The two antibiotic resistance genes on vector pBR322 are for:

The correct answer is Option (1) → ampicillin and tetracycline

A cloning vector needs a selectable marker, a gene that helps pick out the cells that have taken up the vector. Antibiotic resistance genes serve this purpose in E. coli. The vector pBR322 carries two such genes, amp^R and tet^R. Foreign DNA inserted at the BamH I site inactivates the tetracycline gene, while the ampicillin gene still works, and this helps to select the recombinants.

Option (1) ampicillin and tetracycline — Correct. The two resistance genes on pBR322 are for ampicillin (amp^R) and tetracycline (tet^R), as Figure 9.4 shows.

Option (2) ampicillin and chloramphenicol — Incorrect. Ampicillin is correct, but pBR322 has no chloramphenicol resistance gene. Chloramphenicol resistance is a marker used in some other vectors.

Option (3) chloramphenicol and tetracycline — Incorrect. Tetracycline is correct, but chloramphenicol resistance is not carried by pBR322; its other gene is for ampicillin.

Option (4) tetracycline and kanamycin — Incorrect. Tetracycline is correct, but kanamycin resistance is not on pBR322. Kanamycin is only listed in the book as one of the antibiotics whose resistance genes can act as markers.

NCERT Extract:
Cloning Vectors – “… Normally, the genes encoding resistance to antibiotics such as ampicillin, chloramphenicol, tetracycline or kanamycin, etc., are considered useful selectable markers for E. coli.”
Cloning Vectors – “… 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 BamH I site of tetracycline resistance gene in the vector pBR322. The recombinant plasmids will lose tetracycline resistance due to insertion of foreign DNA but can still be selected out from non-recombinant ones by plating the transformants on tetracycline containing medium. The transformants growing on ampicillin containing medium are then transferred on a medium containing tetracycline.”

Figure 9.4 E. coli cloning vector pBR322 showing restriction sites (Hind III, EcoR I, BamH I, Sal I, Pvu II, Pst I, Cla I), ori and antibiotic resistance genes (amp^R and tet^R). rop codes for the proteins involved in the replication of the plasmid