Coding strand pyrimidines determine C + U in mRNA Steps:

• Non-transcribed strand is coding strand with 40% purines (A + G).
• Pyrimidines in coding strand (C + T) = 100% - 40% = 60%.
• Template strand (other strand) is complementary, but primary transcript matches coding strand sequence.
• Transcript replaces T with U, so C + U = 60%.

Why D is correct:

• Transcription produces mRNA complementary to template strand, matching coding strand composition (U for T), so % (C + U) = pyrimidines in coding strand = 60%.

Why the others are wrong:

• A. 20% assumes 80% purines in coding strand.
• B. 30% assumes 70% purines in coding strand.
• C. 40% assumes 60% purines in coding strand.

Final answer: D