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an extrachromosomal self-replicating structure found in bacterial cells that carries genes for a variety of functions not essential for cell growth. Plasmids consist of cyclic double-stranded DNA molecules, replicating independently of the chromosomes and transmitting through successive cell divisions genes specifying such functions as antibiotic resistance (R plasmid); conjugation (F plasmid); the production of enzymes, toxins and antigens; and the metabolism of sugars and other organic compounds. Plasmids can be transferred from one cell to another by conjugation and by transduction. Some plasmids may also become integrated into the bacterial chromosome; these are known as episomes.
conjugative plasmid a plasmid that is transferred from one bacterial cell to another during conjugation.
F plasmid a conjugative plasmid found in F+ (male) bacterial cells that leads with high frequency to its transfer and much less often to transfer of the bacterial chromosome. A cell possessing the F plasmid (F+, male) can form a conjugation bridge (F pilus) to a cell lacking the F plasmid (F−, female), through which genetic material may pass from one cell to another.
F′ plasmid a hybrid F plasmid that contains also a segment of the host chromosome.
R plasmid a conjugative factor in bacterial cells that promotes resistance to agents such as antibiotics, metal ions, ultraviolet radiation, and bacteriophage.
a plasmid that can effect its own intercellular transfer by means of conjugation; this transfer is accomplished by a bacterium's being rendered a donor, usually with specialized pili.
con·ju·ga·tive plas·mid(kon'jŭ-gā-tiv plaz'mid)
A plasmid that can effect its own intercellular transfer by means of conjugation; this transfer is accomplished by a bacterium being rendered a donor, usually with specialized pili.