Cosmids are plasmid particles, into which certain specific DNA sequences, namely those for cos sites are inserted. Since these cos sites enable the DNA to get packed in lambda particle, cosmids allow packaging of the DNA in phage particle in vitro, thus permitting their purification.

Similar to plasmids, cosmids perpetuate in bacteria and do not carry the genes for lytic development. The advantage of using cosmids for cloning is their efficiency is high enough to produce complete genomic library of around 106 clones from1 pg of insert DNA.

They are predominantly plasmids with a bacterial oriV, an antibiotic selection marker and cloning site, and one, or more recently two cos sites derived from bacteriophage lambda (Figure 8).

Figure 8. Scheme of DNA cloning in a cosmid vector
Figure 8. Scheme of DNA cloning in a cosmid vector

The advantages of plasmid and phages are combined in the cosmid vector, as it propogates like plasmid but uses the packaging mechanism of phage lambda to deliver DNA to the bacterial cells.

Despite these apparent advantages, cosmids have not been much used, possibly because various side reactions that reduce efficiency have been associated with their preparation and it is unable to accept more than 40-50 kbp of DNA.