The cosmid vectors consist of modified plasmids carrying cos sites. A DNA sequence required for packaging the DNA or the genome into a phage head during the assembly is known as a cos site. A cosmid vector consists of an origin of replication belonging to E coli (ColE1). It also consists of the ampicillin resistance gene (AmpR). The cosmid is also known as a plasmid carrying the λ (lambda) cos site. The substrates for in vitro packaging of phage DNA involve concatemers of cosmids, linked at their cos sites. The cos sites act as recognizing elements for a λ genome. Hence, the proteins getting packaged into a λ phage particle solely depend on the cos sites.

A specialty of a cosmid is that it does not direct the synthesis of the new phage particle after entering the cell. Hence, a cosmid replicates as a plasmid. Instead of forming plaques (like phage infection), we obtain colonies. They help in identifying the recombinants. The size of the cosmid is 8kb or less. The new DNA insert size i…