Chargaff's rules imply what about base composition in double-stranded DNA?

• A. All bases occur at equal frequencies
• B. %A ≈ %T and %C ≈ %G
• C. %A > %T > %G > %C
• D. Purines pair with pyrimidines only in RNA

Answer: (B)

Chargaff's rules show that in double-stranded DNA, base pairing imposes a balance between the bases on the two strands. Adenine pairs with thymine and cytosine pairs with guanine, so the amount of adenine is about equal to thymine, and the amount of cytosine is about equal to guanine. This pairing keeps A and T in roughly equal percentages and C and G in roughly equal percentages within a given double-stranded DNA sample. The exact values can vary between organisms, but the pairwise equalities hold because of the complementary structure. The idea that all bases are found at the same frequency or that purines only pair with pyrimidines in RNA doesn’t fit this established pairing pattern in DNA.