<P> The term episome was proposed François Jacob and Élie Wollman in 1958 to describe extra-chromosomal genetic material that may replicate autonomously or become integrated into the chromosome . The use of the term, however, has diverged since it was first coined as plasmid became the preferred word for autonomously replicating extrachromosomal DNA as proposed in the symposium in 1968--it was suggested by some that the use of the term episome be abandoned, although others continued to use the term with a shift in meaning . In prokaryotes, episome is now used by some to refer to plasmid that is capable of integrating into the chromosome . The integrative plasmids may be replicated and stably maintained in a cell through multiple generations, but always at some stage they exist as an independent plasmid molecule . </P> <P> In eukaryotes, episomes used to mean non-integrated extrachromosomal closed circular DNA molecule that may be replicated in the nucleus . Viruses are the most common examples of this, such as herpesviruses, adenoviruses, and polyomaviruses, but some are plasmids . Other examples include aberrant chromosomal fragments, such as double minute chromosomes, that can arise during artificial gene amplifications or in pathologic processes (e.g., cancer cell transformation). Episomes in eukaryotes behave similarly to plasmids in prokaryotes in that the DNA is stably maintained and replicated with the host cell . Cytoplasmic viral episomes (as in poxvirus infections) can also occur . Some episomes, such as herpesviruses, replicate in a rolling circle mechanism, similar to bacterial phage viruses . Others replicate through a bidirectional replication mechanism (Theta type plasmids). In either case, episomes remain physically separate from host cell chromosomes . Several cancer viruses, including Epstein - Barr virus and Kaposi's sarcoma - associated herpesvirus, are maintained as latent, chromosomally distinct episomes in cancer cells, where the viruses express oncogenes that promote cancer cell proliferation . In cancers, these episomes passively replicate together with host chromosomes when the cell divides . When these viral episomes initiate lytic replication to generate multiple virus particles, they in general activate cellular innate immunity defense mechanisms that kill the host cell . </P> <P> Some plasmids or microbial hosts include an addiction system or postsegregational killing system (PSK), such as the hok / sok (host killing / suppressor of killing) system of plasmid R1 in Escherichia coli . This variant produces both a long - lived poison and a short - lived antidote . Several types of plasmid addiction systems (toxin / antitoxin, metabolism - based, ORT systems) were described in the literature and used in biotechnical (fermentation) or biomedical (vaccine therapy) applications . Daughter cells that retain a copy of the plasmid survive, while a daughter cell that fails to inherit the plasmid dies or suffers a reduced growth - rate because of the lingering poison from the parent cell . Finally, the overall productivity could be enhanced . </P> <P> In contrast, virtually all biotechnologically used plasmids (such as pUC18, pBR322 and derived vectors) do not contain toxin - antitoxin addiction systems and thus need to be kept under antibiotic pressure to avoid plasmid loss . </P>

Where do plasmids come from and how are they replicated