A mutation introduces a stop codon into the middle of a gene that encodes a polypeptide. Which of the following is not true?

Question

BlackTom AI · Accepted Answer

A nonsense (stop) codon terminates translation, not transcription. Insertion of a stop codon in the middle of a coding sequence creates a premature termination (nonsense) mutation that signals the ribosome to release a truncated polypeptide, but it does not directly truncate the primary RNA transcript.

1) Relevant concepts
- Transcription (DNA → pre-mRNA/mRNA) is carried out by RNA polymerase and ends at specific transcription termination signals; codons in the coding region do not normally cause RNA polymerase to stop.
- Translation (mRNA → polypeptide) is carried out by ribosomes that recognize stop codons (UAA, UAG, UGA) to terminate peptide synthesis.
- Nonsense-mediated decay (NMD) in eukaryotes reduces stability/abundance of mRNAs with premature stop codons, but it degrades full-length transcripts rather than producing shorter transcripts.

2) Step-by-step reasoning
- The mutation changes a codon in the coding sequence to a stop codon.
- During transcription, RNA polymerase continues to transcribe until encountering the normal transcription terminator; therefore the mRNA length (nucleotide count) remains the same as the unmutated gene’s transcript.
- During translation, the ribosome halts at the premature stop codon and releases a truncated polypeptide, so the polypeptide is shorter.
- The biochemical rate of transcription (initiation/elongation) is generally unaffected by the identity of an internal codon, so the rate stays the same. In eukaryotes, the mRNA may be targeted for degradation (NMD), lowering steady-state levels but not producing a shorter transcript; in bacteria, coupling of translation and transcription can sometimes influence termination, but standard interpretation is transcriptional output length is unchanged.

3) Intermediate “steps” (conceptual)
- DNA with nonsense mutation → full-length mRNA produced (contains the premature stop) → translation yields truncated protein; possible mRNA degradation reduces abundance, not length.

4) Why the correct answer is correct
“The mRNA would be shorter.” is not true because a stop codon affects translation termination, not the length of the primary RNA transcript. Any reduction in mRNA level (e.g., by NMD) does not equate to a shorter molecule.

5) Why other options are incorrect/true
- “The polypeptide would be shorter.” True: premature stop halts translation, producing a truncated polypeptide.
- “The rate of transcription would be unaffected.” True in the usual molecular-biology context: internal stop codons do not change RNA polymerase speed or termination signals.
- “All of the above are true.” False because the first statement is false.

Conclusion: The statement that is not true is “The mRNA would be shorter.” This matches the provided correct answer.

A mutation introduces a stop codon into the middle of a gene that encodes a polypeptide. Which of the following is not true?单项选择题

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