<molecular biology> A small, independently replicating, piece of extrachromosomal cytoplasmic DNA that can be transferred from one organism to another. Linear or circular DNA molecules found in both pro and eukaryotes capable of autonomous replication.

Stringent plasmids occur at low copy number in cells, relaxed plasmids at high copy number, ca 10 to 30. Plasmids can become incorporated into the genome of the host or can remain independent. An example is the f factor of E. coli. May transfer genes and plasmids carrying antibiotic resistant genes can spread this trait rapidly through the population.

Described largely from bacteria and protozoa. Some plasmids are capable of integrating into the host genome. A number of artificially constructed plasmids are used as cloning vectors.

(14 Oct 1997)

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