Cosmids

Cosmids are cloning vectors used to capture moderately large fragments of DNA for propagation in Escherichia coli. They are hybrid constructs that combine elements of plasmids with the packaging signals of lambda phage, allowing they to carry sizable inserts while remaining easy to propagate as plasmids in bacterial cells.

A cosmids vector typically contains an origin of replication compatible with plasmids in E. coli, a selectable marker such as an antibiotic resistance gene, and a lambda phage cos site, which provides the ability to package the vector into lambda phage particles. The inserts cloned into cosmids are usually in the range of about 35 to 45 kilobases, somewhat larger than standard plasmid libraries but smaller than typical bacterial artificial chromosomes (BACs).

Cosmids are maintained in bacteria as plasmids, but to generate packaged particles for transduction or library screening, a helper phage or packaging system supplies the missing lambda genes required for phage assembly. This enables cosmid DNA containing large inserts to be transferred into new host cells or used in screen-based workflows.

Applications of cosmids include construction of genomic libraries with relatively large inserts, physical mapping, and interval cloning for sequencing projects. They offer a compromise between the ease of plasmid vectors and the larger clone sizes achievable with phage-based systems. However, packaging efficiency can be variable, inserts may be unstable if large or highly repetitive, and cosmids have largely been superseded in some contexts by fosmids and BACs, which offer different advantages in terms of stability and capacity.