Biotechnology and Bioinformatics (1+1)

2.1.3.1 Plasmids

The occurrence of plasmids in E. coli came to light in the early 1950’s through the pioneering work of Joshua Lederberg in the USA and William Hayes in England.

• Plasmids are gene tic elements that are stably inherited without being a part of the chromosome (s) of their host cells.
• They are found in bacteria and fungi of many kinds but not in higher eukaryotes and are not essential to the survival of the host cell.
• They may be composed of DNA or RNA and may be linear or circular.
• Double-stranded DNA plasmids appear to exist as predominantly covalently closed circular molecules in bacterial cells.
• Both circular and linear plasmids are found in yeast and other fungi. Linear yeast plasmids composed of either RNA or DNA, can encode protein toxins that inhibit the growth of sensitive yeasts.

Plasmids code for molecules that ensure their replication and stable inheritance during cell replication, and they also encode many products of considerable medical, agricultural, and environmental importance. For example, they code for toxins that greatly increase the virulence of pathogenic bacteria. They can also confer resistance to antibiotic s, and they enable bacteria belonging to the genus Rhizobium to fix atmospheric N₂.

Plasmids are widely used in molecular biology because they provide the basis for many vector s that are used to clone and express genes. The smallest bacterial plasmids are about 1.5 kb and the largest are greater than 1500 kb. The vast majority are circular. However, several very large linear DNA plasmids, up to 500 kb long, have been found in species of Streptomyces and Nocardia. Smaller plasmids are much desirable for gene cloning experiments. Larger plasmids are less in number whereas smaller ones are more in number.

The number of molecules of a plasmid found in a single bacterial cell is termed as copy number. It ranges from 1 to more than 50 per cell but this number is specific for a given plasmid residing in bacterial cell.

Plasmids with larger copy number are more useful for gene cloning experiments.

Plasmid PBR 322 is derived from transposon Tn³, plasmid pMBI, and plasmid pSC 101. pMBI- replicon,Tn³ - ampicillin transposon, pSC 101 – tetracycline resistance region.