GATE 2023 BT Q.23: solution and answer

Official answer: ASource: Official answer key, GATE 2023 BT (IIT Kanpur)

1. Hybridoma technology fuses antibody-secreting B cells, which die in culture after a short time, with immortal myeloma cells. The fused cells are grown in HAT medium (hypoxanthine, aminopterin, thymidine).

2. Aminopterin blocks the de novo pathway for purine and thymidylate synthesis, so a cell in HAT medium can survive only if the salvage pathway works.

3. The salvage pathway needs hypoxanthine-guanine phosphoribosyltransferase (HGPRT), which converts hypoxanthine and guanine into IMP and GMP. It also needs thymidine kinase for the thymidine route.

4. The myeloma cells chosen for fusion lack HGPRT. They are selected for resistance to 8-azaguanine, a toxic base analogue that only HGPRT-positive cells convert into a lethal nucleotide. Unfused myeloma cells therefore die in HAT medium.

5. A hybrid survives because the B-cell partner supplies HGPRT and the myeloma partner supplies immortality. HGPRT is therefore the enzyme absent from the myeloma cells, so option (A) is correct.

6. Option (B), alanine aminotransferase, is a general amino acid metabolism enzyme that every cell needs. Myeloma cells have it, so (B) is wrong.

7. Option (C), triose phosphate isomerase, is a glycolytic enzyme. Cells need it to make ATP from glucose, so myeloma cells have it and (C) is wrong.

8. Option (D), glycosyltransferase, is needed to add sugars to proteins. Myeloma cells carry out glycosylation, so they have it and (D) is wrong.