The total amount of purines equals the total amount of pyrimidines in DNA due to base-pairing rules. Which statement is true?

• A. False
• B. True
• C. Not necessarily
• D. Depends on organism

Answer: (B)

In double-stranded DNA, base pairing enforces a balance between the two classes of bases. Each rung of the DNA ladder contains one purine and one pyrimidine: adenine (a purine) pairs with thymine (a pyrimidine), and guanine (a purine) pairs with cytosine (a pyrimidine). Because every base pair has exactly one purine and one pyrimidine, the total number of purines across the molecule equals the total number of pyrimidines. This means A+G = T+C, and equivalently A = T and G = C in a double-stranded DNA molecule. For RNA or single-stranded DNA, this balance does not necessarily hold, but in the common double-stranded DNA context the statement is true.