Figure 1: Illustration of a bacterium with plasmid enclosed showing chromosomal DNA and plasmids.

A plasmid is a DNA molecule that is separate from the chromosomal DNA and that can replicate (copy itself) independently.

The term plasmid was first introduced by the American molecular biologist Joshua Lederberg in 1952.

Plasmids are double stranded and, in many cases, circular. Plasmids usually occur naturally in bacteria, but are sometimes found in eukaryotic organisms (e.g., the 2-micrometre-ring in Saccharomyces cerevisiae).