Also found in: Dictionary, Thesaurus, Medical, Wikipedia.
Related to episome: endogenote, episomal DNA


(ĕp`ĭsōm), unit of genetic material composed of a series of genesgene,
the structural unit of inheritance in living organisms. A gene is, in essence, a segment of DNA that has a particular purpose, i.e., that codes for (contains the chemical information necessary for the creation of) a specific enzyme or other protein.
..... Click the link for more information.
 that sometimes has an independent existence in a host cell and at other times is integrated into a chromosomechromosome
, structural carrier of hereditary characteristics, found in the nucleus of every cell and so named for its readiness to absorb dyes. The term chromosome
..... Click the link for more information.
 of the cell, replicating itself along with the chromosome. Episomes have been studied in bacteria. One group of episomes are actually virusesvirus,
parasite with a noncellular structure composed mainly of nucleic acid within a protein coat. Most viruses are too small (100–2,000 Angstrom units) to be seen with the light microscope and thus must be studied by electron microscopes.
..... Click the link for more information.
 that infect bacteria. As autonomous units they destroy host cells, and as segments integrated into a chromosome they multiply in cell division and are transferred to daughter cells. Episomes called sex factors determine whether chromosome material will be transferred from one bacterium to another. Other episomes carry genes that make bacteria resistant to the inhibitory action of antibiotics. See recombinationrecombination,
process of "shuffling" of genes by which new combinations can be generated. In recombination through sexual reproduction, the offspring's complete set of genes differs from that of either parent, being rather a combination of genes from both parents.
..... Click the link for more information.
The Columbia Electronic Encyclopedia™ Copyright © 2013, Columbia University Press. Licensed from Columbia University Press. All rights reserved. www.cc.columbia.edu/cu/cup/
The following article is from The Great Soviet Encyclopedia (1979). It might be outdated or ideologically biased.



a genetic factor that can exist in a cell either autonomously (in the cytoplasm) or integrated with the chromosome; a molecule of deoxyribonucleic acid. The genome of the temperate lambda bacteriophage, the sex (or F) factor, and some R factors that transmit drug resistance to bacteria, for example, are episomes.

Episomes are not essential constituents of cells, and they can change from one state to another, depending on the type of cell. In E. coli cells, for example, the genome of the temperate lambda bacteriophage may be either integrated or autonomous, whereas in the cells of the causative agent of typhoid fever it is found only in the autonomous state. Most autonomous episomes behave like typical plasmids. Some researchers regard episomes as a transitional link between the structures that determine chromosomal and nonchromosomal heredity.


Stent, G. Molekuliarnaia genetika. Moscow, 1974. (Translated from English.)
Meynell, G. Bakterial’nve plazmidy. Moscow, 1976. (Translated from English.) ’


The Great Soviet Encyclopedia, 3rd Edition (1970-1979). © 2010 The Gale Group, Inc. All rights reserved.


A circular genetic element in bacteria, presumably a deoxyribonucleic acid fragment, which is not necessary for survival of the organism and which can be integrated in the bacterial chromosome or remain free.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.
References in periodicals archive ?
Lieberman and his colleagues found that the three telomeric proteins might help Epstein-Barr virus episomes persist in cells.
The integration of HPV16 was more common than episome genome in the host cells, indicating that continuous HPV infection is the key for esophageal epithelial cell malignant conversion and canceration.
Viral genome is thought to be sustained as low copy number episomes in the basal cells of epithelium after infection and being uncoated.
Although scAAV genomes may persist within cells as episomes, low-frequency genomic integration was observed in previous publication [27, 28].
Since all episomes are generated by amplification of a single initial circle in the infected cells, analysis of the fused terminal fragments may indicate monoclonality, oligoclonality, or polyclonality of EBV-carrying cell populations.
[15] Somewhat suggestively, linearization of the circular viral episomes prior to integration invariably preserves the E6 and E7 genes, but disrupts the regulatory viral E2 gene.
During latency, viral genomes exist as extrachromosomal episomes in the nucleus and, being largely silenced by host-driven methylation of CpG island motifs, are only able to express a small subset of genes including latent proteins with oncogenic potential and viral miRNAs [36].
The molecular basis for this persistence is largely unknown, but 2 recent studies have given evidence in support of persistent circular HBoV episomes (24-26).
In addition, the early and utterly consistent presence of monoclonal EBV episomes in nasopharyngeal carcinoma worldwide suggests a crucial role for the virus in that neoplasm.