Question

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

Hint:

Remember what each part of HAT medium does: aminopterin blocks de novo synthesis, hypoxanthine rescues purine synthesis via HGPRT, thymidine rescues pyrimidine synthesis via thymidine kinase.

Answer 1

The Correct Option is B

Step 1: Understanding the Question:
Monoclonal antibody production uses hybridoma technology, where antibody producing B cells are fused with immortal myeloma cells, and the fused hybridomas are selected using HAT medium (Hypoxanthine, Aminopterin, Thymidine). The question asks what role hypoxanthine specifically plays here.

Step 2: Key Formula or Approach:
Aminopterin in HAT medium blocks dihydrofolate reductase, which shuts down the de novo (from scratch) synthesis of both purines and thymidylate (dTMP). Cells trapped in this medium can only survive if they can make nucleotides through the salvage pathway instead, using hypoxanthine and thymidine supplied in the medium.

Step 3: Detailed Explanation:
The salvage pathway for purines needs the enzyme HGPRT (hypoxanthine-guanine phosphoribosyltransferase), which directly converts free hypoxanthine into inosine monophosphate (IMP), a purine nucleotide precursor.
Myeloma cells used for fusion are deliberately chosen to be HGPRT deficient, so on their own they cannot use hypoxanthine and die in HAT medium once aminopterin blocks the de novo route.
Normal B cells do carry a working copy of HGPRT, but they cannot grow indefinitely outside the body and die out naturally in culture.
Only the fused hybridoma cells inherit both the myeloma cell's immortality and the B cell's functional HGPRT gene, so they alone can use hypoxanthine through the salvage pathway to keep making purines, and survive selection in HAT medium.
So hypoxanthine's role is to allow purine synthesis, but only in cells that still contain a working HGPRT enzyme, exactly what option (B) states.
Option (A) confuses hypoxanthine with thymidine, since thymidine (not hypoxanthine) is the one that bypasses the need for de novo dTMP synthesis, via thymidine kinase.
Option (C) is wrong because cell fusion is triggered by polyethylene glycol (PEG) or Sendai virus, not by hypoxanthine.
Option (D) describes aminopterin's action, not hypoxanthine's; aminopterin is the component that blocks de novo synthesis, while hypoxanthine is the rescue substrate for the salvage pathway.

Step 4: Final Answer:
Hypoxanthine allows purine synthesis through the salvage pathway, but only in cells that still contain a working HGPRT enzyme, which is why only true hybridomas survive HAT selection. \[ \boxed{\text{(B) allows purine synthesis in cells containing HGPRT}} \]