Short note on Plasmid ~ University Notes

1. Structure of Plasmid:

Plasmids are small, circular, double stranded, non-chromosomal DNA molecules. In most plasmids the two ends of the double stranded DNA molecule, that make up plasmids covalently bond together forming a physical circle.

Some plasmids, however, have linear DNA. Their size vary from 1 to over 1000 kbp and contain between 5-100 non-essential genes. They play no role in the in-viability and growth of bacteria and hence called dispensable auto-nous elements.

2. Types of Plasmid:

On the basis of function, there are five types of plasmids:

1. Fertility F-plasmids:

Contain tra genes, capable of conjugation.

2. Resistance Plasmids (R-factor):

Contain genes that can build a resistance against antibiotics or poisons and help bacteria to produce pili. (Historically known as R-factors before the nature of plasmid was understood).

3. Col. Plasmid:

Contain genes that code for bacteriosins, proteins that can kill other bacteria.

4. Degenerative Plasmids:

Unable digestion of substances e.g., toluene, salicylic acid.

5. Virulence Plasmids:

Turns the bacteria into pathogens.

3. Transfer of Plasmids:

Plasmids can be transformed from one bacterium to another by transformation and gene transduction. The most common mechanism of plasmid transfer is conjugation. The plasmids which are transferred from cell to cell are known as conjugative plasmids.

They contains genes coding for proteins involved in both DNA transfer and forming of mating pairs. Plasmids can be considered part of the mobilome because they are often associated with conjugation, a mechanism of horizontal gene transfer.

4. Reproduction in Plasmid:

Plasmids replicate independently of the host chromosome, but some plasmids called episome (def) are able to insert or integrate the host cell chromosome, then their replication is integrated by the chromosome.