Plasmid: Lederber, (1952) first observed the plasmids as extrachromosomal material in Bacterial cell.
It has the following important features.
- These are extrachromosomal. (Other than chromosomal DNA) components.
- These are circular, double-stranded DNA molecules.
- They contain an origin of replication (ori). Hence being able to replicate independently within the cell.
- These are not necessary for the growth and survival of Bacteria.
- These contain specific restriction sites where the desired gene can be inserted.
- Marker genes or marker sites are also found.
- The plasmid may contain three to one thousand genes. Most commonly used plasmid vector is pBR322. In this plasmid, two marker sites TetR (Tetracycline resistant) and AmpR (Ampicillin resistant) are found. It contains recognition sites for 12 different restriction enzymes. Desired DNA (foreign DNA) is inserted in between TetR and AmpR gene with the help of restriction enzyme.