A DNA molecule contains 32% thymine. According to Chargaff's rules, what percentage of cytosine would be present in this DNA molecule?

• A. 32%
• B. 68%
• C. 36%
• D. 18%

Answer: (D)

Verified step by step guidance

1. Recall Chargaff's rules, which state that in a double-stranded DNA molecule, the amount of adenine (A) equals thymine (T), and the amount of guanine (G) equals cytosine (C). This means \(\%A = \%T\) and \(\%G = \%C\).
2. Given that thymine (T) is 32%, use Chargaff's rule to determine that adenine (A) is also 32%, because \(\%A = \%T = 32\%\).
3. Calculate the combined percentage of adenine and thymine: \(\%A + \%T = 32\% + 32\% = 64\%\).
4. Since the total percentage of all four bases must add up to 100%, subtract the combined percentage of A and T from 100% to find the combined percentage of guanine and cytosine: \$100\% - 64\% = 36\%$.
5. Finally, because guanine and cytosine are present in equal amounts, divide the combined percentage by 2 to find the percentage of cytosine: \(\%C = \frac{36\%}{2}\).