COSMID

The way in which DNA is replicated is of particular interest in the development of larger insert cloning vectors termed cosmid. Cosmids are plasmids that contain minimum of 250 bp of DNA consisting of the cos site (sequnence yielding cohesive ends.) these are especially useful for the analysis of highly complex genome mapping projects.
Cosmid vectors have been constructed that incorporate the cos .sites from bacteriophage lemda and also the essential feature of a plasmid , such as the plasmid origin of replication , a gen for drug resistance and several unique restriction sites for insertion of the DNA to be cloned when a cosmid , the produducts will include cocatamers of alternating cosmid vector and inserts. The only requirement for a length of DNA to be packed into viral heads, therefore, is that it should contain cos site spaced the correct distance apart in practice tis spacing can range between 37 to 52 kb. Such DNA can be packed in vitro if is very small, inserts of about 40kb in lenght will be most readily packaged. Once inside the cell, the DNA recircularrises through its , and from, then own wards behave exactly like a plasmid.