The fertility, or F, factor, is a classic example of an episome, a genetic element that exists either as a free circular plasmid, or as a linear sequence integrated into a bacterial chromosome. In the plasmid form, it is a large circular DNA approximately equal to 100,000 base pairs in length. It mediates bacterial conjugation, during which the plasmid and sometimes chromosomal DNA are transferred from one bacterium to another. A cell, which contains the F factor is said to be F-positive and is able to "mate" with a bacterium that is F-negative. Contact between the cells is made through F pili, surface appendages coded by the F factor. Contact between the bacteria is followed by transfer of the F factor and any DNA that it carries. If the F factor exists as a free plasmid, it is transferred and the process converts the F-negative bacterium into...