Write a short note on plasmids.

Explanation

These are small and double-stranded extrachromosomal DNA molecules having an origin of replication. They occur naturally in bacteria. Naturally occurring plasmids are not suitable for the cloning of genes. They are genetically engineered and made suitable for cloning. Such plasmids are called cloning plasmids. A suitable cloning plasmid should preferably have the following properties
(i) It should be non-conjugative.
(ii) It should have a relaxed replication and a high copy number.
(iii) It should have an origin of replication.
(iv) It should have unique restriction sites.
(v) It should have antibiotic marker gene for selection.
The most common cloning plasmid is designated as PBR322, where p stands for the word plasmid and B and R signify the names of its engineers (B. Boliver and R Rodriguez) 322 is a numerical designation that has a relevance to these workers, who worked out the plasmid.