An episome is a portion of genetic material that can exist independent of the main body of genetic material (called the chromosome) at some times, while at other times is able to integrate into the chromosome.
Examples of episomes include insertion sequences and transposons. Viruses are another example of an episome. Viruses that integrate their genetic material into the host chromosome enable the viral nucleic acid to be produced along with the host genetic material in a nondestructive manner. As an autonomous unit (i.e., existing outside of the chromosome) however, the viral episome destroys the host cell as it commandeers the host's replication apparatuses to make new copies of itself.
Another example is an hfr cell (also called hfr strain) which is a bacterium with a conjugative plasmid (often the F-factor) integrated into its genomic DNA. Hfr is the abbreviation for high frequency recombination
An episome is distinguished from other pieces of DNA that are independent of the chromosome (i.e.,plasmids) by their large size.
Plasmids are different from episomes, as plasmid DNA cannot link up with chromosomal DNA. The plasmid carries all the information necessary for its independent replication. While not necessary for bacterial survival, plasmids can be advantageous to a bacterium. For example, plasmids can carry genes that confer resistance to antibiotics or toxic metals, genes that allow the bacterium to degrade compounds that it otherwise could not use as food, and even genes that allow the bacterium to infect an animal or plant cell. Such traits can be passed on to another bacterium.
1 comment:
The definition cited for episome matches a plasmid perfectly. I think this post is wrong. A plasmid is an episome while it's separate from the chromosomal DNA. The instant it integrates into chromosomal DNA, it becomes non-episomal.
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