Bacterial Gene Transfer

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  • Created by: Jess
  • Created on: 17-05-13 14:09

Transformation

Transfromation is the transfer of genetic material as free DNA to recipient cells, with no direct contribution from the intact donor bacterium.

First demonstrated in 1928 by Frederick Griffith. The donor organism was a virulent strain of pneumococcus and the recipient was a non-virulent mutant. Colonies of the virulent strain appear smooth as they synthesis a protective polysaccharide capsule. The non-virulent strain does not have the capsule and appears rough and cannt survive the protective defences of the host.

Non-virulent bacteria can take up DNA from heat killed virulent bacteria. The recipient becomes a merozygote. Recombination events on either side of the cap locus result in the mutant gene being replaced. This is non-reciprocal recombination generating a transformant and associated phenotypes (capsule and virulence) are expressed.

Any DNA fragments remaining are susceptible to enzymatic degradation and are not stably inherited.

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Conjugation

Conjugation is a mechanism of gene transfer that depends on the presence of a conjugative plasmid in the donor organism, and involves cell-to-cell contact between donor and recipient bacteria.

Conjugation involves the production of pili to establish contact between the donor and recipient bacteria. The DNA transfer occurs by rolling circle replication. One of the genes encoded, traZ, encodes an endonuclease which makes a single strand nick at a site called oriT (origin of transfer).

When rolling circle replication occurs attached chromosomal DNA will be transferred from recipient to donor but it tends to break during this process so the whole chromosome is virtually never transferred.

Can produce a genetic map based on the fact genes closest to oriT will be transferred first. By measuring the time of entry it is possible to produce a map gene order and their separation on the chromosome.

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Transduction

Transduction is the transfer of donor bacterial genetic material to recipient cells by a defective (transducing) bacteriophage particle.

Transduction occurs as a result of errrors in the process of intrcellular pahge development. The formation of a transforming particle is a random event in which fragments of degraded bacterial chromosome are accidentally packaged instead of phage DNA. The frequency of such errors is low.

A transducing particle is defective as it carries no phage genes. It can interact with a receptor and inject its genes into a cell but these can do no harm as they are bacterial genes. Instead the recipient cell becomes diploid for part of its genome (merodiploid).

Non-reciprocal recombination will occur for areas of homolgy to produce a transductant. You can then measure co-transductant frequencies.

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