Plasmids are extra-chromosomal molecules of DNA, mostly in the form of double-stranded circles. Plasmids are mostly found in prokaryotes, but may be found in some archaea and eukaryotes. Plasmid usually encodes non-essential genes that are not required during normal growth conditions. The plasmid may encode gene that governs the cell's physiology aspect in order to survive in harsh environment such as antibiotic resistance.

Cross-section of bacteria cell with named internal and external structures. Flagellum shown as long, thick tail at one side of the cell. Pilus presented as shorter and thinner tail-like structure, spread throughout the whole external capsule surface. Cytoplasm, where internal organs are embedded, is surrounded by plasma membrane, which in turn is surrounded by the cell wall that is encapsulated by the capsule. In cytoplasm, Plasmids are shown as small, closed, circular structures, chromosome is shown as much longer, closed, circular structure, taking up most of the space, and ribosomes are shown as small spheres spread through the whole cytoplasm.

Figure 1. Structure of bacteria.

Plasmids vary in size from 1 kb to more than 200 kb. They are frequently used as vectors to transfer recombinant DNA into a host cell.