An episome is a plasmid (a small, circular, self-replicating DNA molecule separate from the bacterial chromosome) capable of integrating into the bacterial chromosome, a genetic element that can replicate either as a plasmid or as part of the bacterial chromosome.

An important example of an episome is the F plasmid, which can undergo reversible incorporation into the cell's chromosome for bacterial conjugation. In addition to some plasmids, temperate virus such as a specific phage (ex. lambda phage) also qualify as episomes. The genomes of these phages replicate sparately in the cytoplasm during the lytic cycle, and as an integral part of the host's chromosome during a lysogenic cycle.

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