A Levels Biology (9700)•9700/13/O/N/18
Explanation
DNA triplet code determines polypeptide length
Steps:
- Double-stranded DNA with 120 total nucleotides equals 60 base pairs (60 nucleotides per strand).
- Only the coding strand's nucleotides code for the polypeptide.
- Each codon consists of 3 nucleotides specifying one amino acid.
- Maximum length: 60 nucleotides / 3 = 20 amino acids.
Why A is correct:
- The genetic code is read in triplets, so one amino acid requires 3 nucleotides per codon.
Why the others are wrong:
- B: Assumes 120 nucleotides on the coding strand, overlooking the double-stranded total.
- C: No calculation yields 60; exceeds triplet division.
- D: Equates nucleotides directly to amino acids, ignoring the 3:1 ratio.
Final answer: A
Topic: Protein synthesis
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