Codons that are closer to the 3’ end of a mature mRNA specify amino acids that are

Question

BlackTom AI · Accepted Answer

The ribosome reads an mRNA in the 5′→3′ direction and builds the polypeptide from its amino (N) terminus toward its carboxyl (C) terminus.

1) Relevant knowledge points and concepts
- Mature mRNA is processed (capping, polyadenylation, splicing) and presents a contiguous coding sequence read by ribosomes.
- Translation initiation positions the start codon (usually AUG) near the ribosome so the first incorporated amino acid becomes the N-terminus of the protein.
- Elongation proceeds codon by codon in the 5′→3′ direction of the mRNA.
- Peptide bond formation extends the growing chain at its carboxyl end, so each new amino acid is added to the C-terminus.

2) Step-by-step reasoning
- The first codon translated (near the 5′ end) specifies the amino acid at the protein’s N-terminus.
- As the ribosome moves toward the 3′ end, it adds successive amino acids to the growing polypeptide.
- Therefore codons encountered later (closer to the 3′ end) specify amino acids that become the downstream (C-terminal) residues of the protein.

3) Intermediate steps (mapping positions)
- Codon at 5′ region → amino acid 1 (N-terminus)
- Next codon → amino acid 2
- ...
- Codon near 3′ region → amino acid near or at the C-terminus

4) Why the correct answer is correct
Because translation progresses 5′→3′ and peptide synthesis extends at the C-terminus, codons nearer the 3′ end encode residues that are located nearer the carboxyl-terminal end of the polypeptide.

5) Why the other options are incorrect
- “closer to the amino terminal end” is the opposite of the correct orientation; those codons are near the 5′ end, not the 3′.
- “spliced out and not found in the polypeptide” misunderstands mature mRNA: splicing removes introns before translation, and the remaining codons in the mature mRNA are translated, not spliced out.
- “duplicated in the polypeptide” is not a general property of codon position; codons do not cause duplication of amino acid segments.

6) Conclusion
Codons closer to the 3′ end of a mature mRNA specify amino acids closer to the carboxyl terminal end of the polypeptide, confirming the provided answer.

Codons that are closer to the 3’ end of a mature mRNA specify amino acids that are单项选择题

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