The sequence of amino acids in a polypeptide chain of protein molecules corresponds to the sequence of nucleotides on mRNA for that protein. If the reading frame of mRNA for a human protein is 993 nucleotides including a stop codon at the end, how many amino acids would be incorporated in the polypeptide chain?
Correct answer: D. 330
- A. 331
- B. 993
- C. 93
- D. 330
Explanation
To determine the number of amino acids in the polypeptide chain encoded by the mRNA sequence, divide the total number of nucleotides by 3 because each codon consists of 3 nucleotides. The mRNA sequence contains 993 nucleotides, which equate to 331 codons. However, among these, one is a stop codon that does not code for an amino acid, leaving 330 codons that do. Hence, 330 amino acids would be incorporated into the polypeptide chain. The incorrect options arise from either misunderstanding the role of the stop codon or miscalculating the division by 3.
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