episomal

Episomal refers to an episome, a genetic element that can replicate independently of the host chromosome but may also integrate into the genome. Episomes are DNA elements that carry a replicon and, in many cases, systems that help them be inherited during cell division. The concept is used across bacteria, yeasts, and certain eukaryotic viruses.

In bacteria and yeasts, plasmids are classic examples of episomal DNA. They exist and replicate extrachromosomally but can sometimes integrate into the chromosome or be maintained as separate replicons. In eukaryotic viruses, some genomes persist as episomes in the nucleus rather than integrating into host chromosomes, enabling long-term maintenance without permanent insertion.

In molecular biology and gene therapy, episomal vectors are designed to express genes without integrating into the host genome. Such vectors rely on replication origins and, in some cases, maintenance elements to persist through cell divisions. Examples include Epstein-Barr virus–based vectors that use oriP and EBNA1 to replicate and segregate in dividing cells, and vectors incorporating scaffold/matrix attachment regions to promote maintenance. Benefits include reduced risk of insertional mutagenesis, while drawbacks include variable copy number, potential loss during cell division, and possible epigenetic silencing in some contexts.

In virology, episomal genomes also describe latent viral DNA that remains separate from host chromosomes, as seen with certain herpesviruses. Episomal maintenance can influence viral latency, reactivation, and pathogenesis, depending on cellular context and available replication/partitioning machinery.