If A equals 25%, what are the approximate percentages of T, G, and C?

• A. T 20%, G 40%, C 40%
• B. T 25%, G 50%, C 25%
• C. T 25%, G 25%, C 25%
• D. T 20%, G 30%, C 50%

Answer: (C)

In double-stranded DNA, base pairing requires A pairs with T and G pairs with C, so the amounts of each paired pair are equal. If A is 25%, then T must also be 25%. Together A and T account for 50%, leaving 50% for G and C. Since G pairs with C, their amounts are equal as well, so each is about 25%. Therefore T ≈ 25%, G ≈ 25%, and C ≈ 25%.