A plasmid is a piece of an extragenomic DNA molecule that exists and replicates independently from the chromosomal DNA within a cell. They are often circular, although linear plasmids also exist. 

Prokaryotic cells may contain extragenomic DNA or DNA that is not part of the chromosome. This extragenomic DNA is found in plasmids, which are small, circular, double-stranded DNA molecules. Plasmids often carry genes that give advantageous traits such as antibiotic resistance, they are essential to the survival of the organism. However, plasmids are not essential for growth and reproduction. Plasmids may carry genes that provide resistance to naturally occurring antibiotics in a competitive environmental niche, or the proteins produced may act as toxins under similar circumstances. 

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Key Points

• Plasmids are small circles of extragenomic DNA that replicates independently from the chromosomal DNA within a cell

• Plasmids are replicons but do not encode genes necessary to encase the genetic material for transfer to a new host.

Key Terms

plasmid: double-stranded DNA that is separate from the chromosomes, which is found in bacteria and protozoa. Most often circular

prokaryotic: an organism whose cell (or cells) are characterized by the absence of a nucleus or any other membrane-bound organelles

extragenomic: inherited genetic information that is not part of an organism’s genome