A gene that encodes a tRNA changes the anticodon from 3'-GAG-5' to 3'-CAG-5'. This mutation does not alter which amino acid that is attached to the mutant tRNA. What effect would this mutation have on translation?

Question

BlackTom AI · Accepted Answer

Anticodon 3'-GAG-5' pairs antiparallel with mRNA codon 5'-C U C-3' (G↔C, A↔U, G↔C). The codon CUC encodes leucine, so the original tRNA is a leucyl-tRNA recognizing leucine codons. When the anticodon is mutated to 3'-CAG-5', it now pairs with mRNA codon 5'-G U C-3' (C↔G, A↔U, G↔C). The codon GUC encodes valine. The problem states the amino acid attached to this tRNA is unchanged (it remains leucine). Thus the mutant tRNA will deliver leucine whenever the ribosome encounters a GUC valine codon, causing leucine to be inserted where valine should be.

Step-by-step reasoning and intermediate steps:
1. Write anticodon and determine complementary codon (anticodon is 3'→5' and pairs antiparallel to codon 5'→3'):
   - Original: anticodon 3'-G A G-5' → codon 5'-C U C-3' → CUC = leucine.
   - Mutant: anticodon 3'-C A G-5' → codon 5'-G U C-3' → GUC = valine.
2. The tRNA’s charged amino acid remains leucine (given), so for GUC codons the ribosome will accept this leucyl-tRNA and incorporate leucine instead of valine.
3. This is a missense (mistranslation) event: leucine substituted for valine at positions specified by GUC codons.

Why the other choices are incorrect:
- “No effect due to wobble rules.” Incorrect: the change alters the anticodon first (3') base that pairs with the codon first (5') base, not the wobble position. Wobble concerns the 5' anticodon / 3' codon position only; this mutation changes codon recognition, so it has an effect.
- “Introduce a leucine where a stop codon occurs.” Incorrect: the mutated anticodon recognizes GUC (valine), not any stop codon (UAA, UAG, UGA).
- “Introduce a valine where a leucine is supposed to be.” Incorrect: the tRNA still carries leucine, so it introduces leucine at valine codons, not the other way around.

Conclusion: The mutation could introduce a leucine where a valine is supposed to be. This matches the provided correct answer.

A gene that encodes a tRNA changes the anticodon from 3'-GAG-5' to 3'-CAG-5'. This mutation does not alter which amino acid that is attached to the mutant tRNA. What effect would this mutation have on translation?单项选择题

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