plasmid

A plasmid is a small, usually circular piece of double-stranded DNA that exists independently of the chromosomal DNA in many bacteria and some archaea. Plasmids replicate autonomously and often carry genes that confer selective advantages under certain conditions, such as antibiotic resistance or unique metabolic capabilities.

Most plasmids are circular and range from a few thousand to several hundred thousand base pairs. They maintain themselves through an origin of replication and regulatory systems that control copy number, which can be high or low. Segregation mechanisms help ensure their transmission to daughter cells during division.

Many plasmids are mobile. Conjugative plasmids can transfer between cells via a mating bridge, spreading traits such as resistance or virulence. Other plasmids are mobilizable or rely on co-resident conjugative plasmids for transfer. Plasmids can also be introduced in the laboratory by transformation or transduction.

Plasmids are classified into incompatibility groups, or Inc groups, based on their replication and maintenance systems. Notable examples include the F plasmid, which promotes conjugation in Escherichia coli; R plasmids carrying antibiotic resistance genes; and Ti plasmids in Agrobacterium tumefaciens involved in plant transformation.

In research and biotechnology, plasmids are widely used as cloning and expression vectors. They can be engineered to carry selectable markers, genes of interest, and regulatory sequences for protein production, functional studies, or gene delivery in experimental models.

The spread of plasmids contributes to horizontal gene transfer and the dissemination of antibiotic resistance, making plasmid biology a key focus of biosafety and public health discussions. Proper containment and surveillance are essential in clinical and agricultural settings.