Synthesis of polypetide terminates when a nonsense codon of mRNA reaches

Options

A. A site
B. P site
C. B site
D. None

Answer

### Step-by-Step Solution:

1. **Understanding the Question**: The question asks about the termination of polypeptide synthesis during translation when a nonsense codon (stop codon) of mRNA is reached.

2. **Identifying Stop Codons**: There are three specific stop codons in mRNA that signal the termination of translation: UAA, UGA, and UAG. These codons do not code for any amino acids and signal the end of the polypeptide chain.

3. **Translation Process Overview**: Translation occurs in the ribosome, which has three sites:
- **A-site (Acceptor Site)**: Where new amino acids are added to the growing polypeptide chain.
- **P-site (Peptidyl Site)**: Where the peptide bond formation occurs between the amino acids.
- **E-site (Exit Site)**: Where the completed polypeptide exits the ribosome.

4. **Role of the Stop Codon**: When a stop codon (e.g., UAA) reaches the A-site during translation:
- The stop codon is recognized by release factors instead of tRNA.
- The presence of the stop codon leads to the termination of the polypeptide synthesis process.

5. **Termination of Polypeptide Chain**: Upon recognition of the stop codon at the A-site:
- The polypeptide chain is cleaved from the tRNA in the P-site.
- The completed polypeptide is then released from the ribosome through the E-site.

6. **Conclusion**: Therefore, the synthesis of the polypeptide terminates when a nonsense codon reaches the A-site of the ribosome.

### Final Answer:
The synthesis of polypeptide terminates when a nonsense codon of mRNA reaches the **A-site** of the ribosome.

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