episome

—episomal, adj. —episomally, adv.

/ep"euh sohm'/, n. Genetics.

bacterial DNA that is extrachromosomal and that may replicate autonomously as a plasmid or become incorporated into the chromosome and replicate with it.

[1930-35; EPI- + -SOME³]

* * *

Any of a group of genetic elements consisting of DNA and capable of giving selective advantage to the bacteria in which they occur.

Episomes may be attached to the bacterial cell membrane or become part of the chromosome. Cells with episomes act like males during conjugation, a mating process in certain bacteria. During conjugation, cells lacking the episome may receive either the episome or the episome plus the genes to which it is attached. Experiments involving gene transfers from cells in which episomes have been incorporated in the chromosomes have been used to determine the locations of genes on the chromosome.

* * *

▪ plasmid

      in bacteria, one of a group of extrachromosomal genetic elements called plasmids, consisting of deoxyribonucleic acid (DNA) and capable of conferring a selective advantage upon the bacteria in which they occur. Episomes may be attached to the bacterial cell membrane (such a cell is designated F⁺) or become integrated into the chromosome (such a cell is designated Hfr). F⁺ and Hfr cells act as donors during conjugation, a mating process in certain bacteria (e.g., Escherichia, Salmonella, Serratia, Pseudomonas). During conjugation, cells lacking the episome (called F^- cells) may receive either the episome (from an F⁺ cell) or the episome plus the chromosomal genes to which it is attached (from an Hfr cell).

      Some bacterial viruses, called temperate phages, carry DNA that can act as an episome. A bacterial cell into whose chromosome the viral DNA has become integrated is called a prophage. See lysogeny.

* * *