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The Collaborative International Dictionary of English v.0.48 (gcide)

Plasmid \Plas"mid\, n. (Molecular genetics)
A piece of DNA, usually circular, functioning as part of the
genetic material of a cell, not integrated with the
chromosome and replicating independently of the chromosome,
but transferred, like the chromosome, to subsequent
generations of daughter cells. In bacteria, plasmids often
carry the genes for antibiotic resistance; they are exploited
in genetic engineering as the vehicles for introduction of
extraneous DNA into cells, to alter the genetic makeup of the
cell. The cells thus altered may produce desirable proteins
which are extracted and used; in the case of genetically
altered plant cells, the altered cells may grow into complete
plants with changed properties, as for example, increased
resistance to disease.
[PJC]