Cosmid

Cosmid

Figure 1 - A typical cosmid

                Cosmids are hybrid plasmid vectors that contain λ cos site. They were first described by Collins and Hohn in 1978. The λ cos site is the sequence needed so that the DNA molecule becomes recognized as part of the ‘λ genome’ by the proteins that package DNA into λ phage particles.

                Inside the cell, the cosmid DNA is incapable of directing the synthesis of new phage particles. As an alternative the cosmid DNA replicates as a plasmid.
The length of the cloned DNA can be as long as the available space within λ cos site. A cosmid can be 8 kb or less in size, so up to 44 kb of new DNA can be inserted before the packaging limit of the λ phage particle is reached. Recombinant DNA is obtained from colonies and can be used to build genomic libraries.