Cosmids as vectors

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

Like plasmids, cosmids also perpetuate in bacteria and do not carry the genes for lytic development.

The advantage of the use of cosmids for cloning is that its efficiency is high enough to produce a complete genomic library of 10⁶ - 10⁷ clones from a mere 1µg of insert DNA. The disadvantage, however, is its inability to accept more than 40-50 kbp of DNA. Bacteriophage PI system and F-factor based vectors (BACs = bacterial artificial chromosomes) can allow cloning of DNA segments, as large as 100 to 1000 kbp (or 1 Mbp = 10⁶bp) length.