Cosmids as Vectors

Cosmids as Vectors - 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 the packaging of DNA in phage particle in vitro, thus permitting their purification.

Like plasmids, these cosmids 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 1 pg 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 can allow cloning of DNA segments, as large as 100 to 1000 kbp (or 1 Mbp = 106bp) length