An alien DNA is ligated into the tetracycline resistance gene of the…

The correct answer is Option (2) → It loses tetracycline resistance but keeps ampicillin resistance.

The vector pBR322 carries two antibiotic resistance genes, amp^R (ampicillin) and tet^R (tetracycline). Alien DNA is ligated at a restriction site inside one of them, for example the BamH I site in tet^R. The inserted DNA breaks up the tetracycline resistance gene, so that gene no longer works. The ampicillin resistance gene is untouched. So the recombinant grows on ampicillin but not on tetracycline, and this is how recombinants are told apart from non-recombinants.

Option (1) It loses ampicillin resistance but keeps tetracycline resistance. — Incorrect. The insert lies in the tetracycline gene, not the ampicillin gene. So it is the tetracycline resistance that is lost, and ampicillin resistance stays.

Option (2) It loses tetracycline resistance but keeps ampicillin resistance. — Correct. The interrupted tet^R gene stops working, while amp^R still works. The recombinant grows on ampicillin but not on tetracycline.

Option (3) It loses both ampicillin and tetracycline resistance. — Incorrect. The ampicillin resistance gene is not touched by the insert, so this resistance is kept.

Option (4) It keeps both ampicillin and tetracycline resistance. — Incorrect. The tetracycline resistance gene is interrupted by the alien DNA, so that resistance cannot be kept.

Note: The book prints “tetracycline containing medium” in the first selection step; the next sentence makes clear the transformants are first grown on ampicillin, as explained above.

NCERT Extract:
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. The recombinants will grow in ampicillin containing medium but not on that 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