Observe the diagram and answer the question.
A transformed E. coli colony with the above shown vector can grow in ampicillin containing media but not on tetracycline containing media. The probable site where foreign DNA got inserted in the vector would be:
(A) Hind III
(B) BamH I
(C) Pvu I
(D) Pvu II
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → (B) Only
The colony grows on ampicillin, so the ampicillin resistance gene is intact; it does not grow on tetracycline, so the tetracycline resistance gene (region D in the figure) has been broken by the foreign DNA. The site inside the tetracycline resistance gene among the options is BamH I.
Option (1) (D) Only — Incorrect. Pvu II lies outside both resistance genes.
Option (2) (B) Only — Correct. In pBR322 the BamH I site lies inside the tetracycline resistance gene. Foreign DNA put there inactivates that gene (insertional inactivation), so the cell still grows on ampicillin but not on tetracycline.
Option (3) (A) Only — Incorrect. Hind III lies at the edge of the gene; the clear site inside it in NCERT is BamH I.
Option (4) (C) Only — Incorrect. Pvu I lies in the ampicillin resistance gene, which is intact here.
NCERT Extract:
Cloning Vectors – “… 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.”
