definition of natural selection
Lambda phage: The phage head is the orange structure at the top. Image: Adenosine

A cosmid is a plasmid that carries bacteriophage lambda's cos gene. Cosmids are used to introduce DNA fragments into the bacterium Escherichia coli. The cos gene allows the fragments to be packaged into lambda phage particles ("phage heads") for infection of prokaryotic cells. Fragments cloned by this method can be larger (as large as 45kb) than in the case of ordinary plasmid cloning vectors.

In addition to the λ cos site, cosmids contain an origin of replication a selectable marker (typically ampr) and a site for the insertion of foreign DNA.

The origin of replication permits autonomous replication of the cosmid after injection into a bacterium. The selectable marker allows isolation of bacteria that carry the cosmid.

See also: cloning vector, yeast artificial chromosome.

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