Biotechnology and Bioinformatics (1+1)

2.1.3.3 Cosmids

Cosmids are the novel cloning vectors which possess properties of both plasmid and phage.

• Cosmids were first developed in 1978 by Barbara Hohn and John Collins.
• Cosmids contain a cos site of phage (which is essential for packaging of nucleic acid into protein coat) plus essential features of plasmid (such as plasmid origin of replication, a gene for drug resistance) and several unique restriction sites for insertion of DNA to be clone d.
• Cosmids can be perpetuated in bacteria in plasmid form, but can be purified by packaging in-vitro into phages.
• Advantage of using cosmid vector is that larger DNA can be cloned than what is possible with phage of plasmid.

For cloning foreign DNA into cosmid vector, cosmid DNA is first linearised by cutting it with appropriate RE. Foreign DNA which is to be cloned is also treated with the same RE. Subsequently, cosmid DNA and foreign DNA fragments are mixed in presence of T₄ DNA ligase .