Which of the following is TRUE about translation ?

Question

BlackTom AI · Accepted Answer

There are three stop codons: UAA, UAG, and UGA.

1) Relevant knowledge points and concepts
- Genetic code: mRNA is read in triplets called codons; each codon specifies an amino acid or a stop signal.
- Codon count: with four nucleotides and triplet codons, there are 4^3 = 64 possible codons.
- Coding vs. termination codons: 61 of those codons specify amino acids; the remaining codons function as termination (stop) signals.
- Translation termination: when a ribosome encounters a stop codon, no tRNA carries a corresponding amino acid; release factors bind and catalyze polypeptide release.

2) Step-by-step reasoning
- Calculate total codons: 4 nucleotides ^ 3 positions = 64 codons.
- Determine coding codons: standard genetic code uses 61 codons to encode the 20 standard amino acids (because of degeneracy).
- Remainder are stop codons: 64 total – 61 coding = 3 stop codons.
- Identify them: the three stop codons in the standard (nuclear) genetic code are UAA, UAG, and UGA.
- Mechanistic consequence: encountering any of these three codons ends elongation; release factors (not tRNAs) recognize them and free the nascent polypeptide.

3) Intermediate “calculation” steps shown above: 4^3 = 64; 64 − 61 = 3.

4) Why the correct answer is correct
The standard genetic code explicitly designates UAA, UAG, and UGA as termination signals. They do not correspond to aminoacyl-tRNAs in cytoplasmic translation; instead they recruit release factors to terminate protein synthesis. This is a universal feature of cytoplasmic translation in most organisms.

5) Why other likely options would be incorrect
- “There is one stop codon” or “There are four stop codons”: incorrect because the standard code has exactly three stop codons (exceptions exist in some mitochondrial codes, but the question refers to translation generally).
- “Translation occurs in the nucleus” (in eukaryotes): incorrect — transcription is nuclear; translation occurs in the cytosol or on rough ER.
- “There are 64 amino acids”: incorrect — there are 20 standard amino acids encoded by the 61 sense codons.

6) Conclusion
The statement “There are three stop codons” is correct and matches the provided answer.

Which of the following is TRUE about translation ?单项选择题

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