What does episome mean?
An episome is a self-replicating genetic element found primarily in bacteria that occupies an unusual position between two better-known categories of DNA. Unlike ordinary chromosomal genes, an episome can live a kind of double life: it may replicate autonomously in the cytoplasm like a plasmid, or it may insert itself into the host cell's chromosome and be replicated along with the rest of the genome. The classic example is the fertility, or F, factor of E. coli, whose integration converts the cell into an Hfr strain that transfers chromosomal genes during conjugation. The term was introduced by French biologists François Jacob and Élie Wollman in 1958 and derives from Greek roots meaning 'upon' and 'body'. Though rooted in classical bacterial genetics, the word persists in modern research, where it also describes viral genomes and engineered vectors maintained outside chromosomes in gene therapy.
nounA genetic element in bacteria, such as the fertility (F) factor, that can replicate either autonomously as a free plasmid-like entity or after becoming integrated into the host cell's chromosome.
- A genetic element, especially in bacteria, capable of existing either as an autonomous extrachromosomal replicon or integrated into the host chromosome.
"The fertility factor of E. coli is the classic example of an episome, capable of living both inside and outside the bacterial chromosome."
"When the F episome integrates into the bacterial chromosome, the cell becomes an Hfr strain capable of transferring chromosomal genes during conjugation."
"In some gene-therapy vectors, therapeutic DNA is maintained as an episome rather than inserting into the host genome."
Standard regular English plural; used when referring to multiple such elements within a cell or across strains.
"Several episomes were detected in the transformed bacterial cells."
The episome was coined in 1958 by Jacob and Wollman to describe DNA with a double life — it thrives floating free in the cell or spliced straight into the chromosome.
Reviewed by Deb Chak, Editor. AI-assisted content curated by RJS Tech Solutions LLP.
Etymology of episome
The term 'episome' was coined around 1958 by French molecular biologists François Jacob and Élie Wollman during their pioneering work on bacterial conjugation. It combines the Greek prefix 'epi-' (meaning 'upon' or 'over') with '-some', from Greek 'sōma' ('body'), placing it in the same family as 'chromosome' and 'ribosome'. The name reflects the element's ability to sit upon, or attach itself to, the bacterial chromosome.
How episome is actually used
Strictly technical vocabulary used in genetics and molecular biology; rare outside scientific literature. Some modern biologists apply 'episome' more loosely to any extrachromosomal DNA element that can persist episomally, including certain viral genomes and engineered vectors, which blurs the older distinction between episomes and plasmids.
Easily confused with episome
An epitome is a person or thing that perfectly embodies a quality or type, while an episome is a genetic element that can integrate into or replicate independently of a bacterial chromosome.
A plasmid replicates only autonomously outside the chromosome, whereas an episome can do both — replicating freely or integrating into the chromosome.
An epistome is a structure near the mouth in certain invertebrates such as bryozoans, unrelated to the genetics sense of episome.