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Plasmids are extrachromosomal DNA molecules that replicate separately from the chromosome. In many cases, they are circular and double-stranded, but linear plasmids also exist. They are very frequently found in Bacteria, some Archaea, and sometimes also in eukaryotic organisms ( e.g., the 2-micrometre-ring in Saccharomyces cerevisiae [w], or yeast ). In Bacteria, they often code for resistance against antibiotics or heavy metals, for virulence factors, or the degradation of environmental pollutants.

The size of plasmids varies from 1 to over 1,000 kilobase pairs. There may be one copy, for large plasmids, to hundreds of copies of the same plasmid in a single cell. Plasmids are part of the mobilome, since they often transfer themselves, or can be mobilized, by conjugation, a mechanism of horizontal gene transfer. - modified from wikipedia

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