A chromatid is one among the two identical copies of DNA making up a replicated chromosome, which are joined at their centromeres, for the process of cell division (mitosis or meiosis). The term is used so long as the centromeres remain in contact. When they separate (during anaphase of mitosis and anaphase 2 of meiosis), the strands are called sister chromatids.
In humans, for example, there are normally 23 pairs of homologous chromosomes in each cell (N=23). However, the quantity of chromatids will be a multiple of 23. It can be either 4N, 2N or 1N. N does not refer to haploid or diploid; it refers to the number of chromatids in the cell as a multiple of the haploid number of chromosomes for the organism. For example, because a human haploid germ cell has 23 chromosomes, then "N" refers to a multiple of 23. (e.g. 2N=46 chromatids). The last is only seen in haploid gametes, with only one of each homologous chromosome pair. Such are created in gametogenesis.
In a cell with 4N chromatids, there are 23 chromosome pairs (46 chromosomes), and each chromosome has 2 chromatids. Thus, there are 92 chromatids in each cell (4N). It occurs after the S phase of interphase. (See cell cycle).
Immediately after a mitosis, where a cell has divided in two, but not yet duplicated its DNA in S phase, there are still 23 chromosome pairs (46 chromosomes). However, each chromosome only has one chromatid. Thus there are 46 chromatids (2xN)
Alternatively, a haploid cell with two chromatids per chromosome also has 46 chromatids. However, this doesn't occur naturally in human somatic cells.
Immediately after meiosis, each cell, called a gamete, only has half the number of chromosomes (23 chromosomes). Furthermore, each chromosome only has one chromatid. Thus, there are 23 chromatids (1xN)
This German entry was created from the translations listed at chromatid. It may be less reliable than other entries, and may be missing parts of speech or additional senses. Please also see Chromatid in the German Wiktionary. This notice will be removed when the entry is checked. (more information) July 2009
[[File:|thumb|220px|Diagram of a duplicated and condensed metaphase eukaryotic chromosome.
(1) Chromatid – one of the two parts of the chromosome after duplication.
(2) Centromere – the point where the two chromatids touch.
(3) Short arm. (4) Long arm.]] Chromatids are the daughter strands of a duplicated chromosome which are joined by a single centromere. When the centromere divides, the chromatids become separate chromosomes.
Each of the two daughter chromatids contains the same DNA and chromatin protein as its original chromosome. But in meiosis, crossing over (exchanges) take place between two of the non-sister chromatids. This has profound consequences: it produces genetic recombination, and increases the variability of gametes.