Question

a non-sense mutation forms a premature stop codon at position 45 in a gene encoding a 180-amino acid protein. a second mutation (a suppressor mutation) in a trna molecule allows read-through of this stop codon. what is the most likely outcome for the final protein product?

○ a shorter protein of exactly 179 amino acids

○ a full-length protein with a different amino acid at position 45 in the polypeptide chain

○ a full-length protein with a different amino acid at position 15 in the polypeptide chain

○ two separate protein fragments of 44 and 135 amino acids

○ a protein with a frame-shift mutation after position 45

Explanation:

1. First, understand nonsense and suppressor mutations: A nonsense mutation introduces a premature stop codon (normally ending translation). A suppressor mutation in tRNA allows the stop codon to be read as a sense codon (encoding an amino acid) instead of stopping.
2. Analyze each option:

• Option 1: A suppressor mutation allows read - through, so the protein won't be shorter by just 1 amino acid. The stop codon is read as an amino acid, not skipped. So this is incorrect.
• Option 2: The premature stop codon is at position 45. The suppressor tRNA will insert an amino acid at position 45 (instead of stopping translation). Since translation can now continue to the normal stop codon (for the 180 - amino acid protein), the protein will be full - length, but with a different amino acid at position 45 (because the tRNA that suppresses the stop codon will carry a specific amino acid to insert at that position).
• Option 3: The mutation is at position 45, not 15. There's no reason for a change at position 15. So this is incorrect.
• Option 4: The suppressor mutation allows read - through, so translation won't stop at position 45, and there won't be two separate fragments. So this is incorrect.
• Option 5: A frameshift mutation is caused by insertions or deletions of non - multiple - of - three nucleotides. This is a nonsense and suppressor mutation situation, not a frameshift. So this is incorrect.

Answer:

B. A full - length protein with a different amino acid at position 45 in the polypeptide chain