(a) : Selectable markers are those genes which help in selecting those host cells which contain vectors (i.e., transformants) and eliminating the non-transformants. The genes encoding resistance to antibiotics such as tetracycline, ampicillin, kanamycin etc., are useful selectable markers for E.coli. Plasmid pBR322 has two resistance genes ampicillin resistance (ampr) and tetracyclin resistance (tetr) which are considered useful for selectable markers.
The presence of restriction sites within the markers tet ‌r and ampr permits an easy selection for cells transformed with the recombinant pBR322. Insertion of the DNA fragment into the plasmid using enzyme Pst I or Pvu I places the DNA insert non-functional. Bacterial cells containing such a recombinant pBR 322 will be unable to grow in the presence of ampicillin, but will grow on tetracycline. Similarly, when restriction enzyme Bam HI or Sal I is used, the DNA insert is placed within the gene tet making it non-functional. Bacterial cells possessing such a recombinant pBR322 will, therefore, grow on ampicillin but not on tetracyclin.