COSMIDS/ what is cosmid

Based on the properties of DNA and col El plasmid DNA, a group of Japanese workers showed that the presence of a small segment of phage ï¬ DNA containing cohesive end on the plasmid molecule is sufficient prerequisite for in vitro packaging of this DNA into infectious particles. The cosmid can be defined as a hybrid vector derived from plasmids which contain cos site of phage l (cosmid = cos site +plasmid). For the first time it was developed by H.A Collins and M.Hohn in 1978.

Figure:- DNA

Cosmid lack gene encoding viral protein; therefore, neither viral particle are formed with the host cell nor cell lysis occurs. Special feature of cosmid similar to plasmid are the presence of: (a)origin of replication, (b) a marker gene coding for antibiotic resistance, (c) a special cleavage site for the insertion of foreign DNA, and (d) the small size.

Extra phage DNA of cos site about 12 bases is present. It help the entire genome in circularization and ligation. The cosmid has the length of about 5 kb, the upper size limit of the foreign DNA fragment that may be inserted in cosmid and packed into phage particles, therefore, approximately 45 kb, much larger than it might be possible to clone in phage lor phage vector (Dahl et al., 1981). According to the size of cos site and upper size limit in the head of phage, the recombinant DNA molecule can be packed into bacteriophage particles in in vitro packaging system consists of packaging enzyme, 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 cyclises through cos site and then replicate as a plasmid and expresses the drug resistance marker. Recently, based on cosmid vector have been determined from E. coli, yeast, and mammalian cells, and gene bank has been constructed.