Cosmids are hybrid plasmids that contain the cos sequence of a Lambda phage (bacteriophage). Structurally these are DNA hybrids of plasmids and a lambda phage. 

Plasmids refer to a small circular DNA in the cytoplasm of bacteria or protozoans. These genetic structures in a cell can replicate independently of the chromosomes. Typically, plasmids are much used in the laboratory manipulation of genes.

In other words, a vector that contains the two cos (cohesive) ends of phage lambda (λ) and one or more selectable markers such as an antibiotic resistance gene of a plasmid is termed as the cosmid.


The first cosmid vector was described by Collins in 1978. Cosmid vectors are developed by combining the features of the plasmid vector and the bacteriophage vector. Origin of replication, multiple cloning site and selectable marker are obtained from the plasmid and only the cohesive site or cos site region is taken from lambda phage. These are fused together to obtain the cosmid vector.
The cos site in cosmid is the only requirement for DNA to be packaged into a phage particle. Cosmids were developed in light of this observation.

Advantage of Cosmid over Lambda Phage

Independent phage head portion can accept between 12 to 20 kb of insert DNA. However, cosmids exploit certain properties of phage lambda (λ) to enable large, 40-50 kb, DNA fragments to be cloned at high efficiency. Cosmids and cosmid recombinants replicate as plasmids.

During the cloning process using cosmid, only about 200 base pairs of lambda phage sequence are cloned into the cosmid vector.

This consists of cosN, cosB and cosQ. 

A cosmid vector may have one or two cos sites. Cosmid vectors are used in the construction of genomic libraries. The cloning of a foreign DNA in cosmid vector involves the following steps:
  • Ligation of the foreign DNA between two cos sites;
  • making a concatemeric DNA;
  • in vitro packaging to introduce the DNA into the phage head to form the matured phage particle; and
  • introduction of the cloned DNA into E. coli by transduction.