This is accomplished by using a vector that has a resistance marker such as ampR or tetR . 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.
Cosmids combine the advantages of a phage and a plasmid vector.
BACs and YACs contain the largest inserts.