F plasmid

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The F plasmid is an example of a large plasmid, which contains genes that allow the plasmids DNA to be transferred between cells. It is found in the bacterium E. coli; E. coli containing this F factor are known as F+ and those without are known as F-. The F stands for fertility and the F factor is around 100000 bases in length. The F+ cells have a tube-like structure called a pilus, which allows it to make contact with F- cells. This joining via a pilus in order to transfer DNA between bacteria is known as conjugation. Therefore the F plasmid is known as a conjugative plasmid. Within the E. coli cells, the F plasmid has one or two copies making it a low-copy-number plasmid. During the cell cycle, it replicates once and segregates to both daughter cells[1].

Transmission of the F plasmid.

Within the F factor are genes, which governs the maintenance and transmission of the F plasmid. As already mentioned, the F plasmid is transferred via conjugation, which occurs due to the pilus known as the F pilus. All the proteins that are associated with the F pilus are transcribed and translated from genes within the F factor. The F plasmid is not transferred to a F- cell via the F pilus, the F pilus merely pulls the two cells together allowing a conjugative junction to form, which contains a pore that allows the DNA to pass from the F+ cell to the F- cell. During the transfer, the F plasmid unwinds and the outer strand breaks, which will be the one that is transferred to the F- cell via the pore in the conjugative junction. Replication of the plasmid then takes place in order to make both single strands of DNA into double-stranded DNA plasmids. In the original F+ cell, the single strand merely undergoes rolling circle replication to once again become double-stranded. In the recipient cell, the linear single-stranded DNA is replicated into a double strand and becomes a circular F plasmid containing the F factor[2].

Both E. coli cells are now considered to be F+ cells and therefore can both now transfer the F plasmid and therefore the F factor. This transfer only requires a few minutes although is not efficient in natural conditions meaning only 10% of naturally occurring E. coli cells contain the F plasmid and hence the F factor[3].

NB: F plasmids are unusually large and can accept large scale inserts (up to 300 kB).

References:

  1. Information and ideas gained from chapter 9, pgs304-305, HartlL. and RuvoloM., (2011) Genetics: Analysis of Genes and Genomes, 8th Edition, Burlington, Jones and Bartlett Learning.
  2. Information and Ideas gained from chapter 9, pgs304-305, Hartl L. and Ruvolo M. (2011) Genetics: Analysis of Genes and Genomes, 8th edition, Burlington, Jones and Bartlett Learning.
  3. Information and ideas gained from Chapter 9, pgs 304-305, Hartl L. and Ruvolo M.(2011)Genetics: Analysis of Genes and Genomes, 8th edition, Burlington, Jones and Bartlett Learning.
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