Gene Pickup and Sex Factors

Bacterial Structure
Part 1
Structure of Fimbriae or Pili
Functions of Fimbriae or Pili
Parts of Flagella
Bacterial Movement
Spirochaetal Movement
Gliding Movement
Nucleoid (Nucleus)
Plasmids
Classes of Plasmids
Gene Pickup and Sex Factors
R Plasmids
Plasmids Conferring Pathogenecity to mammals
Col Plasmids
Degradative Plasmids
Mercury Resistance Plasmids
Tumor Inducing Plasmid in Agrobacterium Tumifaciens
Cryptic Plasmids
Staphylococcal Plasmids

Gene Pickup and Sex Factors

	Gene Pickup and Sex Factors - Infectious transfer of bacterial plasmids such as the F factor takes place during conjugation. Donor cells have plasmids in addition to the chromosome. One strand of the plasmid breaks at a specific site and passes into the recipient cell. The single plasmid strands in both donor and recipient cells now synthesize a complementary strand, giving rise to double-stranded plasmids. Thus identical plasmids are present in both cells, which now become potential donors and are called F cells.
Exchange of DNA segments sometimes take place between the plasmid and the chromosome. Breaks may occur on both strands of plasmid and chromosome DNA in regions where nucleotide sequences are similar. The two sets of ends join, resulting in the insertion of the plasmid into the chromosome. Cells containing chromosomal DNA plus plasmid DNA rings are called Hfr (for high frequency of recombination) cells. The plasmid may sometimes break away from the chromosome. The breakage and reunion may occur at a point different from the area of incorporation of the plasmid.
Under such conditions interchange of segments of DNA takes place between the plasmid and the chromosome. Such plasmids are termed sex factors. The transferred chromosomal fragment combines with the recipient cell chromosome. The first extrachromosomal genetic element recognized was the sex factor F (for fertility) in E. coli K 12. Sex factor F can either exist as an independent and autonomous unit, or may be integrated into the chromosome. Because of its dual existence it is sometimes referred to as the episome to distinguish it from the normal plasmid .

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