Conjunction

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This is the process in which DNA from the donor cell is transferred into the host cell via cell-to-cell contact.

In most cases this is mediated by a conjugative plasmid, it is normally the DNA in this plasmid that is transferred. Occasionally non plasmid DNA can be transferred as long as they contain mobalisation genes.

The main example used for conjunction is a large plasmid called the 'F (fertility) factor', which is found in E. coli bacterium[1]. If a bacterium contains the F factor then it is referred to as F+, and if a bacterium does not contain the F factor, it is referred to as F-. 

Conjunction is first initiated by the donor cell attaching to the host cell via a protruding pilus. This forms a temporary connection between the cells which, when the cells are in close proximity, then causes them to come together and fuse. Through the pore formed by the cells fusing together, the donor cell then transfers one strand of plasmid DNA to the host cell which then synthesizes a complementary strand. The transfer of DNA from the donor cell is then followed by the replication of the plasmid and 'Rolling circle replication' - this is used to replace the transferred single strand and is initiated by the 3' end of the 'nicked' strand[2].   

Conjunction can only take a few minutes to complete in laboratory cultures - however under natural conditions, this process is not very efficient. There are other advantages that conjunction has over other methods of genetic transfer. Firstly, this process hardly causes any disruption to the cellular envelope of the intended target. Secondly this process allows a large amount of genetic information to be transferred from the donor cell to the host cell[3].

Reference

  1. D. L. Hartl (2011) Essential Genetics: A genomics perspective, 5th edition, Jones and Bartlett learning: pages 224-225
  2. D. L. Hartl and M.Ruvolo (2012) Genetics: Analysis of genes and genomes, 8th edition, Jones and Bartlett learning: pages 304-305
  3. Holmes RK, Jobling MG (1996). Genetics: Conjugation. in: Baron's Medical Microbiology (Baron S et al., eds.) (4th ed.). University of Texas Medical Branch.
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