Plasmid

[Becky]

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. 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.

[Tanya Shivari]

A genetic structure in a cell that can replicate independently of the chromosomes, typically a small circular DNA strand in the cytoplasm of a bacterium or protozoan. Plasmids are much used in the laboratory manipulation of genes.