episome, unit of genetic material composed of a series of genes that sometimes has an independent existence in a host cell and at other times is integrated into a chromosome of the cell, replicating itself along with the chromosome. Episomes have been studied in bacteria. One group of episomes are actually viruses that infect bacteria. As autonomous units they destroy host cells, and as segments integrated into a chromosome they multiply in cell division and are transferred to daughter cells. Episomes called sex factors determine whether chromosome material will be transferred from one bacterium to another. Other episomes carry genes that make bacteria resistant to the inhibitory action of antibiotics. See recombination.

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.

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