For monoclonal antibody production, hypoxanthine ____________.
- (A)
allows DNA synthesis in the absence of de novo dTMP synthesis
- (B)
allows purine synthesis in cells containing HGPRT
- (C)
induces cell fusion
- (D)
prevents de novo nucleotide synthesis
GATE 2026 BT Q.18: solution and answer
Answer and solution
ANSWER
In HAT medium, hypoxanthine lets cells that contain HGPRT make purines by the salvage route, so (B) is correct.
Official answer: BSource: Official answer key, GATE 2026 BT (IIT Guwahati)
HAT medium selects hybridomas. It contains hypoxanthine, aminopterin and thymidine. Aminopterin blocks dihydrofolate reductase, so the de novo routes to purines and to dTMP are both shut.
Cells can then survive only through the salvage pathways. Hypoxanthine is the purine salvage substrate: HGPRT converts it to , which gives AMP and GMP without the de novo route.
(B) is correct. Myeloma cells are HGPRT-deficient, so they cannot use hypoxanthine and die in HAT medium. Spleen B cells have HGPRT but do not grow for long in culture. Only fused hybridomas grow, because they carry HGPRT from the B cell and immortality from the myeloma cell.
(A) is wrong. Salvage dTMP synthesis starts from thymidine through thymidine kinase. Hypoxanthine feeds purine synthesis, not dTMP.
(C) is wrong. Cell fusion is induced by polyethylene glycol or Sendai virus, not by hypoxanthine.
(D) is wrong. Aminopterin, not hypoxanthine, blocks de novo nucleotide synthesis. Hypoxanthine supplies purines through the salvage route.
KEY IDEA
In HAT selection, aminopterin shuts the de novo pathways. Hypoxanthine supplies purines through HGPRT, and thymidine supplies dTMP through thymidine kinase. Only HGPRT-positive hybridomas survive.
WATCH OUT FOR
Giving hypoxanthine the job of thymidine (option A, dTMP synthesis), or the job of aminopterin (option D, blocking de novo synthesis).
From the GATE 2026 BT paper. Every question of this paper · Download the full paper · All 3 Hybridomas & transgenics questions, with notes