Plasmids are small circular DNA molecules occuring in bacteria, which can be exchanged between different cells under natural conditions.

PlasmidMost of the genetic information contain in bacteria is found in a single, circular chromosome. Plasmids are additional cassettes of information that can divide autonomously and exist in several copies scattered throughout the cell.

Bacterial cells do not need plasmids to survive, but they often contain genes that can give the bacteria some kind of advantage. Plasmids may contain genes needed for coping with challenges like heavy metals or antibiotics.

Plasmids are commonly used as "tools" for genetic engineering:

• Plasmids can be used as cloning vectors to amplify certain genes. The gene of interest is integrated into a specially designed plasmid (cloned) and is thereby multiplied every time the carrier cell divides.
• Plasmids are also used as transporters (vectors) for sending foreign genes into target organisms. The delivery of genes contained in plasmids for plant genetic engineering can be facilitated by Agrobacterium tumefaciens.