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(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.
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 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
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 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.
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 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.
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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.) ’



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.
References in periodicals archive ?
cells across blood drawn from multiple donors with non-integrating episomal vectors.
However, PyVs have alternate mechanisms of replication incompetence, and late gene suppression might occur in episomal virus.
As a positive control, the same mutants were transformed with an episomal vector, pYES2, containing URA3 as a selectable marker (Figure 5A).
Damage to episomal HPV DNA, such as that caused by oxidative stress, is a critical step triggering genomic integration of the virus and expression of genes that promote keratinocyte proliferation and inhibit differentiation (Jones and Wells 2006).
Similarly, vancomycin resistant Enterococcus faecalis transferred episomal DNA to previously vancomycin susceptible Staphylococcus aureus, (11) revealing that antibiotic resistance can cross both bacterial species and genus barriers.
During the early phases of infection, the copy number of viral genome is between 50 and 100, and the viral genome exists as extrachromosomal plasmid or episomal form that replicates as the host cell chromosomes replicate.
Various recombinant parasites carrying a reporter gene either as an episomal copy or after its integration in a defined locus, generally the rDNA locus against leishmaniasis are currently available (Table I).
In vivo gene gun-mediated transduction into rat heart with Epstein-Barr virus-based episomal vectors.
2) Low-risk episomal HPVs (HPV 6/11) induce proliferation of the epithelium.
While in latency, HHV8 exists as circular episomal DNA and expresses limited gene products, including LANA-1 (or LNA-1).
Other topics discussed include gene silencing by double-stranded RNA in Apis mellifera, induction of RNAi by episomal transcriptionally silent homologous DNA, and polymers for siRNA delivery in vivo.
An earlier statement in the text, on page 2281, reiterates this notion for the polyomaviruses: "No information is available, however, on the mechanisms of viral persistence*, whether the viral DNA is episomal or integrated, what triggers viral synthesis and multiplication " [1].