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