An episome is a special type of
plasmid
A plasmid is a small, extrachromosomal DNA molecule within a cell that is physically separated from chromosomal DNA and can replicate independently. They are most commonly found as small circular, double-stranded DNA molecules in bacteria and ...
, which remains as a part of the eukaryotic genome without integration. Episomes manage this by replicating together with the rest of the genome and subsequently associating with
metaphase chromosomes during mitosis. Episomes do not degrade, unlike standard plasmids, and can be designed so that they are not epigenetically silenced inside the eukaryotic cell nucleus. Episomes can be observed in nature in certain types of long-term infection by
adeno-associated virus or
Epstein-Barr virus. In 2004, it was proposed that non-viral episomes might be used in
genetic therapy for long-term change in
gene expression
Gene expression is the process (including its Regulation of gene expression, regulation) by which information from a gene is used in the synthesis of a functional gene product that enables it to produce end products, proteins or non-coding RNA, ...
.
As of 1999, there were many known sequences of
DNA (deoxyribonucleic acid) that allow a standard plasmid to become episomally retained. One example is the
S/MAR sequence.
The length of episomal retention is fairly variable between different genetic constructs and there are many known features in the sequence of an episome which will affect the length and stability of genetic expression of the carried transgene. Among these features is the number of
CpG sites which contribute to epigenetic silencing of the transgene carried by the episome.
Mechanism of episomal retention
The mechanism behind episomal retention in the case of S/MAR episomes is generally still uncertain.
As of 1985, in the case of latent Epstein-Barr virus infection, episomes seemed to be associated with nuclear proteins of the host cell through a set of viral proteins.
Episomes in prokaryotes
Episomes in prokaryotes are special sequences which can divide either separate from or integrated into the prokaryotic chromosome.
References
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Molecular biology