Cosmid

A cosmid is a type of hybrid plasmid that contains a Lambda phage cos sequence.

[3] Unlike plasmids, they can also be packaged in vitro into phage capsids, a step which requires cohesive ends, also known as cos sites also used in cloning with a lambda phage as a vector, however nearly all the lambda genes have been deleted with the exception of the cos sequence.

This instability can largely be counteracted by using a host bacterium with specific mutations affecting DNA recombination (N.B.

The cloning procedure involves the generation of two vector arms which are then joined to the foreign DNA.

After the construction of recombinant lambda or cosmid libraries the total DNA is transferred into an appropriate E. coli host via a technique called in vitro packaging.

This is usually done by partial restriction followed by either size fractionation or dephosphorylation (using calf-intestine phosphatase) to avoid chromosome scrambling, i.e. the ligation of physically unlinked fragments.