Plasmids -

Plasmids are small, circular, double stranded, autonomously replicating and covalently bonded DNA molecules that occur naturally in bacteria. Like all natural DNA molecules, plasmids contain a special region in their DNA called an origin of replication.

The origin serves as a start signal for DNA polymerase and ensures that the plasmid DNA molecules will be replicated by the cell. Many kinds of plasmids have been discovered. Plasmids differ in length and in the genes contained in their DNA. Some of the smaller plasmids which are popular for gene cloning contain about 5,000 nucleotide pairs, or enough DNA to code about five average size proteins.

Several plasmids have been found to carry genetic factors for fertility, resistance to antibiotics, ability to ferment sugars and even hydrocarbons such as petroleum, and for the production of bacteriocins and hemolysins. All these types of plasmids have been duly altered to yield smaller plasmids suitable as small vehicles. They are classified on the basis of their compatibility characteristics.

Plasmids are considered incompatible when they fail to establish themselves due to competition for essential replication sites on the cell membrane. One special kind of plasmid, called the Ti plasmid, has been used extensively in plant genetic engineering programmes. We shall discuss it in detail later. Other plasmids, such as the F plasl\1id, R plasmid, col plasmid, are also available but not much used in genetic engineering.