What is a plasmid?

What a plasmid is

A plasmid is a piece of DNA that is separate from the cell’s main chromosome and can copy itself independently. Plasmids are typically small and circular and occur most often in bacteria, where a cell may carry one or many copies. The genes a plasmid carries are not usually essential for basic survival, but can give the host useful traits — antibiotic resistance being a well-known example — and plasmids can be passed between bacteria.

Plasmids as vectors

In genetic engineering, plasmids are the most common type of vector — a vehicle for carrying a chosen piece of DNA into a host cell. A researcher cuts the plasmid open with a restriction enzyme, inserts the DNA of interest, and seals it to form recombinant DNA.

When the modified plasmid is taken up by a bacterium, the cell copies the plasmid and its inserted gene each time it divides, producing many copies — a process called cloning. Plasmids often include a selectable marker so that cells carrying them can be identified.

Significance in research

Plasmids are foundational tools of molecular biology and biotechnology. They are used to clone and store DNA fragments, to produce proteins in bacteria, and to deliver genes in many laboratory experiments. Because plasmids are shared between laboratories, consistent description of their sequences, features, and selectable markers supports reproducible and reusable research.