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.