Knowing Chargaff's rule, if 20% of the nucleotides are Adenine, what percentage of each of the other 3 bases would be found in this molecule?

• A. Thymine 30%, Guanine 30%, Cytosine 40%
• B. Thymine 25%, Guanine 25%, Cytosine 50%
• C. Thymine 20%, Guanine 50%, Cytosine 30%
• D. Thymine 20%, Guanine 30%, Cytosine 30%

Answer: (D)

Chargaff's rule states that in double-stranded DNA, adenine pairs with thymine and guanine pairs with cytosine, so A equals T and G equals C, with all percentages adding up to 100%. If adenine is 20%, thymine must also be 20%. The remaining 60% is split equally between guanine and cytosine, giving 30% each. So the base composition is A 20%, T 20%, G 30%, C 30%. Distributions that violate A = T or G = C, or that don’t sum to 100%, would not fit this rule.