • Germ Cells and Fertilization -- Molecular Biology of the Cell -- NCBI Bookshelf 11 January 2010 10:35 UTC www.ncbi.nlm.nih.gov [Source type: Academic]

^ Cytokinesis refers to the division of a eukaryotic cell.

^ Homologous chromosomes Chromosomes that synapse during meiosis.

.A regular diploid human cell contains 46 chromosomes and is considered 2N because it contains 23 pairs of homologous chromosomes.^ Humans normally have 46 chromosomes.

^ How many chromosomes does a diploid human cell have?
  • TE Cell Division and Reproduction - CK12 - Flexbooks 12 October 2009 14:26 UTC authors.ck12.org [Source type: Academic]

^ In man, the diploid number of chromosomes is 46.

.However, after meiosis I, although the cell contains 46 chromatids, it is only considered as being N, with 23 chromosomes.^ Nucleus -the region of the cell that contains the chromosomes.
  • Technical Glossary of Breast Cancer Terms 12 October 2009 14:26 UTC www.cancerlynx.com [Source type: Academic]

^ Each of these cells contain 23 chromosomes.
  • Overview of Genetic Concepts. 12 October 2009 14:26 UTC members.shaw.ca [Source type: Academic]

^ Meiosis converts a diploid cell (46 chromosomes) to a haploid (23 chromosomes) egg or sperm cell.
  • Overview of Genetic Concepts. 12 October 2009 14:26 UTC members.shaw.ca [Source type: Academic]

.This is because later, in Anaphase I, the sister chromatids will remain together as the spindle pulls the pair toward the pole of the new cell.^ During anaphase of the cell cycle, sister chromatids migrate to opposite poles.
  • TE Cell Division and Reproduction - CK12 - Flexbooks 12 October 2009 14:26 UTC authors.ck12.org [Source type: Academic]

^ Anaphase is the phase in which the sister chromatids separate.
  • TE Cell Division and Reproduction - CK12 - Flexbooks 12 October 2009 14:26 UTC authors.ck12.org [Source type: Academic]

^ The two sister chromatid copies in each pair are held together by a link.
  • Overview of Genetic Concepts. 12 October 2009 14:26 UTC members.shaw.ca [Source type: Academic]

.In meiosis II, an equational division similar to mitosis will occur whereby the sister chromatids are finally split, creating a total of 4 haploid cells (23 chromosomes, N) per daughter cell from the first division.^ Each of these cells contain 23 chromosomes.
  • Overview of Genetic Concepts. 12 October 2009 14:26 UTC members.shaw.ca [Source type: Academic]

^ Events through the first cell division of meiosis Figure 20-6 .
  • Germ Cells and Fertilization -- Molecular Biology of the Cell -- NCBI Bookshelf 11 January 2010 10:35 UTC www.ncbi.nlm.nih.gov [Source type: Academic]

^ Meiosis converts a diploid cell (46 chromosomes) to a haploid (23 chromosomes) egg or sperm cell.
  • Overview of Genetic Concepts. 12 October 2009 14:26 UTC members.shaw.ca [Source type: Academic]

Prophase I

.During prophase I, DNA is exchanged between homologous chromosomes in a process called homologous recombination.^ Homologous chromosomes -chromosomes that pair during meiosis.
  • Technical Glossary of Breast Cancer Terms 12 October 2009 14:26 UTC www.cancerlynx.com [Source type: Academic]

^ Recombination occurs during the prophase of meiosis 1.

^ Recombination between nonhomologous chromosomes.
  • Mnd1/Hop2 Facilitates Dmc1-Dependent Interhomolog Crossover Formation in Meiosis of Budding Yeast -- Henry et al. 26 (8): 2913 -- Molecular and Cellular Biology 11 January 2010 10:35 UTC mcb.asm.org [Source type: Academic]

.This often results in chromosomal crossover.^ The same result is expected, however, if the two mutant genes are far apart on the same chromosome, as one or more crossover events will separate them at meiosis.
  • Germ Cells and Fertilization -- Molecular Biology of the Cell -- NCBI Bookshelf 11 January 2010 10:35 UTC www.ncbi.nlm.nih.gov [Source type: Academic]

.The new combinations of DNA created during crossover are a significant source of genetic variation, and may result in beneficial new combinations of alleles.^ In the way we traditionally think about genetic inheritance, genes in germ cells (in an egg or in a sperm cell) carry a mutation and it is passed on to offspring when sexual union introduces the mutated DNA into the genetic mix that creates the new "embryo."
  • Overview of Genetic Concepts. 12 October 2009 14:26 UTC members.shaw.ca [Source type: Academic]

^ During meiosis, the chromosomes of the double chromosome set exchange DNA by genetic recombination before being shared out, in new combinations, into single chromosome sets.
  • Germ Cells and Fertilization -- Molecular Biology of the Cell -- NCBI Bookshelf 11 January 2010 10:35 UTC www.ncbi.nlm.nih.gov [Source type: Academic]

^ As will be seen, crossing over may result in the incorporation of new combinations of specific segments into one chromosome that were previously carried separately in the parental homologous chromosomes.
  • PROCEDURES FOR USE OF GENIC MALE STERILITY IN PRODUCTION OF COMMERCIAL HYBRID MAIZE - Patent 3861079 12 October 2009 14:26 UTC www.freepatentsonline.com [Source type: Academic]

.The paired and replicated chromosomes are called bivalents or tetrads, which have two chromosomes and four chromatids, with one chromosome coming from each parent.^ The homologue recognizes its partner homologue and pairs up to form a bundle of four chromatids called a tetrad (or a bivalent pair).
  • Overview of Genetic Concepts. 12 October 2009 14:26 UTC members.shaw.ca [Source type: Academic]

^ Bivalents, each composed of two chromosomes (four chromatids) align at the metaphase plate.
  • Overview of Genetic Concepts. 12 October 2009 14:26 UTC members.shaw.ca [Source type: Academic]

^ Each duplicated chromosome pairs with its duplicated homolog, forming a structure called a bivalent , which contains four chromatids.
  • Germ Cells and Fertilization -- Molecular Biology of the Cell -- NCBI Bookshelf 11 January 2010 10:35 UTC www.ncbi.nlm.nih.gov [Source type: Academic]

.At this stage, non-sister chromatids may cross-over at points called chiasmata (plural; singular chiasma).^ The combination of the chiasma and the tight attachment of the sister chromatids holds the two duplicated homologs together.
  • Germ Cells and Fertilization -- Molecular Biology of the Cell -- NCBI Bookshelf 11 January 2010 10:35 UTC www.ncbi.nlm.nih.gov [Source type: Academic]

^ During ________, genetic material on a section of a chromosome is swapped between non-sister chromatids during prophase I of meiosis.
  • TE Cell Division and Reproduction - CK12 - Flexbooks 12 October 2009 14:26 UTC authors.ck12.org [Source type: Academic]

^ Pairing of homologous chromosomes and cohesion between sister chromatids must be established during different stages of meiosis to ensure accurate segregation of chromosomes (1) .
  • STAG3, a novel gene encoding a protein involved in meiotic chromosome pairing and location of STAG3-related genes flanking the Williams-Beuren syndrome deletion -- PEZZI et al. 14 (3): 581 -- The FASEB Journal 12 October 2009 14:26 UTC www.fasebj.org [Source type: Academic]

Leptotene
.The first stage of prophase I is the leptotene stage, also known as leptonema, from Greek words meaning "thin threads".[1] During this stage, individual chromosomes begin to condense into long strands within the nucleus.^ First DNA condenses to create chromosomes.
  • Student poems - mitosis 11 January 2010 10:35 UTC www.starsandseas.com [Source type: FILTERED WITH BAYES]

^ Prophase is the first stage that is when .
  • Student poems - mitosis 11 January 2010 10:35 UTC www.starsandseas.com [Source type: FILTERED WITH BAYES]

^ Prophase begins with leptotene , when the duplicated paired homologs condense.
  • Germ Cells and Fertilization -- Molecular Biology of the Cell -- NCBI Bookshelf 11 January 2010 10:35 UTC www.ncbi.nlm.nih.gov [Source type: Academic]

.However the two sister chromatids are still so tightly bound that they are indistinguishable from one another.^ At this stage, each of the chromosomes is still composed of two sister chromatids.
  • Germ Cells and Fertilization -- Molecular Biology of the Cell -- NCBI Bookshelf 11 January 2010 10:35 UTC www.ncbi.nlm.nih.gov [Source type: Academic]

^ The sister chromatids in each homolog are kept tightly packed together, with their DNA extending from their own side of the protein ladder in a series of loops.
  • Germ Cells and Fertilization -- Molecular Biology of the Cell -- NCBI Bookshelf 11 January 2010 10:35 UTC www.ncbi.nlm.nih.gov [Source type: Academic]

^ The combination of the chiasma and the tight attachment of the sister chromatids holds the two duplicated homologs together.
  • Germ Cells and Fertilization -- Molecular Biology of the Cell -- NCBI Bookshelf 11 January 2010 10:35 UTC www.ncbi.nlm.nih.gov [Source type: Academic]

Zygotene
.The zygotene stage, also known as zygonema, from Greek words meaning "paired threads",[1] occurs as the chromosomes approximately line up with each other into homologous chromosomes.^ Pairs of chromosomes are lined up at the center.
  • Student poems - mitosis 11 January 2010 10:35 UTC www.starsandseas.com [Source type: FILTERED WITH BAYES]

^ As the chromosomes line up in a silly order .
  • Student poems - mitosis 11 January 2010 10:35 UTC www.starsandseas.com [Source type: FILTERED WITH BAYES]

^ Synapsis The pairing of homologous chromosomes in meiosis.

.This is called the bouquet stage because of the way the telomeres cluster at one end of the nucleus.^ The crossovers are possible because of a small region of homology between the X and the Y at one end of these chromosomes.
  • Germ Cells and Fertilization -- Molecular Biology of the Cell -- NCBI Bookshelf 11 January 2010 10:35 UTC www.ncbi.nlm.nih.gov [Source type: Academic]

^ The Ball-of-Light Particle Model predicts there are galactic clusters because one large ball-of-light decaying into smaller balls-of-light.

^ During early stages in meiosis, the ends of chromosomes, called telomeres, associate with the envelope of the nucleus and undergo highly dynamic movements.
  • PLoS Genetics: Csm4, in Collaboration with Ndj1, Mediates Telomere-Led Chromosome Dynamics and Recombination during Yeast Meiosis 12 October 2009 14:26 UTC www.plosgenetics.org [Source type: Academic]

.At this stage, the synapsis (pairing/coming together) of homologous chromosomes takes place.^ Synapsis The pairing of homologous chromosomes in meiosis.

^ Homologous chromosomes -chromosomes that pair during meiosis.
  • Technical Glossary of Breast Cancer Terms 12 October 2009 14:26 UTC www.cancerlynx.com [Source type: Academic]

^ During meiosis I, homologous chromosomes pair; come together and line up in a process called synapsis -During synapsis, the two sets of paired chromosomes lay alongside each other as two pairs (bivalents) (also called tetrads) -While paired up, the chromosomes can have exchanges of genetic material; this is called crossing over -After crossing-over occurs, sister chromatids of a chromosome are no longer identical In Prophase I: The nucleus disappears, spindle fibers form, and the chromatid pairs form.
  • Overview of Genetic Concepts. 12 October 2009 14:26 UTC members.shaw.ca [Source type: Academic]

Pachytene
.The pachytene stage, also known as pachynema, from Greek words meaning "thick threads",[1] contains the following chromosomal crossover.^ (Mitosis, which refers to the nuclear division that occurs during an ordinary mitotic cell division (discussed in Chapter 18 ), is from the Greek word mitos, meaning “a thread.” The term refers to the threadlike appearance of the chromosomes as they condense during nuclear division—a process that occurs in both meiotic and mitotic divisions.
  • Germ Cells and Fertilization -- Molecular Biology of the Cell -- NCBI Bookshelf 11 January 2010 10:35 UTC www.ncbi.nlm.nih.gov [Source type: Academic]

^ At the pachytene stage of meiosis in such a plant, homologous chromosome segments are intimately paired.
  • PROCEDURES FOR USE OF GENIC MALE STERILITY IN PRODUCTION OF COMMERCIAL HYBRID MAIZE - Patent 3861079 12 October 2009 14:26 UTC www.freepatentsonline.com [Source type: Academic]

^ This synchrony is consistent with a delay in cell cycle progression at prophase II. Figure 8 A shows a dyad at this stage with both cells at prophase II. This dyad also contained a chromosome or chromosome fragment (Fig.
  • The Arabidopsis Gene Tardy Asynchronous Meiosis Is Required for the Normal Pace and Synchrony of Cell Division during Male Meiosis -- Magnard et al. 127 (3): 1157 -- PLANT PHYSIOLOGY 11 January 2010 10:35 UTC www.plantphysiol.org [Source type: Academic]

.Nonsister chromatids of homologous chromosomes randomly exchange segments of genetic information over regions of homology.^ Crossing over The exchange of genetic material between homologous chromosomes.

^ During synapsis (see above), genetic information is exchanged between paired homologous chromosomes.
  • Overview of Genetic Concepts. 12 October 2009 14:26 UTC members.shaw.ca [Source type: Academic]

^ The replicated pairs of chromosomes, known as homologs, recombine allowing exchange of genetic information between sister chromatids of originally paternally and maternally-derived homologs before the first meiotic division (Coop and Przeworski, 2007 ).
  • Meiosis in oocytes: predisposition to aneuploidy and its increased incidence with age -- Jones, 10.1093/humupd/dmm043 -- Human Reproduction Update 11 January 2010 10:35 UTC humupd.oxfordjournals.org [Source type: Academic]

.Sex chromosomes, however, are not wholly identical, and only exchange information over a small region of homology.^ However, in addition, polyploids with 24,48 and 60 chromosomes are found only in the New Zealand populations of Spirotrichosoma .
  • Evolution as a Self-Limiting Process 12 October 2009 14:26 UTC www.uvm.edu [Source type: FILTERED WITH BAYES]

^ Exchange of DNA is unequal, such that one homologue winds up with three copies of a region of the chromosome, while the other homologue has only one.

^ SRY The "sex-determining region of Y" - a gene on the Y chromosome that is believed to direct the differentiation of male gonads in mammals.

.Exchange takes place at sites where recombination nodules (the chiasmata) have formed.^ Chiasmata form at sites where programmed Spo11-catalyzed DNA double-stranded breaks (DSBs), induced early in meiotic prophase, are repaired to form crossovers [1] .
  • PLoS Genetics: The pch2Δ Mutation in Baker's Yeast Alters Meiotic Crossover Levels and Confers a Defect in Crossover Interference 12 October 2009 14:26 UTC www.plosgenetics.org [Source type: Academic]

^ In higher plants, meiosis takes place in specialized cells, the sporocytes, which are formed in the anthers and ovules.
  • The Arabidopsis-mei2-Like Genes Play a Role in Meiosis and Vegetative Growth in Arabidopsis -- Kaur et al. 18 (3): 545 -- THE PLANT CELL 11 January 2010 10:35 UTC www.plantcell.org [Source type: Academic]

^ It takes place during the long prophase of meiotic division I (prophase I), in which parts of homologous chromosomes are exchanged.
  • Germ Cells and Fertilization -- Molecular Biology of the Cell -- NCBI Bookshelf 11 January 2010 10:35 UTC www.ncbi.nlm.nih.gov [Source type: Academic]

.The exchange of information between the non-sister chromatids results in a recombination of information; each chromosome has the complete set of information it had before, and there are no gaps formed as a result of the process.^ The chromosomes have been formed, but no spindle has appeared.
  • Dissertation of Michael F. Guyer on Spermatogenesis in Pigeons 12 October 2009 14:26 UTC qspace.library.queensu.ca [Source type: FILTERED WITH BAYES]

^ Note that the sister chromatids of the same chromosome do not exchange with each other.
  • Germ Cells and Fertilization -- Molecular Biology of the Cell -- NCBI Bookshelf 11 January 2010 10:35 UTC www.ncbi.nlm.nih.gov [Source type: Academic]

^ Recombination between nonhomologous chromosomes.
  • Mnd1/Hop2 Facilitates Dmc1-Dependent Interhomolog Crossover Formation in Meiosis of Budding Yeast -- Henry et al. 26 (8): 2913 -- Molecular and Cellular Biology 11 January 2010 10:35 UTC mcb.asm.org [Source type: Academic]

.Because the chromosomes cannot be distinguished in the synaptonemal complex, the actual act of crossing over is not perceivable through the microscope.^ Because the chromosomes cannot be distinguished in the synaptonemal complex, the actual act of crossing over is not perceivable through the microscope.
  • Meiosis - Hwiki 11 January 2010 10:35 UTC hwiki.fzk.de [Source type: Academic]

^ We, however, will tend in this class to employ a more-narrow definition of eukaryotic chromosome , i.e., nuclear DNA -protein complexes that are visible through a light microscope .

^ As the synaptonemal complex disassembles in late prophase, each chromosome pair becomes visible as a tetrad, or group of four chromatids.
  • CHAPTER 13 MEIOSIS AND SEXUAL LIFE CYCLES 11 January 2010 10:35 UTC jade.ccccd.edu [Source type: Academic]

Diplotene
.During the diplotene stage, also known as diplonema, from Greek words meaning "two threads",[1] the synaptonemal complex degrades and homologous chromosomes separate from one another a little.^ Chromosomes become condensed during this stage.

^ Homologous chromosomes -chromosomes that pair during meiosis.
  • Technical Glossary of Breast Cancer Terms 12 October 2009 14:26 UTC www.cancerlynx.com [Source type: Academic]

^ But males have one X and one Y chromosome, and these chromosomes are not homologous.
  • Germ Cells and Fertilization -- Molecular Biology of the Cell -- NCBI Bookshelf 11 January 2010 10:35 UTC www.ncbi.nlm.nih.gov [Source type: Academic]

.The chromosomes themselves uncoil a bit, allowing some transcription of DNA. However, the homologous chromosomes of each bivalent remain tightly bound at chiasmata, the regions where crossing-over occurred.^ Chromosomes uncoil to become DNA. .
  • Student poems - mitosis 11 January 2010 10:35 UTC www.starsandseas.com [Source type: FILTERED WITH BAYES]

^ The sister chromatids in each homolog are kept tightly packed together, with their DNA extending from their own side of the protein ladder in a series of loops.
  • Germ Cells and Fertilization -- Molecular Biology of the Cell -- NCBI Bookshelf 11 January 2010 10:35 UTC www.ncbi.nlm.nih.gov [Source type: Academic]

^ As will be seen, crossing over may result in the incorporation of new combinations of specific segments into one chromosome that were previously carried separately in the parental homologous chromosomes.
  • PROCEDURES FOR USE OF GENIC MALE STERILITY IN PRODUCTION OF COMMERCIAL HYBRID MAIZE - Patent 3861079 12 October 2009 14:26 UTC www.freepatentsonline.com [Source type: Academic]

The chiasmata remain on the chromosomes until they are severed in Anaphase I.
.In human fetal oogenesis all developing oocytes develop to this stage and stop before birth.^ In human females, for example, oogonia proliferate only in the fetus, enter meiosis before birth, and become arrested as oocytes in the first meiotic prophase, in which state they may remain for up to 50 years.
  • Germ Cells and Fertilization -- Molecular Biology of the Cell -- NCBI Bookshelf 11 January 2010 10:35 UTC www.ncbi.nlm.nih.gov [Source type: Academic]

^ [CrossRef] [Web of Science] [Medline] Garcia M, Dietrich AJ, Freixa L, Vink AC, Ponsa M, Egozcue J. Development of the first meiotic prophase stages in human fetal oocytes observed by light and electron microscopy.
  • Meiosis in oocytes: predisposition to aneuploidy and its increased incidence with age -- Jones, 10.1093/humupd/dmm043 -- Human Reproduction Update 11 January 2010 10:35 UTC humupd.oxfordjournals.org [Source type: Academic]

^ Electron micrographs of developing primary oocytes in the rabbit ovary (A) An early stage of primary oocyte development.
  • Germ Cells and Fertilization -- Molecular Biology of the Cell -- NCBI Bookshelf 11 January 2010 10:35 UTC www.ncbi.nlm.nih.gov [Source type: Academic]

.This suspended state is referred to as the dictyotene stage and remains so until puberty.^ These cells enter the prophase stage of reduction division (meiosis I) but do not complete it until after the female reaches puberty.
  • Overview of Genetic Concepts. 12 October 2009 14:26 UTC members.shaw.ca [Source type: Academic]

.In males, only spermatogonia (spermatogenesis) exist until meiosis begins at puberty.^ In human males, by contrast, meiosis and spermatogenesis do not begin in the testes until puberty and then go on continuously in the epithelial lining of very long, tightly coiled tubes, called seminiferous tubules.
  • Germ Cells and Fertilization -- Molecular Biology of the Cell -- NCBI Bookshelf 11 January 2010 10:35 UTC www.ncbi.nlm.nih.gov [Source type: Academic]

^ An intriguing feature of spermatogenesis is that the developing male germ cells fail to complete cytoplasmic division (cytokinesis) during mitosis and meiosis.
  • Germ Cells and Fertilization -- Molecular Biology of the Cell -- NCBI Bookshelf 11 January 2010 10:35 UTC www.ncbi.nlm.nih.gov [Source type: Academic]

^ A ) produced only tetrads in male meiosis at either 22°C or 27°C ( Fig.
  • Progression through Meiosis I and Meiosis II in Arabidopsis Anthers Is Regulated by an A-Type Cyclin Predominately Expressed in Prophase I -- Wang et al. 136 (4): 4127 -- PLANT PHYSIOLOGY 11 January 2010 10:35 UTC www.plantphysiol.org [Source type: Academic]

Diakinesis
.Chromosomes condense further during the diakinesis stage, from Greek words meaning "moving through".[1] This is the first point in meiosis where the four parts of the tetrads are actually visible.^ Chromosomes become condensed during this stage.

^ The stage of meiosis during which crossing-over occurs is .
  • TE Cell Division and Reproduction - CK12 - Flexbooks 12 October 2009 14:26 UTC authors.ck12.org [Source type: Academic]

^ Homologous chromosomes -chromosomes that pair during meiosis.
  • Technical Glossary of Breast Cancer Terms 12 October 2009 14:26 UTC www.cancerlynx.com [Source type: Academic]

.Sites of crossing over entangle together, effectively overlapping, making chiasmata clearly visible.^ Chiasmata- site of crossing over.
  • Mitosis and Meiosis 11 January 2010 10:35 UTC www.und.nodak.edu [Source type: FILTERED WITH BAYES]

^ Sites of crossing over entangle together, effectively overlapping, making chiasma clearly visible.
  • Meiosis - Hwiki 11 January 2010 10:35 UTC hwiki.fzk.de [Source type: Academic]

^ Sister relationships are important in three respects: First, sister chromatid cohesion distal to the site of exchange is vital for the stabilization of the physical manifestations of crossing over, chiasmata, which hold the homologous pair together [74] – [76] .
  • PLoS Genetics: Csm4, in Collaboration with Ndj1, Mediates Telomere-Led Chromosome Dynamics and Recombination during Yeast Meiosis 12 October 2009 14:26 UTC www.plosgenetics.org [Source type: Academic]

.Other than this observation, the rest of the stage closely resembles prometaphase of mitosis; the nucleoli disappear, the nuclear membrane disintegrates into vesicles, and the meiotic spindle begins to form.^ The next step is the disappearance of the nuclear membrane and the attachment of the chromosomes to the spindle.
  • Dissertation of Michael F. Guyer on Spermatogenesis in Pigeons 12 October 2009 14:26 UTC qspace.library.queensu.ca [Source type: FILTERED WITH BAYES]

^ Telophase, Telophase envelop forms And the spindle fiber begin to disappear .
  • Student poems - mitosis 11 January 2010 10:35 UTC www.starsandseas.com [Source type: FILTERED WITH BAYES]

^ It is when new nuclear envelope forms and spindle fibers disappear What is it?
  • Student poems - mitosis 11 January 2010 10:35 UTC www.starsandseas.com [Source type: FILTERED WITH BAYES]

Synchronous processes
.During these stages, two centrosomes, containing a pair of centrioles in animal cells, migrate to the two poles of the cell.^ Notably, the frequency of ectopic recombination between these two loci in mcp6 cells was twice (202%) (in the M26 and 469 pair) that of wild-type cells ( Fig.
  • Mcp6, a meiosis-specific coiled-coil protein of Schizosaccharomyces pombe, localizes to the spindle pole body and is required for horsetail movement and recombination -- Saito et al. 118 (2): 447 -- Journal of Cell Science 12 October 2009 14:26 UTC jcs.biologists.org [Source type: Academic]

^ Our work suggests that cut11 + is essential for the formation of a bipolar mitotic spindle because it helps to anchor the spindle poles to the nuclear envelope during cell division.
  • cut11+: A Gene Required for Cell Cycle-dependent Spindle Pole Body Anchoring in the Nuclear Envelope and Bipolar Spindle Formation in Schizosaccharomyces pombe -- West et al. 9 (10): 2839 -- Molecular Biology of the Cell 12 October 2009 14:26 UTC www.molbiolcell.org [Source type: Academic]

^ These pairs fail to segregate normally, and many of the resulting gametes contain too many or too few chromosomes.
  • Germ Cells and Fertilization -- Molecular Biology of the Cell -- NCBI Bookshelf 11 January 2010 10:35 UTC www.ncbi.nlm.nih.gov [Source type: Academic]

.These centrosomes, which were duplicated during S-phase, function as microtubule organizing centers nucleating microtubules, which are essentially cellular ropes and poles.^ Microtubule-organizing centers and nucleating sites in land plants.
  • {gamma}-Tubulin and microtubule organization during meiosis in the liverwort Ricciocarpus natans (Ricciaceae) -- Brown and Lemmon 95 (6): 664 -- American Journal of Botany 11 January 2010 10:35 UTC www.amjbot.org [Source type: Academic]

^ Our work suggests that cut11 + is essential for the formation of a bipolar mitotic spindle because it helps to anchor the spindle poles to the nuclear envelope during cell division.
  • cut11+: A Gene Required for Cell Cycle-dependent Spindle Pole Body Anchoring in the Nuclear Envelope and Bipolar Spindle Formation in Schizosaccharomyces pombe -- West et al. 9 (10): 2839 -- Molecular Biology of the Cell 12 October 2009 14:26 UTC www.molbiolcell.org [Source type: Academic]

^ Calmodulin localizes to the spindle pole body of Schizosaccharomyces pombe and performs an essential function in chromosome segregation.
  • cut11+: A Gene Required for Cell Cycle-dependent Spindle Pole Body Anchoring in the Nuclear Envelope and Bipolar Spindle Formation in Schizosaccharomyces pombe -- West et al. 9 (10): 2839 -- Molecular Biology of the Cell 12 October 2009 14:26 UTC www.molbiolcell.org [Source type: Academic]

.The microtubules invade the nuclear region after the nuclear envelope disintegrates, attaching to the chromosomes at the kinetochore.^ The centromere is the chromosomal region over which the kinetochore is organized.

^ Oscillatory nuclear movement in fission yeast meiotic prophase is driven by astral microtubules, as revealed by continuous observation of chromosomes and microtubules in living cells.
  • Mcp6, a meiosis-specific coiled-coil protein of Schizosaccharomyces pombe, localizes to the spindle pole body and is required for horsetail movement and recombination -- Saito et al. 118 (2): 447 -- Journal of Cell Science 12 October 2009 14:26 UTC jcs.biologists.org [Source type: Academic]

^ Complete serial sections demonstrated that this SPB was not attached to the nuclear envelope; it is, therefore, interpreted as free floating in the nucleoplasm.
  • cut11+: A Gene Required for Cell Cycle-dependent Spindle Pole Body Anchoring in the Nuclear Envelope and Bipolar Spindle Formation in Schizosaccharomyces pombe -- West et al. 9 (10): 2839 -- Molecular Biology of the Cell 12 October 2009 14:26 UTC www.molbiolcell.org [Source type: Academic]

.The kinetochore functions as a motor, pulling the chromosome along the attached microtubule toward the originating centriole, like a train on a track.^ Microtubules that attach to the kinetochores are known as kinetochore microtubules .
  • Meiosis - Hwiki 11 January 2010 10:35 UTC hwiki.fzk.de [Source type: Academic]

^ Kinetochore fibers shorten and pull the chromosomes toward the poles.
  • Mitosis and the Cell Cycle 11 January 2010 10:35 UTC jade.ccccd.edu [Source type: Academic]

^ The kinetochore functions as a motor, pulling the chromosome along the attached microtubule toward the originating centriole, like a train on a track.
  • Meiosis - Hwiki 11 January 2010 10:35 UTC hwiki.fzk.de [Source type: Academic]

There are four kinetochores on each tetrad, but the pair of kinetochores on each sister chromatid fuses and functions as a unit during meiosis I. [2][3]
.Microtubules that attach to the kinetochores are known as kinetochore microtubules.^ Microtubules that attach to the kinetochores are known as kinetochore microtubules .
  • Meiosis - Hwiki 11 January 2010 10:35 UTC hwiki.fzk.de [Source type: Academic]

^ Microtubules from one pole are attached to the kinetochore of one chromosome of each tetrad, while those from the other pole are attached to the other.
  • CHAPTER 13 MEIOSIS AND SEXUAL LIFE CYCLES 11 January 2010 10:35 UTC jade.ccccd.edu [Source type: Academic]

^ The kinetochores of sister chromatids attach to microtubules extending from opposite poles.
  • CHAPTER 13 MEIOSIS AND SEXUAL LIFE CYCLES 11 January 2010 10:35 UTC jade.ccccd.edu [Source type: Academic]

.Other microtubules will interact with microtubules from the opposite centriole: these are called nonkinetochore microtubules or polar microtubules.^ Mutations can cause problems in the manufacture of these proteins that can disrupt their function and the function of other proteins they interact with.
  • Overview of Genetic Concepts. 12 October 2009 14:26 UTC members.shaw.ca [Source type: Academic]

^ These interact epistatically [The suppression of a gene by the effect of an unrelated gene] with other genes and gene variants to predispose to disease.
  • Overview of Genetic Concepts. 12 October 2009 14:26 UTC members.shaw.ca [Source type: Academic]

^ Proteins often function in clumps called protein complexes and often complexes interact with each other.
  • Overview of Genetic Concepts. 12 October 2009 14:26 UTC members.shaw.ca [Source type: Academic]

.A third type of microtubules, the aster microtubules, radiates from the centrosome into the cytoplasm or contacts components of the membrane skeleton.^ DNA is taken directly into the cell cytoplasm either through these pores or as a consequence of the redistribution of membrane components that accompanies closure of the pores.
  • Plant artificial chromosome compositions and methods - Patent 6900012 12 October 2009 14:26 UTC www.freepatentsonline.com [Source type: Academic]

^ The smaller centrosome, together with material of cytoplasmic origin, finally comes to lie inside of the nuclear membrane.

^ The centrosome persists and together with the tip of the spindle moves out into the cytoplasm to one side of the nucleus after the nuclear membrane is formed (Fig.
  • Dissertation of Michael F. Guyer on Spermatogenesis in Pigeons 12 October 2009 14:26 UTC qspace.library.queensu.ca [Source type: FILTERED WITH BAYES]

Metaphase I

.Homologous pairs move together along the metaphase plate: As kinetochore microtubules from both centrioles attach to their respective kinetochores, the homologous chromosomes align along an equatorial plane that bisects the spindle, due to continuous counterbalancing forces exerted on the bivalents by the microtubules emanating from the two kinetochores of homologous chromosomes.^ A. four homologous chromosomes B. four sex chromosomes C. two sister chromatids attached by the centromere D. two sister chromatids attached by the centrioles 8.
  • TE Cell Division and Reproduction - CK12 - Flexbooks 12 October 2009 14:26 UTC authors.ck12.org [Source type: Academic]

^ The chiasmata hold together pairs of homologous chromosomes, or bivalents as they are usually called.
  • The Cell Nucleus | NZETC 12 October 2009 14:26 UTC www.nzetc.org [Source type: FILTERED WITH BAYES]

^ Bivalents, each composed of two chromosomes (four chromatids) align at the metaphase plate.
  • Overview of Genetic Concepts. 12 October 2009 14:26 UTC members.shaw.ca [Source type: Academic]

.The physical basis of the independent assortment of chromosomes is the random orientation of each bivalent along the metaphase plate, with respect to the orientation of the other bivalents along the same equatorial line.^ Independent assortment The random distribution of chromosomes to gametes during meiosis.

^ Note how the chromosomes are lined up at the metaphase plate.
  • TE Cell Division and Reproduction - CK12 - Flexbooks 12 October 2009 14:26 UTC authors.ck12.org [Source type: Academic]

^ Random assortment of chromosomes .

Anaphase I

.Kinetochore microtubules shorten, severing the recombination nodules and pulling homologous chromosomes apart.^ Meiosis involves several phases in two major cell divisions, one after the other, producing four daughter cells, each with only one chromosome (chromatid) from each homologous pair.
  • Overview of Genetic Concepts. 12 October 2009 14:26 UTC members.shaw.ca [Source type: Academic]

^ In recombination-proficient organisms, chromosome disjunction at meiosis I generally occurs by chiasma formation between the homologs (chiasmate meiosis).
  • Live observation of fission yeast meiosis in recombination-deficient mutants: a study on achiasmate chromosome segregation -- Molnar et al. 114 (15): 2843 -- Journal of Cell Science 11 January 2010 10:35 UTC jcs.biologists.org [Source type: Academic]

^ This interpretation is supported by the observation that, during the selection of templates for recombinational DNA repair in vivo, sister chromatids are preferred to homologous chromosomes (Kadyk and Hartwell, 1992 ).
  • Fission yeast Pds5 is required for accurate chromosome segregation and for survival after DNA damage or metaphase arrest -- Wang et al. 115 (3): 587 -- Journal of Cell Science 12 October 2009 14:26 UTC jcs.biologists.org [Source type: Academic]

.Since each chromosome has only one functional unit of a pair of kinetochores[3], whole chromosomes are pulled toward opposing poles, forming two haploid sets.^ Haploid -the chromosome number of a normal gamete, with only one member of each chromosome pair.
  • Technical Glossary of Breast Cancer Terms 12 October 2009 14:26 UTC www.cancerlynx.com [Source type: Academic]

^ Each pole will receive one set of chromosomes of 2 chromatids each.
  • Overview of Genetic Concepts. 12 October 2009 14:26 UTC members.shaw.ca [Source type: Academic]

^ Two chromosomes can fuse together to form one.
  • Evolution as a Self-Limiting Process 12 October 2009 14:26 UTC www.uvm.edu [Source type: FILTERED WITH BAYES]

.Each chromosome still contains a pair of sister chromatids.^ At the end of anaphase I, each chromosome still contains 2 chromatids.
  • Mitosis and the Cell Cycle 11 January 2010 10:35 UTC jade.ccccd.edu [Source type: Academic]

^ This pairing will produce two tetrads containing 4 sister chromatids each.

^ Sister chromatids of each chromosome are joined at the centromere.
  • Meiosis and Fertilization -- The Cell -- NCBI Bookshelf 11 January 2010 10:35 UTC www.ncbi.nlm.nih.gov [Source type: Academic]

Nonkinetochore microtubules lengthen, pushing the centrioles farther apart. .The cell elongates in preparation for division down the center.^ All these and undoubtedly other preparations for cell division set the stage for the initiation of mitosis.
  • The Cell Nucleus | NZETC 12 October 2009 14:26 UTC www.nzetc.org [Source type: FILTERED WITH BAYES]

^ They approach each other but remain distinct until after the membrane of each pronucleus has broken down in preparation for the zygote's first mitotic division ( Figure 20-34 ).
  • Germ Cells and Fertilization -- Molecular Biology of the Cell -- NCBI Bookshelf 11 January 2010 10:35 UTC www.ncbi.nlm.nih.gov [Source type: Academic]

^ A somatic cell with a diameter of 10–20 μm typically takes about 24 hours to double its mass in preparation for cell division.
  • Germ Cells and Fertilization -- Molecular Biology of the Cell -- NCBI Bookshelf 11 January 2010 10:35 UTC www.ncbi.nlm.nih.gov [Source type: Academic]

Telophase I

.The last meiotic division effectively ends when the chromosomes arrive at the poles.^ If we consider chromosome segregation, chromatids only finish segregation by telophase II, consistent with an end of a nuclear division, as defined in mitosis.
  • Progression through Meiosis I and Meiosis II in Arabidopsis Anthers Is Regulated by an A-Type Cyclin Predominately Expressed in Prophase I -- Wang et al. 136 (4): 4127 -- PLANT PHYSIOLOGY 11 January 2010 10:35 UTC www.plantphysiol.org [Source type: Academic]

^ Also in this study, when possible, we used RH and meiotic linkage mapping to position chromosome-specific markers that had been FISH-mapped to both the proximal and distal ends of each chromosome.
  • Chromosome-Specific Single-Locus FISH Probes Allow Anchorage of an 1800-Marker Integrated Radiation-Hybrid/Linkage Map of the Domestic Dog Genome to All Chromosomes 12 October 2009 14:26 UTC www.pubmedcentral.nih.gov [Source type: Academic]

^ During the second meiotic division, sister chromatids segregate to the opposite poles (equational segregation).
  • Activation of the pheromone-responsive MAP kinase drives haploid cells to undergo ectopic meiosis with normal telomere clustering and sister chromatid segregation in fission yeast -- Yamamoto et al. 117 (17): 3875 -- Journal of Cell Science 11 January 2010 10:35 UTC jcs.biologists.org [Source type: Academic]

.Each daughter cell now has half the number of chromosomes but each chromosome consists of a pair of chromatids.^ The separated chromatids are now known as daughter chromosomes, each of which has only one chromatid.
  • Mitosis and the Cell Cycle 11 January 2010 10:35 UTC jade.ccccd.edu [Source type: Academic]

^ Each chromosome consists of two sister chromatids.
  • CHAPTER 13 MEIOSIS AND SEXUAL LIFE CYCLES 11 January 2010 10:35 UTC jade.ccccd.edu [Source type: Academic]

^ Half the sperm cells receive an X chromosome, and half receive a Y chromosome.
  • Chapter 15 - The Chromosomal Basis of Inheritance 12 October 2009 14:26 UTC homepage.mac.com [Source type: Academic]

.The microtubules that make up the spindle network disappear, and a new nuclear membrane surrounds each haploid set.^ The next step is the disappearance of the nuclear membrane and the attachment of the chromosomes to the spindle.
  • Dissertation of Michael F. Guyer on Spermatogenesis in Pigeons 12 October 2009 14:26 UTC qspace.library.queensu.ca [Source type: FILTERED WITH BAYES]

^ It is when new nuclear envelope forms and spindle fibers disappear What is it?
  • Student poems - mitosis 11 January 2010 10:35 UTC www.starsandseas.com [Source type: FILTERED WITH BAYES]

^ This effect produces a variety of responses from the neuro-synrchromatic enzyme, also known as NSE. The microtubules that make up the spindle network disappear, and a new nuclear membrane surrounds each haploid set.
  • Meiosis - Hwiki 11 January 2010 10:35 UTC hwiki.fzk.de [Source type: Academic]

.The chromosomes uncoil back into chromatin.^ The chromosomes uncoil back into chromatin.
  • Meiosis - Hwiki 11 January 2010 10:35 UTC hwiki.fzk.de [Source type: Academic]

^ During prophase I the nuclear chromatin becomes transformed into chromosomes as in somatic mitosis, the nucleolus likewise gradually disappears, and the nuclear membrane breaks down.
  • The Cell Nucleus | NZETC 12 October 2009 14:26 UTC www.nzetc.org [Source type: FILTERED WITH BAYES]

^ For this discussion, complexity-reduction mechanisms will be separated into 3 levels: the chromatin (or sub-chromosomal) level, the chromosome level, and the nuclear level.
  • Why Meiosis is my Favorite Biological Problem 11 January 2010 10:35 UTC www.ocf.berkeley.edu [Source type: Academic]

.Cytokinesis, the pinching of the cell membrane in animal cells or the formation of the cell wall in plant cells, occurs, completing the creation of two daughter cells.^ Cytokinesis, the pinching of the cell membrane in animal cells or the formation of the cell wall in plant cells, occurs, completing the creation of two daughter cells.
  • Meiosis - Hwiki 11 January 2010 10:35 UTC hwiki.fzk.de [Source type: Academic]

^ It is two new daughter cells What is it?
  • Student poems - mitosis 11 January 2010 10:35 UTC www.starsandseas.com [Source type: FILTERED WITH BAYES]

^ Cytokinesis continues and cell division is completed.
  • Mitosis and the Cell Cycle 11 January 2010 10:35 UTC jade.ccccd.edu [Source type: Academic]

Sister chromatids remain attached during telophase I.
.Cells may enter a period of rest known as interkinesis or interphase II. No DNA replication occurs during this stage.^ The cell grows and replicates its DNA and centrioles.

^ It is during interphase that DNA replication and gene expression occur.

^ The period in the cell cycle when DNA is replicated in the nucleus; followed by mitosis.

Meiosis II

.Meiosis II is the second part of the meiotic process.^ Formation of the actual gamete nuclei can now proceed simply through a second cell division, division II of meiosis , without further DNA replication.
  • Germ Cells and Fertilization -- Molecular Biology of the Cell -- NCBI Bookshelf 11 January 2010 10:35 UTC www.ncbi.nlm.nih.gov [Source type: Academic]

^ First, it takes part in the same biological process as the target genes (nucleobase biosynthesis), and second, it physically interacts with the module's regulatory protein Ppr1.
  • PLoS Genetics: Understanding Gene Sequence Variation in the Context of Transcription Regulation in Yeast 11 January 2010 10:35 UTC www.plosgenetics.org [Source type: Academic]

^ Prophase of meiosis I is termed prophase I to distinguish it from the prophase of meiosis II, or prophase II; likewise for the other phases of the two meiotic divisions.
  • The Cell Nucleus | NZETC 12 October 2009 14:26 UTC www.nzetc.org [Source type: FILTERED WITH BAYES]

Much of the process is similar to mitosis. The end result is production of four haploid cells (23 chromosomes, 1N in humans) from the two haploid cells (23 chromosomes, 1N * each of the chromosomes consisting of two sister chromatids) produced in meiosis I. The four main steps of Meiosis II are: Prophase II, Metaphase II, Anaphase II, and Telophase II.
.Prophase II takes an inversely proportional time compared to prophase I.[citation needed] In this prophase we see the disappearance of the nucleoli and the nuclear envelope again as well as the shortening and thickening of the chromatids.^ Nonetheless, it is evident that chromosomal movement is largely hampered in mcp6 cells compared with the vigorous nuclear movements in wild-type cells that occur several times after karyogamy ( Fig.
  • Mcp6, a meiosis-specific coiled-coil protein of Schizosaccharomyces pombe, localizes to the spindle pole body and is required for horsetail movement and recombination -- Saito et al. 118 (2): 447 -- Journal of Cell Science 12 October 2009 14:26 UTC jcs.biologists.org [Source type: Academic]

^ This SPB localization correlates with the time in the cell cycle when SPBs are inserted into the nuclear envelope.
  • cut11+: A Gene Required for Cell Cycle-dependent Spindle Pole Body Anchoring in the Nuclear Envelope and Bipolar Spindle Formation in Schizosaccharomyces pombe -- West et al. 9 (10): 2839 -- Molecular Biology of the Cell 12 October 2009 14:26 UTC www.molbiolcell.org [Source type: Academic]

^ The nuclear envelope remains intact and disappears only when the meiotic spindle begins to form, as prophase I gives way to metaphase I. After prophase I is completed, two successive cell divisions follow without an intervening period of DNA synthesis.
  • Germ Cells and Fertilization -- Molecular Biology of the Cell -- NCBI Bookshelf 11 January 2010 10:35 UTC www.ncbi.nlm.nih.gov [Source type: Academic]

.Centrioles move to the polar regions and arrange spindle fibers for the second meiotic division.^ Selected frames of the second meiotic division.
  • Live observation of fission yeast meiosis in recombination-deficient mutants: a study on achiasmate chromosome segregation -- Molnar et al. 114 (15): 2843 -- Journal of Cell Science 11 January 2010 10:35 UTC jcs.biologists.org [Source type: Academic]

^ The first of these are the chromosomes, the second are the centromeres, and the third are the centrioles, the self-replicating structures at the ends of the spindle on which the chromosomes move.
  • Evolution as a Self-Limiting Process 12 October 2009 14:26 UTC www.uvm.edu [Source type: FILTERED WITH BAYES]

^ Through experimental gynogenesis one can detect heterozygosity by employing heterozygous females and then inhibiting the second meiotic division.
  • An Evolutionary Manifesto 12 October 2009 14:26 UTC www.uvm.edu [Source type: FILTERED WITH BAYES]

.In metaphase II, the centromeres contain two kinetochores that attach to spindle fibers from the centrosomes (centrioles) at each pole.^ Each centrosome has a complex called a kinetochore, which attaches to a fiber that's connected to only one spindle pole.
  • Basics: How can chromosome numbers change? : Pharyngula 12 October 2009 14:26 UTC scienceblogs.com [Source type: General]

^ In particular, two out of four complete reconstructions showed a single SPB with a monopolar spindle attached free floating in the nucleoplasm.
  • cut11+: A Gene Required for Cell Cycle-dependent Spindle Pole Body Anchoring in the Nuclear Envelope and Bipolar Spindle Formation in Schizosaccharomyces pombe -- West et al. 9 (10): 2839 -- Molecular Biology of the Cell 12 October 2009 14:26 UTC www.molbiolcell.org [Source type: Academic]

^ Here the pericentromeric crossover may interfere with centromeric cohesion and/or kinetochore attachment to allow both homologs to move to the same pole (Fig.
  • Meiosis in oocytes: predisposition to aneuploidy and its increased incidence with age -- Jones, 10.1093/humupd/dmm043 -- Human Reproduction Update 11 January 2010 10:35 UTC humupd.oxfordjournals.org [Source type: Academic]

.The new equatorial metaphase plate is rotated by 90 degrees when compared to meiosis I, perpendicular to the previous plate.^ A most important factor governing a regular division of the nucleus in meiosis and mitosis is the co-orientation of the chromosome or bivalent centromeres to form the so called metaphase plate.
  • The Cell Nucleus | NZETC 12 October 2009 14:26 UTC www.nzetc.org [Source type: FILTERED WITH BAYES]

.This is followed by anaphase II, where the centromeres are cleaved, allowing microtubules attached to the kinetochores to pull the sister chromatids apart.^ Anaphase is the phase in which the sister chromatids separate.
  • TE Cell Division and Reproduction - CK12 - Flexbooks 12 October 2009 14:26 UTC authors.ck12.org [Source type: Academic]

^ Comparison of the mechanisms of chromosome alignment (at metaphase) and separation (at anaphase) in meiotic division I and meiotic division II The ungluing of the sister chromatid arms allows the duplicated homologs to separate at anaphase I, while an ungluing of the chromosomes at their centromeres allows the sister chromatids to separate at anaphase II. In contrast, at anaphase in mitosis, both the arms and the centromeres come apart at the same time (discussed in Chapter 18 ).
  • Germ Cells and Fertilization -- Molecular Biology of the Cell -- NCBI Bookshelf 11 January 2010 10:35 UTC www.ncbi.nlm.nih.gov [Source type: Academic]

^ This absence of kinetochore fission in the first meiotic division (MI) reflects a difference in the molecular mechanics of centromere–microtubule attachment, a consequence of the geometry of sister-chromatid placement when homologs are paired.
  • The Evolution of Meiosis From Mitosis -- Wilkins and Holliday 181 (1): 3 -- Genetics 11 January 2010 10:35 UTC www.genetics.org [Source type: Academic]

.The sister chromatids by convention are now called sister chromosomes as they move toward opposing poles.^ During these movements the two sister chromatids (now chromosomes in their own right) fall apart along their lengths and move away from each other and away from the equator towards opposite poles, headed, as in prometaphase, by the centromeres (Fig.
  • The Cell Nucleus | NZETC 12 October 2009 14:26 UTC www.nzetc.org [Source type: FILTERED WITH BAYES]

^ Pds5p is an essential chromosomal protein required for both sister chromatid cohesion and condensation in Saccharomyces cerevisiae.
  • Fission yeast Pds5 is required for accurate chromosome segregation and for survival after DNA damage or metaphase arrest -- Wang et al. 115 (3): 587 -- Journal of Cell Science 12 October 2009 14:26 UTC jcs.biologists.org [Source type: Academic]

^ During ________, genetic material on a section of a chromosome is swapped between non-sister chromatids during prophase I of meiosis.
  • TE Cell Division and Reproduction - CK12 - Flexbooks 12 October 2009 14:26 UTC authors.ck12.org [Source type: Academic]

.The process ends with telophase II, which is similar to telophase I, and is marked by uncoiling and lengthening of the chromosomes and the disappearance of the spindle.^ The last process of mitosis is Telophase, This is when chromosomes are at each end going face to face.
  • Student poems - mitosis 11 January 2010 10:35 UTC www.starsandseas.com [Source type: FILTERED WITH BAYES]

^ Telophase the last phase The chromosomes uncoil I see something new New nuclear envelopes form I cant see the spindle fibers because They broke down and disappeared I see a completed mitosis I see it I really do .
  • Student poems - mitosis 11 January 2010 10:35 UTC www.starsandseas.com [Source type: FILTERED WITH BAYES]

^ (As bacteria have only one chromosome, there is no spindle, no sorting of the chromosomes, and no migration to opposite ends of the cell.
  • TE Cell Division and Reproduction - CK12 - Flexbooks 12 October 2009 14:26 UTC authors.ck12.org [Source type: Academic]

.Nuclear envelopes reform and cleavage or cell wall formation eventually produces a total of four daughter cells, each with a haploid set of chromosomes.^ Four haploid cells are therefore produced from each cell that enters meiosis.
  • Germ Cells and Fertilization -- Molecular Biology of the Cell -- NCBI Bookshelf 11 January 2010 10:35 UTC www.ncbi.nlm.nih.gov [Source type: Academic]

^ HAPLOID daughter cells produced .
  • Meiosis / Flashcards - Create Free Flashcards 11 January 2010 10:35 UTC www.proprofs.com [Source type: Academic]

^ Assume that this cell has four pairs of chromosomes.
  • TE Cell Division and Reproduction - CK12 - Flexbooks 12 October 2009 14:26 UTC authors.ck12.org [Source type: Academic]

.Meiosis is now complete and ends up with four new daughter cells.^ The two new cells are called daughter cells.

^ Meiosis begins with one diploid cell and ends with four haploid cells.
  • TE Cell Division and Reproduction - CK12 - Flexbooks 12 October 2009 14:26 UTC authors.ck12.org [Source type: Academic]

^ Each cell that begins meiosis gives rise to four mature gametes rather than one.
  • Germ Cells and Fertilization -- Molecular Biology of the Cell -- NCBI Bookshelf 11 January 2010 10:35 UTC www.ncbi.nlm.nih.gov [Source type: Academic]

Significance

.Meiosis facilitates stable sexual reproduction.^ Encyclopedia II - Meiosis - Significance of meiosis Meiosis facilitates stable sexual reproduction.

^ Meiosis is the first step in sexual reproduction.
  • Mitosis and Meiosis 11 January 2010 10:35 UTC www.und.nodak.edu [Source type: FILTERED WITH BAYES]

^ The sexual reproductive cycle involves an alternation of diploid and haploid states: diploid cells divide by meiosis to form haploid cells, and the haploid cells from two individuals fuse in pairs at fertilization to form new diploid cells.
  • Germ Cells and Fertilization -- Molecular Biology of the Cell -- NCBI Bookshelf 11 January 2010 10:35 UTC www.ncbi.nlm.nih.gov [Source type: Academic]

.Without the halving of ploidy, or chromosome count, fertilization would result in zygotes that have twice the number of chromosomes as the zygotes from the previous generation.^ If an egg carrying chromosome in the standard form shown in FIG. 33 is fertilized by a sperm carrying the homologous inversion-bearing chromosome shown in FIG. 34, the resulting plant is heterozygous for the inversion.
  • PROCEDURES FOR USE OF GENIC MALE STERILITY IN PRODUCTION OF COMMERCIAL HYBRID MAIZE - Patent 3861079 12 October 2009 14:26 UTC www.freepatentsonline.com [Source type: Academic]

^ As will be seen, crossing over may result in the incorporation of new combinations of specific segments into one chromosome that were previously carried separately in the parental homologous chromosomes.
  • PROCEDURES FOR USE OF GENIC MALE STERILITY IN PRODUCTION OF COMMERCIAL HYBRID MAIZE - Patent 3861079 12 October 2009 14:26 UTC www.freepatentsonline.com [Source type: Academic]

^ This would lead to the conclusion that perhaps in some cases the chromosomes rearranged themselves for the second division without passing through a resting stage.
  • Dissertation of Michael F. Guyer on Spermatogenesis in Pigeons 12 October 2009 14:26 UTC qspace.library.queensu.ca [Source type: FILTERED WITH BAYES]

.Successive generations would have an exponential increase in chromosome count.^ In this instance, the non-transmissibility of recombinant chromatids would be the result of the generation of new kinds of chromosome deficiency as the result of crossing over.
  • PROCEDURES FOR USE OF GENIC MALE STERILITY IN PRODUCTION OF COMMERCIAL HYBRID MAIZE - Patent 3861079 12 October 2009 14:26 UTC www.freepatentsonline.com [Source type: Academic]

^ Irregular division of chromosomes would be likely to produce marked variation, but as Guyer himself observes, these irregularities increase with the degree of infertility.

^ With successive generations, the size of the repeated section keeps increasing and patients commonly show symptoms at an earlier age and / or they may show more severe symptoms.
  • Overview of Genetic Concepts. 12 October 2009 14:26 UTC members.shaw.ca [Source type: Academic]

.In organisms that are normally diploid, polyploidy, the state of having three or more sets of chromosomes, results in extreme developmental abnormalities or lethality [4].^ Polyploidy -the condition of a cell or organism with more than two complete sets of chromosomes.
  • Technical Glossary of Breast Cancer Terms 12 October 2009 14:26 UTC www.cancerlynx.com [Source type: Academic]

^ There can be a benefit to having more chromosomes.
  • Basics: How can chromosome numbers change? : Pharyngula 12 October 2009 14:26 UTC scienceblogs.com [Source type: General]

^ Polysomy -the condition of a diploid cell or organism having three or more copies of a particular chromosome.
  • Technical Glossary of Breast Cancer Terms 12 October 2009 14:26 UTC www.cancerlynx.com [Source type: Academic]

.Polyploidy is poorly tolerated in most animal species.^ In most animals, the developing oocyte is surrounded by specialized accessory cells that help to isolate and nourish it (not shown).
  • Germ Cells and Fertilization -- Molecular Biology of the Cell -- NCBI Bookshelf 11 January 2010 10:35 UTC www.ncbi.nlm.nih.gov [Source type: Academic]

.Plants, however, regularly produce fertile, viable polyploids.^ Only the tassels of the adjacent normal, male fertile Y plants would produce pollen for pollinating X and formation of hybrid XY seed on X plants would be assured.
  • PROCEDURES FOR USE OF GENIC MALE STERILITY IN PRODUCTION OF COMMERCIAL HYBRID MAIZE - Patent 3861079 12 October 2009 14:26 UTC www.freepatentsonline.com [Source type: Academic]

^ It is not necessary, however, that all deficient or duplicate-deficient eggs or zygotes produced on an ear be viable.
  • PROCEDURES FOR USE OF GENIC MALE STERILITY IN PRODUCTION OF COMMERCIAL HYBRID MAIZE - Patent 3861079 12 October 2009 14:26 UTC www.freepatentsonline.com [Source type: Academic]

^ The plants also were allowed to self-fertilize and the seed they produced was collected (a list of the seed stock of informative individuals used for tetrad analysis is given in FIG. 4 ).
  • Plant artificial chromosome compositions and methods - Patent 6900012 12 October 2009 14:26 UTC www.freepatentsonline.com [Source type: Academic]

.Polyploidy has been implicated as an important mechanism in plant speciation.^ A further object of the present invention is to provide an apomictic hybrid plant that produces progeny identical to itself by transferring an apomictic mechanism from a wild species to a cultivated plant.
  • US Patent 5811636 - Apomixis for producing true-breeding plant progenies 12 October 2009 14:26 UTC www.wikipatents.com [Source type: Academic]

^ SUMMARY OF THE INVENTION It is, therefore, an object of the present invention to provide a true-breeding plant which is apomictic by transferring an apomictic mechanism from a wild species to a cultivated plant.
  • US Patent 5811636 - Apomixis for producing true-breeding plant progenies 12 October 2009 14:26 UTC www.wikipatents.com [Source type: Academic]

^ Description FIELD OF THE INVENTION This invention relates to plants that breed true by transferring the apomictic mechanism of reproduction from a wild plant species to a cultivated plant to form a true-breeding hybrid.
  • US Patent 5811636 - Apomixis for producing true-breeding plant progenies 12 October 2009 14:26 UTC www.wikipatents.com [Source type: Academic]

.Most importantly, recombination and independent assortment of homologous chromosomes allow for a greater diversity of genotypes in the population.^ Thus, it enhances the chance that a chromosome encounters its correct partner and thereby promotes the linkage of homologous pairs of chromosomes through homologous recombination.
  • Mcp6, a meiosis-specific coiled-coil protein of Schizosaccharomyces pombe, localizes to the spindle pole body and is required for horsetail movement and recombination -- Saito et al. 118 (2): 447 -- Journal of Cell Science 12 October 2009 14:26 UTC jcs.biologists.org [Source type: Academic]

^ Each homologous pair of chromosomes aligns and orients independently of the other pairs, thus, the first meiotic division results in an independent assortment of maternal and paternal chromosomes.
  • Overview of Genetic Concepts. 12 October 2009 14:26 UTC members.shaw.ca [Source type: Academic]

^ The distribution of one pair of alleles is independent of other genes on non-homologous chromosomes.

.This produces genetic variation in gametes that promote genetic and phenotypic variation in a population of offspring.^ Genome -the complete DNA sequence containing the entire genetic information of a gamete, an individual, a population, or a species.
  • Technical Glossary of Breast Cancer Terms 12 October 2009 14:26 UTC www.cancerlynx.com [Source type: Academic]

^ In some populations, selection pressure on some traits is so low that they "drift", and so much of the variation in phenotypes we see is just the result of what we call neutral theory: .
  • Basics: How can chromosome numbers change? : Pharyngula 12 October 2009 14:26 UTC scienceblogs.com [Source type: General]

^ De or dE.) Having more chromosomes means the same genetic diversity from a parent can be arranged in more combinations in its gametes (sperm & eggs) and more variation means a better chance of having an offspring match the selective conditions it is about to face.
  • Basics: How can chromosome numbers change? : Pharyngula 12 October 2009 14:26 UTC scienceblogs.com [Source type: General]

Nondisjunction

.The normal separation of chromosomes in meiosis I or sister chromatids in meiosis II is termed disjunction.^ Meiosis 2 separates chromatids as .
  • Student poems - mitosis 11 January 2010 10:35 UTC www.starsandseas.com [Source type: FILTERED WITH BAYES]

^ Anaphase is the phase in which the sister chromatids separate.
  • TE Cell Division and Reproduction - CK12 - Flexbooks 12 October 2009 14:26 UTC authors.ck12.org [Source type: Academic]

^ Disjunction The separation of chromosomes at anaphase.

.When the separation is not normal, it is called nondisjunction.^ When the separation is not normal, it is called nondisjunction.

^ Encyclopedia II - Meiosis - Nondisjunction The normal separation of chromosomes in Meiosis I or sister chromatids in meiosis II is termed disjunction.

^ Occasionally during meiosis, chromosomes fail to separate normally into the four haploid cells, a phenomenon known as nondisjunction.
  • Germ Cells and Fertilization -- Molecular Biology of the Cell -- NCBI Bookshelf 11 January 2010 10:35 UTC www.ncbi.nlm.nih.gov [Source type: Academic]

.This results in the production of gametes which have either too many or too few of a particular chromosome, and is a common mechanism for trisomy or monosomy.^ Nondisjunction results in monosomy and/or trisomy.

^ These pairs fail to segregate normally, and many of the resulting gametes contain too many or too few chromosomes.
  • Germ Cells and Fertilization -- Molecular Biology of the Cell -- NCBI Bookshelf 11 January 2010 10:35 UTC www.ncbi.nlm.nih.gov [Source type: Academic]

^ Chromosomal abnormalities: include structural and numerical defects (too many or too few chromosomes) (example: Down Syndrome, Trisomy 21- an extra whole chromosome 21) .
  • Overview of Genetic Concepts. 12 October 2009 14:26 UTC members.shaw.ca [Source type: Academic]

.Nondisjunction can occur in the meiosis I or meiosis II, phases of cellular reproduction, or during mitosis.^ The stage of meiosis during which crossing-over occurs is .
  • TE Cell Division and Reproduction - CK12 - Flexbooks 12 October 2009 14:26 UTC authors.ck12.org [Source type: Academic]

^ Asynchronous M-phase during meiosis II in tam .
  • The Arabidopsis Gene Tardy Asynchronous Meiosis Is Required for the Normal Pace and Synchrony of Cell Division during Male Meiosis -- Magnard et al. 127 (3): 1157 -- PLANT PHYSIOLOGY 11 January 2010 10:35 UTC www.plantphysiol.org [Source type: Academic]

^ Asynchronous M-phase during meiosis I in tam .
  • The Arabidopsis Gene Tardy Asynchronous Meiosis Is Required for the Normal Pace and Synchrony of Cell Division during Male Meiosis -- Magnard et al. 127 (3): 1157 -- PLANT PHYSIOLOGY 11 January 2010 10:35 UTC www.plantphysiol.org [Source type: Academic]

This is a cause of several medical conditions in humans (such as):

Meiosis in mammals

.In females, meiosis occurs in cells known as oogonia (singular: oogonium).^ The tam mutant is defective in cell cycle progression during male meiosis, and as a consequence dyads are formed when cytokinesis occurs.
  • The Arabidopsis Gene Tardy Asynchronous Meiosis Is Required for the Normal Pace and Synchrony of Cell Division during Male Meiosis -- Magnard et al. 127 (3): 1157 -- PLANT PHYSIOLOGY 11 January 2010 10:35 UTC www.plantphysiol.org [Source type: Academic]

^ In each Arabidopsis flower, male meiosis occurs synchronously in about 500 microspore mother cells in the four medial anthers.
  • The Arabidopsis Gene Tardy Asynchronous Meiosis Is Required for the Normal Pace and Synchrony of Cell Division during Male Meiosis -- Magnard et al. 127 (3): 1157 -- PLANT PHYSIOLOGY 11 January 2010 10:35 UTC www.plantphysiol.org [Source type: Academic]

^ Chromosome breakage and reunion are known to occur spontaneously (and can be induced) outside meiosis, but whether or not such a process occurs in crossing-over and chiasma formation has not been entirely proven.
  • The Cell Nucleus | NZETC 12 October 2009 14:26 UTC www.nzetc.org [Source type: FILTERED WITH BAYES]

.Each oogonium that initiates meiosis will divide twice to form a single oocyte and two polar bodies.^ The polar bodies do not form gametes.
  • TE Cell Division and Reproduction - CK12 - Flexbooks 12 October 2009 14:26 UTC authors.ck12.org [Source type: Academic]

^ The polar body from meiosis I divides again to form 2 polar bodies.
  • TE Cell Division and Reproduction - CK12 - Flexbooks 12 October 2009 14:26 UTC authors.ck12.org [Source type: Academic]

^ During the process of oogenesis, mature eggs (ova) are formed by meiosis of precursor cells called primary oocytes.
  • TE Cell Division and Reproduction - CK12 - Flexbooks 12 October 2009 14:26 UTC authors.ck12.org [Source type: Academic]

[5] .However, before these divisions occur, these cells stop at the diplotene stage of meiosis I and lie dormant within a protective shell of somatic cells called the follicle.^ The stage of meiosis during which crossing-over occurs is .
  • TE Cell Division and Reproduction - CK12 - Flexbooks 12 October 2009 14:26 UTC authors.ck12.org [Source type: Academic]

^ Somatic mutation A mutation that occurs in a somatic cell.

^ Here, we describe an Arabidopsis mutant, named tardy asynchronous meiosis ( tam ), that exhibits a phenotype of delayed and asynchronous cell divisions during male meiosis.
  • The Arabidopsis Gene Tardy Asynchronous Meiosis Is Required for the Normal Pace and Synchrony of Cell Division during Male Meiosis -- Magnard et al. 127 (3): 1157 -- PLANT PHYSIOLOGY 11 January 2010 10:35 UTC www.plantphysiol.org [Source type: Academic]

.Follicles begin growth at a steady pace in a process known as folliculogenesis, and a small number enter the menstrual cycle.^ This capability will greatly enhance the efficiency with which new mutations will be studied as the number of known mutations begins to plateau.
  • Multinational Coordinated Arabidopsis thaliana Genome Research Project 12 October 2009 14:26 UTC arabidopsis.info [Source type: Academic]

^ Chromosome loss was measured in the progeny from a single ade + cell after a known number of generations during which selection for adenine prototrophy had been relaxed by growth on YE agar.
  • Fission yeast Pds5 is required for accurate chromosome segregation and for survival after DNA damage or metaphase arrest -- Wang et al. 115 (3): 587 -- Journal of Cell Science 12 October 2009 14:26 UTC jcs.biologists.org [Source type: Academic]

.Menstruated oocytes continue meiosis I and arrest at meiosis II until fertilization.^ Completion of meiosis II at fertilization .
  • Meiosis in oocytes: predisposition to aneuploidy and its increased incidence with age -- Jones, 10.1093/humupd/dmm043 -- Human Reproduction Update 11 January 2010 10:35 UTC humupd.oxfordjournals.org [Source type: Academic]

^ In most vertebrates, oocyte maturation proceeds to metaphase of meiosis II and then arrests until fertilization.
  • Germ Cells and Fertilization -- Molecular Biology of the Cell -- NCBI Bookshelf 11 January 2010 10:35 UTC www.ncbi.nlm.nih.gov [Source type: Academic]

^ In fact, the pre-egg cell does not complete meiosis until after fertilization has occurred.
  • Inside the Cell: Chapter 4: Cellular Reproduction: Multiplication by Division - National Institute of General Medical Sciences 11 January 2010 10:35 UTC publications.nigms.nih.gov [Source type: FILTERED WITH BAYES]

.The process of meiosis in females occurs during oogenesis, and differs from the typical meiosis in that it features a long period of meiotic arrest known as the Dictyate stage and lacks the assistance of centrosomes.^ This process, known as meiosis or chromosome reduction, occurs in two steps.
  • An Evolutionary Manifesto 12 October 2009 14:26 UTC www.uvm.edu [Source type: FILTERED WITH BAYES]

^ During female meiosis the embryo sac typically fails to differentiate.
  • Dissecting plant meiosis using Arabidopsis thaliana mutants -- Caryl et al. 54 (380): 25 -- Journal of Experimental Botany 12 October 2009 14:26 UTC jxb.oxfordjournals.org [Source type: Academic]

^ The stage of meiosis during which crossing-over occurs is .
  • TE Cell Division and Reproduction - CK12 - Flexbooks 12 October 2009 14:26 UTC authors.ck12.org [Source type: Academic]

.In males, meiosis occurs in precursor cells known as spermatogonia that divide twice to become sperm.^ Here, we describe an Arabidopsis mutant, named tardy asynchronous meiosis ( tam ), that exhibits a phenotype of delayed and asynchronous cell divisions during male meiosis.
  • The Arabidopsis Gene Tardy Asynchronous Meiosis Is Required for the Normal Pace and Synchrony of Cell Division during Male Meiosis -- Magnard et al. 127 (3): 1157 -- PLANT PHYSIOLOGY 11 January 2010 10:35 UTC www.plantphysiol.org [Source type: Academic]

^ Here, the primordial germ cells start to develop into either eggs, if the gonad is becoming an ovary, or sperm, if the gonad is becoming a testis.
  • Germ Cells and Fertilization -- Molecular Biology of the Cell -- NCBI Bookshelf 11 January 2010 10:35 UTC www.ncbi.nlm.nih.gov [Source type: Academic]

^ During the process of oogenesis, mature eggs (ova) are formed by meiosis of precursor cells called primary oocytes.
  • TE Cell Division and Reproduction - CK12 - Flexbooks 12 October 2009 14:26 UTC authors.ck12.org [Source type: Academic]

.These cells continuously divide without arrest in the seminiferous tubules of the testicles.^ These haploid spermatids then undergo morphological differentiation into sperm ( Figure 20-27 ), which escape into the lumen of the seminiferous tubule ( Figure 20-28 ).
  • Germ Cells and Fertilization -- Molecular Biology of the Cell -- NCBI Bookshelf 11 January 2010 10:35 UTC www.ncbi.nlm.nih.gov [Source type: Academic]

^ These data indicate that SPB duplication, as measured by Sad1p staining, continues in cut11 ts cells at the restrictive temperature.
  • cut11+: A Gene Required for Cell Cycle-dependent Spindle Pole Body Anchoring in the Nuclear Envelope and Bipolar Spindle Formation in Schizosaccharomyces pombe -- West et al. 9 (10): 2839 -- Molecular Biology of the Cell 12 October 2009 14:26 UTC www.molbiolcell.org [Source type: Academic]

^ Spermatogenesis also depends on testosterone secreted by Leydig cells, located between the seminiferous tubules.
  • Germ Cells and Fertilization -- Molecular Biology of the Cell -- NCBI Bookshelf 11 January 2010 10:35 UTC www.ncbi.nlm.nih.gov [Source type: Academic]

Sperm is produced at a steady pace. .The process of meiosis in males occurs during spermatogenesis.^ The stage of meiosis during which crossing-over occurs is .
  • TE Cell Division and Reproduction - CK12 - Flexbooks 12 October 2009 14:26 UTC authors.ck12.org [Source type: Academic]

^ New mutations can occur in the genes of these cells at any stage during this process.
  • Overview of Genetic Concepts. 12 October 2009 14:26 UTC members.shaw.ca [Source type: Academic]

^ During the process of oogenesis, mature eggs (ova) are formed by meiosis of precursor cells called primary oocytes.
  • TE Cell Division and Reproduction - CK12 - Flexbooks 12 October 2009 14:26 UTC authors.ck12.org [Source type: Academic]

.In female mammals, meiosis begins immediately after primordial germ cells migrate to the ovary in the embryo, but in the males, meiosis begins years later at the time of puberty.^ The subsequent production in this embryo of new primordial germ cells begins the cycle again.
  • Germ Cells and Fertilization -- Molecular Biology of the Cell -- NCBI Bookshelf 11 January 2010 10:35 UTC www.ncbi.nlm.nih.gov [Source type: Academic]

^ These primordial germ cells migrate to the developing gonads, which will form the ovaries in females and the testes in males.
  • Germ Cells and Fertilization -- Molecular Biology of the Cell -- NCBI Bookshelf 11 January 2010 10:35 UTC www.ncbi.nlm.nih.gov [Source type: Academic]

^ Migration of mammalian primordial germ cells Figure 20-16 .
  • Germ Cells and Fertilization -- Molecular Biology of the Cell -- NCBI Bookshelf 11 January 2010 10:35 UTC www.ncbi.nlm.nih.gov [Source type: Academic]

.It is retinoic acid, derived from the primitive kidney (mesonephros) that stimulates meiosis in ovarian oogonia.^ Loss of cyp26b1 expression in females between 10–12.5 days pc, allows RA to stimulate oogonia commitment to meiosis.
  • Meiosis in oocytes: predisposition to aneuploidy and its increased incidence with age -- Jones, 10.1093/humupd/dmm043 -- Human Reproduction Update 11 January 2010 10:35 UTC humupd.oxfordjournals.org [Source type: Academic]

.Tissues of the male testis suppress meiosis by degrading retinoic acid, a stimulator of meiosis.^ We postulated that devising some way to artificially delay cytokinesis in tam male meiosis might suppress the dyad phenotype.
  • The Arabidopsis Gene Tardy Asynchronous Meiosis Is Required for the Normal Pace and Synchrony of Cell Division during Male Meiosis -- Magnard et al. 127 (3): 1157 -- PLANT PHYSIOLOGY 11 January 2010 10:35 UTC www.plantphysiol.org [Source type: Academic]

^ The swi1-2 allele, which affects both male and female meiosis, was generated by EMS and has a stop codon after amino acid 389 (of the long transcript).
  • Dissecting plant meiosis using Arabidopsis thaliana mutants -- Caryl et al. 54 (380): 25 -- Journal of Experimental Botany 12 October 2009 14:26 UTC jxb.oxfordjournals.org [Source type: Academic]

.This is overcome at puberty when cells within seminiferous tubules called Sertoli cells start making their own retinoic acid.^ Within cells is a chemically and structurally complex material called protoplasm which comprises the living material of the cell.
  • PROCEDURES FOR USE OF GENIC MALE STERILITY IN PRODUCTION OF COMMERCIAL HYBRID MAIZE - Patent 3861079 12 October 2009 14:26 UTC www.freepatentsonline.com [Source type: Academic]

^ First, it goes through interphase: The cell makes a copy of its DNA and the chromosomes start to appear.
  • Student poems - mitosis 11 January 2010 10:35 UTC www.starsandseas.com [Source type: FILTERED WITH BAYES]

^ The germ-cells start out apparently to perform their functions normally, but later succumb to the conflicting forces at work within their boundaries.

.Sensitivity to retinoic acid is also adjusted by proteins called nanos and DAZL.[6][7] Meoisis involves Spermatocytes.^ Exons encode a 14,000 base pair mRNA that is further processed to yield the 3,686 amino acid protein called dystrophin.
  • Overview of Genetic Concepts. 12 October 2009 14:26 UTC members.shaw.ca [Source type: Academic]

^ As the mRNA is read, the codons tell the factory what amino acid to add next in the new protein chain: this process is called translation .
  • Overview of Genetic Concepts. 12 October 2009 14:26 UTC members.shaw.ca [Source type: Academic]

See also

References

  1. ^ a b c d e Snustad, D. Peter and Simmons, Michael J. 2006. Principles of genetics. 4th ed, Wiley.
  2. ^ Raven, Peter H.; Johnson, George B.; Mason, Kenneth A.; Losos, Jonathan & Singer, Susan. Biology, 8th ed. McGraw-Hill 2007.
  3. ^ a b Petronczki M, Siomos MF, Nasmyth K (February 2003). "Un ménage à quatre: the molecular biology of chromosome segregation in meiosis". Cell 112 (4): 423–40. doi:10.1016/S0092-8674(03)00083-7. PMID 12600308. 
  4. ^ BIL 104 - Lecture 15
  5. ^ Rosenbusch B (November 2006). "The contradictory information on the distribution of non-disjunction and pre-division in female gametes". Hum. Reprod. 21 (11): 2739–42. doi:10.1093/humrep/del122. PMID 16982661. 
  6. ^ Lin Y, Gill ME, Koubova J, Page DC (December 2008). "Germ cell-intrinsic and -extrinsic factors govern meiotic initiation in mouse embryos". Science 322 (5908): 1685–7. doi:10.1126/science.1166340. PMID 19074348. 
  7. ^ Suzuki A, Saga Y (February 2008). "Nanos2 suppresses meiosis and promotes male germ cell differentiation". Genes Dev. 22 (4): 430–5. doi:10.1101/gad.1612708. PMID 18281459. 

External links


Study guide

Up to date as of January 14, 2010

From Wikiversity

Here is the link to the ITunes U Lecture from Berkeley. Meiosis
I will post my notes soon. Please feel free to add details or make changes where necessary. Contact me via email if you need help. Thanks, April

Simple English

Meiosis is a special type of cell division. Unlike mitosis, the way normal body cells divide, meiosis results in cells that only have half the usual number of chromosomes, one from each pair. For that reason, meiosis is often called reduction division. In the long run, meiosis increases genetic variation, in a way which will be explained later.

Sexual reproduction takes place when a sperm fertilizes an egg. The eggs and sperm are special cells called gametes, or sex cells. Gametes are haploid; they have only half the number of chromosomes as a normal body cell (called a somatic cell). Fertilization restores the chromosomes in body cells to the diploid number.

The basic number of chromosomes in the body cells of a species is called the somatic number and is designated 2n. Thus, in humans 2n = 46. We have 46 chromosomes. In the sex cells the chromosome number is n (humans: n = 23).[1] So, in normal diploid organisms, chromosomes are present in two copies, one from each parent (23x2=46). The only exception are the sex chomosomes. In mammals, the female has two X chromosomes, and the male one X and one Y chromosome.

A karyotype is the characteristic chromosome number of a eukaryote species.[1][2][3] The preparation and study of karyotypes is part of cytogenetics, the genetics of cells.

All eukaryotes that reproduce sexually use meiosis. This also includes single-celled organisms. Meiosis does not occur in archaea or bacteria, which reproduce via asexual processes such as binary fission.

Contents

Meiosis as part of sexual reproduction

The significance of fertilization

The offspring gets a set of chromosomes from each parent so that, overall, half of its heredity comes from each parent. But the two sets of chromosomes are not identical with the parental chromosomes. This is because they are changed during the reduction division by a process called crossing-over.

The significance of meiosis

This is two-fold:

1. First, to reduce the chromosomes in each egg or sperm to one set only.
2. To allow crossing-over to take place between each pair of parental chromosomes.
Although the gametes have only one set of chromosomes, that set is a mixture of genetic material from both parents. Every single egg or sperm has a different selection from the parental chromosomes. The basic reason is that at any particular locus[4] on the chromosomes there may be different alleles (forms of the gene) on the two chromosomes. Thus crossing-over changes some of the alleles on a particular chromosome.[5]
Take two deck of cards. Keep spades and hearts from each pack; discard the rest. Take the two suits from pack 1. Now select one ace, one king, one queen (etc) at random from one or other of the two suits, until 13 cards have been selected. You now have 13 cards from ace to deuce, but taken from two suits. Now do the same thing with the two suits from the second pack. Then you have another 13 cards. When you put the two sets together (fertilization) you have 26 cards drawn from the two suits, and yet they are not the same 26 cards you started with. Sometimes you will have genes from both parents (colour of suits); sometimes from the same parent.
Crossing-over between chromosomes takes place in meiosis, and it is called recombination. It means that every sperm and every egg is a bit different in its genes. That is why offspring of two parents look similar, but not identical.[6]

As with shuffling a deck of cards, many different combinations of genes can be produced without a change (mutation) in any individual gene. This greatly increases the variety of the offspring, and the variety gives at least some of the offspring a better chance of surviving in difficult times.

"It is not difficult to think of reasons why sexual populations should have long-term advantages over asexual ones. They can evolve more rapidly to meet changing circumstances..." [7]

The recombination (crossing-over) which takes place in meiosis may be the reason why sexual reproduction exists at all. [8]

Special cases

Cyclical parthenogenesis

Several quite large taxa (groups of organisms) use cyclical parthenogenesis. This is when several generations are born by virgin birth, and then a generation occurs with normal sexual reproduction. Examples include aphids, and cladocerans (small crustacea called water fleas). Aphids usually operate as follows: when the weather is good, and their plant hosts are at their best, they use parthenogenesis. At the end of the season, when the weather gets worse, they use sexual reproduction. This system of reproduction is called apogamy.

In parthenogenesis, the eggs contain only the mother's genetic material, and they are not fertilized. The egg cells may be produced either by meiosis or mitosis. When meiosis occurs, crossing-over produces a genetic fingerprint which differs somewhat from the mother's. So, the parthenogenetic greenfly offspring are not identical, and do show some genetic variation: some chromosome segments differ because of meiosis. Mitosis would produce identical offspring.[9]

Amongst these parthenogenetic taxa are a number of species which have entirely abandoned sex.

Loss of sexuality

A few eukaryote organisms have completely lost the ability for sexual reproduction, and so do not have meiosis. These include the Bdelloid rotifers, which only reproduce by parthenogenesis. Little or nothing is known about this loss of sexuality.

Details of Meiosis

Meiosis can be divided into Meiosis I and Meiosis II.

Meiosis I

The function of the first division is to permit crossing over. Just like Mitosis, meiosis includes Prophase, Metaphase, Anaphase and Telophase.

Prophase I: The chromosomes become visible, the nuclear envelope disappears and the centrioles (located at the top and bottom of the nucleus) begin forming spindle fibres that envelope the chromosomes.

At this stage each chromosome is split into two sister chromatids, held together by the centromeres. The paired chromosomes now have four chromatids (2 sets of 'sisters') pressed together. Crossing over takes place between two of the non-sister chromatids; the other two remain uncrossed. The crossover results in the exchange of segments of each of the participating chromatids, DNA and associated chromatin protein. Genetically, the process is called recombination.

File:Meiosis
Events involving meiosis, showing chromosomal crossover

Metaphase I: The chromosomes line up along the equatorial (the central line) of the spindle fibres in homologous pairs.

Anaphase I: The chromosomes are divided so that there are equal amounts on either side of the cell. As there are 46 chromosomes in a human cell, 23 end up on either side.

(Cytokinesis, the division of cells into two, takes place. The cell divides.)

Telophase I: The two daughter cells are completely divided, a nucleic envelope forms and the chromosomes become less visible. There are 23 chromosomes in each of these cells.

Meiosis II

The two cells prepare to divide again in a stage known as Interkinesis or Interphase II. Both of these cells will go through meiosis II.

Prophase II: The chromosomes become visible, the nuclear envelope disappears and the centrioles form the spindle fibres.

Metaphase II: The chromosomes line up along the middle line on the spindle fibres

Anaphase II: The chromosomes get split into its two chromatids. Chromatids are the two strands of DNA (deoxyribo-nucleic acid) that make up the chromosome. They are joined by a mid-way connection called a centromere.

(Cytokinesis takes place. The cell divides.)

Telophase II: The cells are completely divided. The nucleic envelope reforms and four new cells with different DNA are created.

In males, all four cells become sperm. In females, only one becomes a mature egg, while the remaining three become re-absorbed into the body.

Problems

In humans, there are certain conditions that are caused by a meiosis gone wrong. Examples are:

  • Down Syndrome - trisomy of chromosome 21
  • Patau Syndrome - trisomy of chromosome 13
  • Edward Syndrome - trisomy of chromosome 18
  • Klinefelter Syndrome - extra X chromosomes in males - ie XXY, XXXY, XXXXY
  • Turner Syndrome - lacking of one X chromosome in females - ie XO
  • Triple X syndrome - an extra X chromosome in females
  • XYY Syndrome - an extra Y chromosome in males

References

  1. 1.0 1.1 White M.J.D. 1973. The chromosomes. 6th ed, Chapman & Hall, London. p28
  2. Stebbins G.L. 1950. Variation and evolution in plants. Chapter XII: The Karyotype. Columbia University Press N.Y.
  3. King R.C., Stansfield W.D. and Mulligan P.K. 2006. A dictionary of genetics. 7th ed, Oxford U.P Oxford & N.Y. p242
  4. position of gene on chromosome
  5. Whitehouse H.L.K. 1965. Towards an understanding of the mechanism of heredity. Arnold, London.
  6. Identical twins occur only when a fertilized egg splits before development begins, producing two offspring with absolutely identical genes
  7. Maynard Smith J. 1999. Evolutionary genetics. Oxford. Discussion: p234–240; quote: p241
  8. Mayr, Ernst 1982. The growth of biological thought: diversity, evolution & inheritance. Harvard. Meiosis, p761–769.
  9. The evolution of these variations has been widely discussed: Maynard Smith, John. 1978. The Evolution of Sex. Cambridge: Cambridge University Press. ISBN 0-521-29302-2. Schlupp, I. 2005. The evolutionary ecology of gynogenesis. Annu. Rev. Ecol. Evol. Syst. 36: 399-417. Simon, Jean-Christophe; Rispe, Claude & Sunnucks, Paul. 2002. Ecology and evolution of sex in aphids. Trends in Ecology & Evolution, 17, 34-39.


Citable sentences

Up to date as of December 23, 2010

Here are sentences from other pages on Meiosis, which are similar to those in the above article.








Got something to say? Make a comment.
Your name
Your email address
Message