plasmid -
A circle of double-stranded DNA that is separate from the chromosomes, and which is found in bacteria and protozoa.
Wikipedia
A plasmid is a DNA molecule that is separate from, and can replicate independently of, the chromosomal DNA. They are double stranded and, in many cases, circular. Plasmids usually occur naturally in bacteria, but are sometimes found in eukaryotic organisms (e.g.
OmegaWiki Dictionary
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plasmid A small DNA molecule within a cell that is physically separated from a chromosomal DNA and can replicate independently.