Which statement describes Chargaff's rule?

• A. A = T and G = C
• B. A = C and G = T
• C. A = T and G ≠ C
• D. A ≠ T and G ≠ C

Answer: (A)

Chargaff's rule states that in double-stranded DNA the amount of adenine equals thymine and the amount of guanine equals cytosine. This follows from the specific base pairing: A pairs with T and G pairs with C, so their numbers balance across the two strands. Therefore, the description that A equals T and G equals C is the correct way to express this rule. The other statements would imply incorrect pairings or unequal amounts, which don’t match how DNA strands pair.