The second problem is seperating religated vector from one with a cloned insert. If a fragment is cloned in the tetR gene it will be inactive . The addition of a Multiple Cloning Site or polylinker has greatly facilitated the number of restrictgion enzymes that can be used for cloning. Virus and Bacteraphage can be used as vectors. Cloning using lambda or charons is a good way to make a gene library.
Cosmids combine the advantages of a phage and a plasmid vector.
BACs and YACs contain the largest inserts. There are two primary sources of DNA for cloning. DNA purified from almost any cell is identical. In addition it lacks introns and can be expressed in prokaryotes. Production of the library.