Plasmids are extrachromosomal circular pieces of DNA found in some bacterial cells.

They contain genes which convey some form of selective advantage, such as the ability to engage in the recombinant event called conjugation, or the production of enzymes which prevent the activity of antibiotics such as cell wall inhibitors (penicillin and ampicillin), and protein synthesis inhibitors (kanamycin, tetracycline, and chloramphenicol).

Plasmids are used in biotechnology as vectors, DNA molecules carrying foreign genes into a host cell.