A plasmid is a small, often circular DNA molecule found in bacteria and other cells. Plasmids are separate from the bacterial chromosome and replicate independently of it. They generally carry only a small number of genes, notably some associated with antibiotic resistance. Plasmids may be passed between different bacterial cells.
Small pieces of DNA, such as human DNA, can be attached to appropriate elements, circularized, and then introduced into bacteria, where they are propagated--or in other words, copied--along with the host bacterial chromosome. These small circles containing the cloned DNA are called plasmids. Each bacterial cell typically produces many copies of a plasmid, in contrast to making only one copy of its own chromosome. The fact that plasmids are smaller and in greater number than the host chromosome make plasmids easier to isolate in pure form, which is why researchers commonly use them for studying DNA in the laboratory. Plasmids are thus a fundamental tool of recombinant DNA technology.- Eric D. Green, M.D., Ph.D.