Cosmid Vector Library Construction
Cosmid vectors Cosmid are plasmid type of vector (example- c2RB, pJB8) Cosmid vectors are hybrids of plasmid and bacteriophage lambda DNA (a small plasmid containing the plasmid origin of replication (ori), an antibiotic resistance gene such as ampR and a suitable restriction site for cloning along with the COS sequences from phage DNA. Because of the COS sequence, cosmid recombinants can be packaged into viral particles allowing high efficiency transformation but the distance between two cos sites should be 45-50 kb i.e., if the insert size is smaller than this then it can’t go for packaging. Since the recombinant DNA does not encode any lambda proteins, cosmids do not form viral particles (or plaques) but rather forms large circular plasmids and the colonies that arise can be selected on antibiotic plates, like other plasmid DNA recombinants. Advantage of cosmids are that they can insert large DNA compared to plasmid and the transformation efficiency is very high as phage can infec