1-4.3 Cosmid:

•  A cosmid, first described by Collins and Hohn in 1978, is a type of hybrid plasmid with a bacterial “ori” sequence and a “ cos” sequences derived from the lambda phage.

•  Cos site is the sequence required by a DNA molecule in order to be recognized as a ‘λ genome' by the proteins that package DNA into λ phage particles.

•  Cosmid DNA containing particles are as transmittable as real λ phages, but once inside the cell, the cosmid cannot control synthesis of new phage particles and instead replicates as a plasmid.

•  Recombinant DNA is therefore obtained from colonies rather than plaques. They frequently also contain a gene for selection such as antibiotic resistance.

•  They are able to load 37 to 52 kb of DNA, while normal plasmids are able to carry only 1–26 kb.

•  Sometimes helper phage is used to assist in packaging of cosmid inside phage. Helper phage provides the essential proteins required for packaging which are lacked by cosmid vector.

•  For packaging into a phage, concatemer formation is required (cosmid-insert-cosmid). This is generated by using two cos sites flanked by the insertion site for foreign DNA. Providing the inserted DNA in the right size, in vitro packaging cleaves the cos sites and replaces the recombinant cosmids in mature phage particles.

•  Recombinant λ phages are used to infect an E. coli culture. Infected cells are plated on a selective medium and antibiotic-resistant colonies are grown. All colonies are recombinants, as non-recombinant linear cosmids are too small to be packaged into λ heads.

•  Cosmids are widely exploited to build genomic libraries. The upper limit for the length of the cloned DNA is set by the space available within the λ phage particle. New DNA insert of size upto 44 kb can be inserted before the packaging of the λ phage particle is reached.