Explain the 'cloning sites' in cloning vectors.

(N/A) $(iii)$ Cloning Sites: To link the alien $DNA$,the vector needs to have very few,preferably single,recognition sites for the commonly used restriction enzymes. Presence of more than one recognition site within the vector will generate several fragments,which will complicate the gene cloning process.
The ligation of alien $DNA$ is carried out at a restriction site present in one of the two antibiotic resistance genes. For example,you can ligate a foreign $DNA$ at the $BamHI$ site of the tetracycline resistance gene in the vector $pBR322$.
The recombinant plasmids will lose tetracycline resistance due to the insertion of foreign $DNA$,but can still be selected out from non-recombinant ones by plating the transformants on an ampicillin-containing medium.
The transformants growing on the ampicillin-containing medium are then transferred to a medium containing tetracycline. The recombinants will grow in the ampicillin-containing medium but not on that containing tetracycline. But non-recombinants will grow on the medium containing both the antibiotics.
In this case,one antibiotic resistance gene helps in selecting the transformants,whereas the other antibiotic resistance gene gets 'inactivated' due to the insertion of alien $DNA$,and helps in the selection of recombinants.