ABSTRACT
Using restriction enzymes, ligase, and simple polymerase chain reaction (PCR) is sufficient for many problems but becomes unwieldy or even impossible under certain situations. If the insert to be cloned is too large to purify using standard gel techniques, or if it is unstable or recombines, some other approaches can make it tractable. A bacteriophage or phage is a virus that infects bacteria. Phage are grown by simply incubating them with the appropriate strain of E. coli. The mixture can then be mixed with agar and plated onto a petri dish. The way phage may be used as cloning vectors is to take advantage of the way they replicate their genomes, and of the fact that most of the genome of a phage can be removed without affecting its ability to make viral particles. A hybrid of phage and plasmid technology is the cosmid, which is a plasmid containing phage cos sites so that it can concatemerize to form libraries.
