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Cosmid Vectors - Cosmids are essentially plasmids that contain a minimum of 250 bp of λ. DNA, which includes

(1) the cos site (the sequence yielding cohesive ends) and

(2) sequences needed for binding of and cleavage by terminase so that under appropriate conditions they are packaged in vitro into empty λ phage particles.

A typical cosmid has
(1) replication origin,

(2) unique restriction sites and

(3) selectable markers from the plasmid; therefore, selection strategy for obtaining the recombinant DNA is based on that for the contributing plasmid.

Cosmid vectors are constructed using recombinant DNA techniques.

The cosmid vectors are opened by the appropriate restriction enzyme at a unique site, are then mixed with DNA inserts prepared by using the same enzyme and annealed. Among the several types of products, long cancatemers are present, which are the appropriate precursors for packaging in λ. particles.

This procedure selects for long DNA inserts since for packaging the distance between two cos sites must be between 38 and 52 kb. Cosmids can accommodate upto 45 kb long DNA inserts. Packaged cosmids infect host cells like λ particles, but once inside the host they replicate and propagate like plasmids.

The typical features of cosmids areas follows:
(1) they can be used to clone, DNA inserts of upto 45 kb.

(2) They can be packaged into A. particles that infect host cells, which is many-fold more efficient than plasmid transformation.

(3) Selection for recombinant DNA is based on the procedure applicable to the plasmid making up the cosmid.

(4) Finally, these vectors are amplified and maintained in the same manner as the contributing plasmid.

 
 

 
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