The selectable markers are ampR (resistance to ampicillin) and tetR (resistance to tetracycline). Selectable markers help to select the host cells which contain the vector (transformants) and eliminating non-transformants.
If a foreign DNA ligates at the BamHI site of tetracycline resistance gene in the vector pBR322, the recombinant plasmid loses the tetracycline resistance due to insertion of DNA. It can still be selected out from non-recombinant.
