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.
Related Theory
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.