episomes

An episome is a genetic element that can exist and replicate independently as a plasmid or be maintained in a host cell integrated into the chromosome. When integrated, it behaves like part of the host genome; when excised, it can re-form an autonomous replicating plasmid. The term highlights the element’s dual life cycle and its ability to switch between extrachromosomal and chromosomal states.

In bacteria, episomes commonly refer to plasmids that can integrate into the host chromosome. The F plasmid in Escherichia coli is a classic example: it can insert into the chromosome to produce a high-frequency recombination (Hfr) strain. After excision, the plasmid may carry some chromosomal genes as an F′ plasmid. As episomes, these elements use their own origin of replication to persist and can also be transmitted to daughter cells during cell division.

In virology and eukaryotic biology, episomes describe viral or plasmid DNA that resides in the nucleus as circular, nonintegrated DNA during latent or nonproductive infection. Epigenetic maintenance involves host and viral factors that ensure replication with the cell cycle and proper segregation to daughter cells. Epstein-Barr virus and Kaposi’s sarcoma–associated herpesvirus are prominent examples of viral episomes that persist as extrachromosomal DNA in infected cells.

Maintenance of episomes relies on partitioning systems and, in some cases, specific viral or plasmid-encoded proteins that tether the episome to the host’s replication and segregation machinery. This contrasts with proviruses or integrated elements, which reside within the chromosomal DNA.

Applications include the use of episomal vectors in gene expression and gene therapy, where the goal is long-term expression without chromosomal integration and the associated risk of insertional mutagenesis.