Why do cosmids need cos sites?
The cos sites (cos = ‘cohesive’) are 12-base overhangs at the 5′-ends of the dsDNA form of the single phage genome. The ends are complementary to each other such that once phage DNA is injected into an E. coli host, the ends mediate circularization of the genome. Once covalently closed by DNA ligase, DNA polymerase catalyses the synthesis of a concatenated polymer of phage DNA. An endonuclease recognizes the cos sites, catalyzes their cutting to release a single copy of phage DNA. Cutting is coupled to packaging, so… to package DNA as a lambda phage, you need to have cos sites. To further clarify what a cosmid is, it is basically a plasmid that happens to have cos sites. The cos sites allow it to be packaged in vitro as a phage virion which can then infect a bacterium (thus you get high efficiency transformation). Once inside a bacterium, however, the cosmid lacks all of the phage structural genes, so it is basically trapped as a plasmid and cannot code for the coat proteins, etc. th
