A Levels Biology (9700)•9700/11/M/J/19

Explanation
DNA coding sequence length determines polypeptide size via triplet code
Steps:
- Double-stranded DNA with 6000 nucleotides means 3000 nucleotides per strand.
- The coding sequence is on one strand, so it contains 3000 nucleotides.
- Transcription yields mRNA with 3000 nucleotides.
- Translation uses 3 nucleotides per codon to specify 1 amino acid, so 3000 / 3 = 1000 amino acids.
Why B is correct:
- The triplet genetic code dictates that 3 nucleotides encode 1 amino acid, so 3000 coding nucleotides produce 1000 amino acids.
Why the others are wrong:
- A: Divides total DNA nucleotides (6000) by 12, ignoring the single coding strand and triplet code.
- C: Assumes the full 6000 nucleotides are the coding sequence, overlooking double-stranded structure.
- D: Assumes 1 nucleotide per amino acid, violating the triplet code.
Final answer: B
Topic: Protein synthesis
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