If 35% of an organism's DNA is thymine, what is the percentage of guanine?

• A. 20%
• B. 30%
• C. 15%
• D. 35%

Answer: (C)

Verified step by step guidance

1. Recall Chargaff's rules, which state that in double-stranded DNA, 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) makes up 35% of the DNA, adenine (A) must also be 35% because \(\%A = \%T\).
3. Calculate the combined percentage of adenine and thymine: \(\%A + \%T = 35\% + 35\% = 70\%\).
4. Since the total percentage of all four bases must equal 100%, subtract the combined A and T percentage from 100% to find the combined percentage of guanine and cytosine: \$100\% - 70\% = 30\%$.
5. Because guanine and cytosine are present in equal amounts, divide the combined G and C percentage by 2 to find the percentage of guanine: \(\%G = \frac{30\%}{2}\).