Plasmids

The circular double-stranded DNA molecule that is present in the bacterial cytoplasm apart from the original chromosome is called a plasmid.

Lederberg (1952) first discovered plasmids in E. coli bacterial cells. DNA molecules of plasmids can replicate independently. Plasmids are located in most bacteria as an extra, smaller piece of DNA (chromosome) outside the main chromosome. Their number can range from 1–1000 per cell.

Fig: Plasmid

General Properties of Plasmids

1. Plasmids are circular (circular) double-stranded DNA molecules.

2. Its molecular mass is about 100–200 × 10⁶ Dalton.

3. Plasmids contain a small number of genes.

4. Restriction enzymes can cut out specific sites in ideal plasmids.

5. They are easily transferred to other bacteria by conjugation.

6. Genes in some plasmids can synthesize specific chemicals, such as colicin, vibriocin, etc.

Types of Plasmids

Plasmids are mainly of three types:

(i) F and F Plasmids

• These plasmids are responsible for transferring genetic material from one bacterium to another.

• F (fertility) and F-plasmids produce pili in the bacterial body, which help them reproduce sexually.

(ii) R-Plasmids

• These plasmids contain genes with antibiotic resistance potential.

• R-plasmid is resistant to 6 important antibiotics.

(iii) Col Plasmids

• Plasmids that contain colicin-producing genes are called Col plasmids.

• Colicin is a type of protein that can kill susceptible E. coli cells.

• Another type of plasmid similar to the Col plasmid contains the vibriocin-producing gene.

• Vibriocin destroys susceptible Vibrio cholerae cells.

Use of Plasmids

Plasmids are widely used in various fields of molecular genetics research. They are very useful vectors for genetic engineering, gene cloning, and other applications.

Plasmid DNA has led to unprecedented success in many areas of modern biotechnology; for example:

• Human insulin gene cloning

• Production of disease-resistant and insect-resistant plants