Question

Aminopterin is added in HAT medium to:

• A. Block the de novo pathway of nucleotide synthesis
• B. Activate the de novo pathway of nucleotide synthesis
• C. Block the salvage pathway of nucleotide synthesis
• D. Activate the salvage pathway of nucleotide synthesis

Hint:

Remember: \[ \text{Aminopterin} \rightarrow \text{Blocks } \textit{de novo} \text{ nucleotide synthesis} \]
• HAT medium is used in hybridoma selection.
• Only cells with a functional salvage pathway survive in HAT medium. \[ \text{HAT} = \text{Hypoxanthine + Aminopterin + Thymidine} \]

Answer 1

The Correct Option is A

Concept: HAT medium stands for: \[ \text{HAT} = \text{Hypoxanthine} + \text{Aminopterin} + \text{Thymidine} \] It is commonly used in hybridoma technology for the selection of hybrid cells. Aminopterin is a folic acid analogue that inhibits the enzyme dihydrofolate reductase (DHFR), thereby blocking the de novo synthesis of nucleotides. As a result, cells are forced to use the salvage pathway for DNA synthesis.

Step 1: Understanding the role of aminopterin in HAT medium. Aminopterin inhibits: \[ \text{Dihydrofolate reductase (DHFR)} \] This prevents the formation of tetrahydrofolate required for: \[ \textit{de novo} \text{ nucleotide synthesis} \] Hence: \[ \boxed{\text{Aminopterin blocks the } \textit{de novo} \text{ pathway of nucleotide synthesis}} \]

Step 2: Importance of the salvage pathway in HAT medium. Since the de novo pathway is blocked, cells survive only if they can utilize: \[ \text{Salvage pathway} \] Hypoxanthine and thymidine present in HAT medium help cells synthesize nucleotides through the salvage pathway.

Step 3: Eliminating the incorrect options.
• Option (B): Aminopterin inhibits rather than activates the de novo pathway.
• Option (C): Aminopterin does not block the salvage pathway.
• Option (D): Aminopterin itself does not activate the salvage pathway; it indirectly forces cells to depend on it. Therefore, option (A) is correct.