Cosmid

A cosmid, first described by Collins and Hohn in 1978, is a type of plasmid (often used as a cloning vector) constructed by the insertion of cos sequences, DNA-Sequences of the Lambda phage.

Cos sequences are single stranded sequences of DNA, which have been split from the parent molecule by a specific restriction enzyme in such a way that the ends have specific affinity for each other, and hence are known as cohesive ends.

These DNA-Sequences make it possible to pack genes with up to 44K base pairs, while normal plasmids are able to carry only 10-15K base pairs. Cosmids are packaged in phage structures consisting of proteins, which allows the foreign genes to be inserted into the bacteria using transduction. If the Cosmids contain, for example, genes for resistance against antibiotics, the transfected bacteria are then able to survive and to spawn in a nutrient solution containing the antibiotic and can thus be selected. Cosmids can be used to build genomic libraries.