A plasmid is a small circular DNA molecule that replicates independently of the bacterial chromosome. Bacteria use plasmids to share genes, including antibiotic resistance genes. Scientists use them as vectors to carry genes into host cells. The plasmid is one of the most important tools in genetic engineering.

A typical lab plasmid has several features. An origin of replication allows it to replicate in the host. A selectable marker, usually an antibiotic resistance gene, allows cells that took up the plasmid to be identified. A multiple cloning site provides a place to insert the gene of interest. A promoter drives expression of the inserted gene. Some plasmids also carry tags for protein purification or reporters for visualization.

Plasmids are used in cloning, protein expression, gene therapy, and CRISPR. They can be designed to express a gene in bacteria, yeast, mammalian cells, or plants. They can carry multiple genes. They can be engineered with inducible promoters that turn expression on or off. The versatility is why plasmids are found in almost every molecular biology lab.

Plasmids are also used in nature. They spread antibiotic resistance between bacteria, which is a major public health concern. The same mechanism that makes them useful in the lab makes them dangerous in the clinic.