An episome is a special type of
plasmid, 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
Metaphase ( and ) is a stage of mitosis in the eukaryotic cell cycle in which chromosomes are at their second-most condensed and coiled stage (they are at their most condensed in anaphase). These chromosomes, carrying genetic information, alig ...
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
Adeno-associated viruses (AAV) are small viruses that infect humans and some other primate species. They belong to the genus ''Dependoparvovirus'', which in turn belongs to the family ''Parvoviridae''. They are small (approximately 26 nm in di ...
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.
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
The term S/MAR (scaffold/matrix attachment region), otherwise called SAR (scaffold-attachment region), or MAR (matrix-associated region), are sequences in the DNA of eukaryotic chromosomes where the nuclear matrix attaches. As architectural DNA co ...
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 site
The CpG sites or CG sites are regions of DNA where a cytosine nucleotide is followed by a guanine nucleotide in the linear sequence of bases along its 5' → 3' direction. CpG sites occur with high frequency in genomic regions called CpG isl ...
s 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 mitotically divide either separate from or integrated into the prokaryotic chromosome.
References
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Molecular biology