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F factor[′ef ‚fak·tər]
(also F element, sex factor, and fertility factor), a genetic element that determines sex differentiation in bacteria and controls their ability to transfer chromosomal genes during conjugation from the donor, or male, cell to the recipient, or female, cell. The F factor has a molecular weight of about 4 × 107 and consists of ring-shaped deoxyribonucleic acid (DNA) formed by a double helix.
The F factor is an episome; that is, it can exist either autonomously in cytoplasm or as an integral part of the DNA bacterial chromosome. When it exists autonomously, the F factor behaves like a plasmid and is frequently transferred during conjugation to recipient cells, whose properties it alters. Bacterial genes are rarely transferred during this type of conjugation.
When it is an integral part of the DNA bacterial chromosome, the F factor is responsible for the efficient transfer of bacterial genes but is usually not transferred itself. The integrated F factor can become autonomous and can incorporate the genes in the adjacent portions of the bacterial DNA. Such an F factor is designated Hfr because it transfers incorporated bacterial genes with high frequency.
V. G. LIKHODED