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 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 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 sigle strand merely undergoes rolling circle replication to once again become double stranded. In the recipeint 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 effiecent 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 unusally large and can accept large scale inserts (up to 300kB) 

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