In a DNA molecule, the total amount of purine nucleotides is $40\%$ of the total bases. What is the percentage of cytosine in this DNA molecule?

Solution

1. According to Chargaff's rule, the total amount of purines equals the total amount of pyrimidines, so purines = $50\%$ and pyrimidines = $50\%$.
2. The problem states purines are $40\%$, which implies an error in the premise or a non-standard DNA; however, assuming standard base pairing where $$\begin{array}{rl}\text{A}+\text{G}& =\text{T}+\text{C}\\ & =50\%\end{array}$$.
3. If the total pyrimidines are $60\%$ (as $100\%-40\%=60\%$), then $\text{C}+\text{T}=60\%$.
4. Assuming $\text{C}=\text{T}$, then $2\text{C}=60\%$, so $\text{C}=30\%$.
5. Hence the answer is (C).