Antibiotic resistance gene in a vector helps in the selection of :

The correct answer is Option (2) → Transformed cells

Note: NTA accepted both Option (1) Recombinant cells and Option (2) Transformed cells: the antibiotic resistance gene selects the transformed cells, and a second resistance gene, inactivated by the inserted DNA, helps in the selection of recombinants.

Option (1) Recombinant cells — Also accepted by NTA. In a vector such as pBR322, foreign DNA ligated inside the tetracycline resistance gene inactivates it, so recombinants can be told from non-recombinants.

Option (2) Transformed cells — Correct. Only cells that have taken up the vector carry the resistance gene and grow on the antibiotic.

Option (3) Competent cells — Incorrect. Cells are made competent (with divalent cations such as calcium) before the DNA is put in; the marker works after that step.

Option (4) Totipotent cells — Incorrect. Totipotency is the capacity of a plant cell to grow into a whole plant, unrelated to vectors.

NCERT Extract: “(ii) Selectable marker : In addition to ‘ori’, the vector requires a selectable marker, which helps in identifying and eliminating non-transformants and selectively permitting the growth of the transformants.”
Insertion of Recombinant DNA into the Host Cell/organism – “… Since, due to ampicillin resistance gene, one is able to select a transformed cell in the presence of ampicillin.”