Cosmid Vectors

Cosmid are essentially plasmids that contain minimum of 250 bp of DNA which includes (1) the cos site (the sequence yielding cohesive ends and (2) sequences needed for binding of and cleavage by terminaase so that under appropriate condition they are packaged in vitro into empty phage particles cosmid has (1) replication origin (2) unique restriction sites and (3) selecetable markers from the plasmid therefore selection stratgy for obtaining the recombinant vectors 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 precusors a for packaging in particles. The procedure selects for long DNA insert since for packaging the distance between two cos sites must between 38 and 50 kb. Cosmid can accomodate upto 45 bp long DNA inserts packaged cosmids infect host cells like particles but once inside the host they replicate and propagate like plasmids.
The typical features of cosmids are as follows (1) they can be used to clone DNA inserts of upto 45 kb (2) THey can be packaged into particles which infect host cells, which many fold more efficient than plasmid transformation (3) Selection for recombinant vector 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.