The plasmid is the extrachromosomal DNA that can independently replicate. Most commonly plasmids are found in bacteria as double-stranded circular DNA. Sometime plasmid can be found in multicellular organisms.

Plasmid classification:

• Conjugative and Non-conjugative Plasmid

Conjugative plasmids are characterized by the ability to promote sexual conjugation between bacterial cells.

Non-conjugative plasmid can’t make sexual conjugation.

Transfer of plasmid is controlled by a set of genes known as TRA genes. These genes are only present in Conjugative plasmids.

Specific types of plasmid

There are five types of plasmids

1. F plasmid
2. R plasmid
3. Col plasmid
4. Degradative plasmid
5. Virulence plasmid

• F plasmid

Known as fertility plasmid. These plasmids carry tra (transfer) genes. These plasmids can transfer genes from one bacteria to another bacteria thorough conjugation.  During the process of conjugation, only DNA is passed to the recipient from the donor. Therefore, cytoplasm and other cell materials are not transferred to the donor.

• R plasmid

Known as resistance plasmid. These plasmids carry genes conferring on host bacteria resistance to one or more antibiotic agents. R plasmid helps the bacteria to defend against antibiotics and other environmental factors.

• Col plasmid

Col plasmid can make colicins proteins. These proteins belong to a group of toxins known as bacteriocins. It allows the bacterium to kill other bacteria and defend host bacteria.

E.g., Col B, Col E2 & E3

• Degradative plasmid

Degradative plasmid allows the host to metabolize/degrade the unusual molecules such as salicylic acids, oils.

E.g., pseudomonas putida (1^st patented microorganism/ oil digesting bacteria by Ananda Mohan Chakrabarti)

• Virulence plasmid

If we compare these plasmids to other plasmids of harmless bacteria Virulence plasmid tend to confer pathogenicity. These can infect hosts. It helps the bacteria to adapt to the environment.

Applications/Advantages:

• Used in genetic engineering.
• Because of the high copy number and self-replicating, it can produce, numerous copies of genes.
• Antibiotic-resistant.
• Some plasmid can resist the growth of other bacteria (Col plasmid)
• Used as a vector for cloning.
• Used to express a gene.