COSMIDS (cosmid = cos site + plasmid)

[aayush_005]

Based on the properties of DNA and Col E1 plasmid DNA, a group of Japanese workers (Fukumaki et al., 1976) showed that the presence of a small segment of phage λ DNA containing cohesive end on the plasmid molecule is a sufficient prerequisite for in vitro packaging of this DNA into infectious particles. The cosmids can be defined as the hybrid vectors derived from plasmids which contain cos site of phage λ. (cosmid = cos site + plasmid). For the first time it was developed by Collins and Hohn (1978).
Cosmids lack genes encoding viral proteins; therefore, neither viral particles are formed with the host cell nor cell lysis occurs. Special features of cosmids similar to plasmids are the presence of:
(i) Origin of replication,
(ii) A marker gene coding for antibiotic resistance,
(iii) A special cleavage site for the insertion of foreign DNA, and
(iv) The small size. Character dissimilar to plasmid is the presence of extra
phage DNA, the cos site, which has about 12 bases. It helps the whole genome in circularization and ligation.
The cosmids have a length of about 5 Kb, the upper size limit of the foreign DNA fragment that may be inserted in cosmids and packed into phage particles is, therefore, approximately 45 Kb, much larger than it would be possible to clone in phage λ or plasmid vector. According to the size of cos sites and upper size limit in the head of phage, the recombinant DNA molecules can be packed into bacteriophage particles in in vitro packaging system consisting of packaging enzymes, head and tail proteins.
Procedure of DNA cloning by using cosmid vector is shown in Fig. Upon transfection of E. coli by bacteriophage, the recombinant DNA cyclizes through cos sites and then replicates as a plasmid and expresses the drug resistance marker. Recently, based on cosmid vectors, a number of cosmid vectors have been determined from E. coli, yeast, and mammalian cells, and gene bank has been constructed.