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.