C6 Transfer of genes between bacteria | 22 | v4

ABSTRACT

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Overview

Bacteria do not show Mendelian inheritance, because they are haploid and lack meiosis. However they can undergo recombination. An artificial process for achieving this is transformation, where DNA from one bacterial strain is mixed with another. DNA is taken up only by competent cells, and can alter their phenotype. Frequencies of cotransduction can be used to map closely linked genes. Processes involving bacterial viruses and plasmids also bring about recombination.

Plasmid-mediated gene transfer

Plasmids are chromosome-like structures found in bacterial cells, but which are not essential to bacterial growth. Some can move from one bacterium to another. The best studied is the F plasmid. This is an episome, an element that can replicate either independently or integrate into the bacterial chromosome. In the chromosome it can cause transfer of the entire chromosome to a recipient cell. A bacterium with an integrated F factor is called an Hfr strain. During conjugation, the time at which each gene enters the recipient can be used to create a gene map. When the F plasmid is excised from the bacterial chromosome it sometimes incorporates part of the chromosome into its own structure. It is then known as an F factor. This also transfers bacterial genes but at lower frequency.

Bacteriophages and gene transfer

Bacteriophages are involved in transduction. Virulent phages replicate in the bacterium. An occasional phage particle will contain a small fragment of bacterial DNA rather than bacteriophage DNA. The bacteriophage will transfer it to a recipient cell. This is generalized transduction.

Temperate bacteriophages can integrate into the bacterial chromosome, and remain in a dormant lysogenic state. Integration occurs only at specific sites. Dormancy is lost at a low frequency and the bacteriophage genome is excised and begins to replicate. In some cases the excision is not accurate and the resulting bacteriophage particles incorporate some host DNA sequences. These can be transduced to any cell the bacteriophage subsequently infects. The bacteriophage can only incorporate host DNA that is contiguous to its site of integration, hence this is termed restricted transduction.

Recombination in bacteria

Recipient bacteria are converted to partial diploids or merozygotes by conjugation and transduction. Recombination to give new stable genotypes takes place by double crossovers between the host chromosome and the donated DNA.