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Sex Factors

	Sex Factors - In E. coli, fertility is determined by the presence of a fertility factor or sex factor which is a double stranded DNA containing element that can either be autonomous or integrated in the bacterial chromosome. When E. coli, strains having the F factor in an integrated strain were crossed with F- cells, the yield of recombinants was about a thousand times higher than in F+ x F- crosses. Such strains were designated as Hfr strains (or high frequency of recombination) and these were found to mobilize the transfer of their chromosomal DNA faster than the F+ cells.
During mating, these strains rarely transfer their f factor or their entire chromosome to the recipient cells. However, Hfr cells can become F+ cells by the F factor becoming autonomous and occasionally F+ cells can also become Hfr by the reverse process. However, F- cells do not become either F+ or Hfr spontaneously.
In a cross between F+ and F- cells, the F.L. factor is transferred to the recipient without any chromosomal DNA being transferred. Because of this, F- recipient cells become F+ at a high frequency and the F factor behaves like a. infectious agent. This occurs because the F factor in a F+ cell can initiate conjugation and then transfer itself to the recipient. The isolation of Hfr strains and the recognition that they can transfer their genetic material at a higher frequency made it possible to better understand the process of conjugation in bacteria. Experiments carded out using Hfr strains with a number of genetic markers show that gene transfer by this process is, highly ordered, but the order of transfer of markers from Hfr strains varies from one strain to another.
Although tile transfer is linear, it has a fixed point of origin and different Hfr strains donate DNA from different regions William, Hayes, making use of this finding carried out extensive genetic mapping in E. coli and was able to conclude that the linkage (chromosomal) map in E. coli was circular. The order of genes on the chromosome of E. coli determined by the time of transfer was consistent with the frequency of recombination between various points on the bacterial chromosome.

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