Transduction (genetics): Wikis

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Transduction

Transduction is the process by which DNA is transferred from one bacterium to another by a virus. It also refers to the process whereby foreign DNA is introduced into another cell via a viral vector. This is a common tool used by molecular biologists to stably introduce a foreign gene into a host cell's genome.

When bacteriophages (viruses that infect bacteria) infect a bacterial cell, their normal mode of reproduction is to harness the replicational, transcriptional, and translation machinery of the host bacterial cell to make numerous virions, or complete viral particles, including the viral DNA or RNA and the protein coat.

Contents

Lytic and lysogenic (temperate) cycles

Transduction happens through either the lytic cycle or the lysogenic cycle.

If the lysogenic cycle is adopted, the phage chromosome is integrated into the bacterial chromosome, where it can remain dormant for thousands of generations. If the lysogen is induced (by UV light for example), the phage genome is excised from the bacterial chromosome and initiates the lytic cycle, which culminates in lysis of the cell and the release of phage particles. The lytic cycle leads to the production of new phage particles which are released by lysis of the host.

Transduction as a method of transfer genetic material

The packaging of bacteriophage DNA has low fidelity and small pieces of bacterial DNA, together with the bacteriophage genome, may become packaged into the bacteriophage genome. At the same time, some phage genes are left behind in the bacterial chromosome.

There are generally three types of recombination events that can lead to this incorporation of bacterial DNA into the viral DNA, leading to two modes of recombination.

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Generalized transduction

Generalized transduction may occur in two main ways, recombination and headful packaging.

If bacteriophages undertake the lytic cycle of infection upon entering a bacterium, the virus will take control of the cell’s machinery for use in replicating its own viral DNA. If by chance bacterial chromosomal DNA is inserted into the viral capsid used to encapsulate the viral DNA, the mistake will lead to generalized transduction.

If the virus replicates using 'headful packaging', it attempts to fill the nucleocapsid with genetic material. If the viral genome results in spare capacity, viral packaging mechanisms may incorporate bacterial genetic material into the new virion.

The new virus capsule now loaded with part bacterial DNA continues to infect another bacterial cell. This bacterial material may become recombined into another bacterium upon infection.

When the new DNA is inserted into this recipient cell it can fall to one of three fates

  1. The DNA will be absorbed by the cell and be recycled for spare parts.
  2. If the DNA was originally a plasmid, it will re-circularize inside the new cell and become a plasmid again.
  3. If the new DNA matches with a homologous region of the recipient cell’s chromosome, it will exchange DNA material similar to the actions in conjugation.

This type of recombination is random and the amount recombined depends on the size of the virus being used.

It is worth asking whether generalized transduction can occur by lysogenic phages. Two possible scenarios might be imagined[citation needed] to cause generalized transduction though literature references have not been found to confirm or dispute them:

  1. A lysogenic phage whose site of integration is randomly chosen, which occasionally brings along adjacent DNA because of an erroneous excision process.
  2. A lysogenic phage that goes into its lytic phase and randomly incorporates cell DNA.

Specialized transduction

Here lysogenic phage infect the bacterium rather than lytic as in generalised type of transduction.

The second type of recombination event is called specialized transduction. If a virus removes itself from the chromosome incorrectly, some of the bacterial DNA can be packaged into the virion. There are three possible results from specialized transduction:Lambda is a good example of specialized transducer.(sehr)

  1. DNA can be absorbed and recycled for spare parts.
  2. The bacterial DNA can match up with a homologous DNA in the recipient cell and exchange it. The recipient cell now has DNA from both itself and the other bacterial cell.
  3. DNA can insert itself into the genome of the recipient cell as if still acting like a virus resulting in a double copy of the bacterial genes.

Example of specialized transduction is λ phages in Escherichia coli.

RNA, DNA

Viruses with RNA genomes are not able to package DNA and so do not usually make this mistake.

Upon lysis of the host cell, the mispackaged virions containing bacterial DNA can attach to other bacterial cells and inject the DNA they have packaged, thus transferring bacterial DNA from one cell to another. This DNA can become part of the new bacterium's genome and thus be stably inherited.

More general uses of the term

More generally, transduction is the process by which genetic material, e.g. DNA or siRNA, is inserted into a cell. Common techniques in molecular biology are the use of viral vectors (including bacteriophages), electroporation, or chemical reagents that increase cell permeability. Transfection and transformation are more common terms, although these sometimes imply expression of the genetic material as well.

Discovery

Transduction was discovered by Norton Zinder and Joshua Lederberg at the University of Wisconsin–Madison in 1951.[1]

References

External links


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