Extrachromosomal circular DNA located in the cytoplasm of bacteria is called plasmid. It can range from 1-30 micrometers. Lederberg (1952) first used the term plasmid.
Characteristics of Plasmid?
1. Plasmids are circular double-stranded DNA molecules.
2. Its molecular weight is 106-200×106 Daltons.
3. It contains small amount of genes.
4. Certain parts of the plasmid can be cut by restriction enzymes.
5. Some plasmids synthesize specific chemicals. Such as Colicin, Vibrin.
6. Number of plasmids is 1-1000 per cell.
7. Replication of plasmids can occur in a semiconservative process.
8. They can move from one bacterial cell to another.
9. It is capable of recombination with other plasmids or roots.
10. It is smaller and different from the original bacterial chromosome.
Types of Plasmids Based on Function
1. F and F- Plasmids: Plasmids that transfer genetic material from one bacterial cell to another are called F plasmids. Plasmid which helps in sexual reproduction by forming pili in bacterial cells is called F © plasmid.
2. R Plasmid: Plasmids that contain antibiotic resistance genes are called R plasmids. R6 plasmid is resistant to 6 important antibiotics. It is a gene marker. It contains the tetracycline antibiotic resistance gene.
3. Col Plasmid: Plasmids that produce a toxic protein called colicin are called Col plasmids. Colicin can kill susceptible E. coli cells. Plasmids contain genes producing vibriocin. Vibriocin can kill susceptible Vibrio cholerae cells.
4. Virulence Plasmid: Plasmids that cause disease are called virulence plasmids. Ti plasmid of Agrobacterium tumefaciens causes crown gall disease in dicotyledonous plants.
5. Degradative Plasmids: Plasmids which can rapidly convert the abnormal substances of bacteria like toluene, salicylic acid etc. are called degradative plasmids.
Classification based on the number of plasmids
1. Single Copy Plasmid: If there is only one plasmid in the bacterial cell, it is called single copy plasmid.
2. Multicopy Plasmid: If there are many plasmids (1000) in the bacterial cell, it is called multicopy plasmid.
Use of Plasmid
1. Plasmids are widely used in molecular genetics research.
2. Plasmids serve as useful vectors in genetic engineering and gene cloning.
3. Plasmids have a role in producing high yielding crops.
4. Plasmids are used in biotechnology to produce disease resistant varieties.
5. Plasmids are used to make insulin, interferon, enzymes etc.
6. Plasmids have applications in the diagnosis of a variety of diseases.
7. Plasmids are used to create insect and weed resistant plants.
