What does Chargaff's rule state about base composition in DNA?

• A. In any species, the amount of adenine equals thymine and the amount of guanine equals cytosine; A=T and G=C.
• B. A pairs with C and G pairs with T.
• C. All bases exist in equal amounts.
• D. Only purines pair with purines.

Answer: (A)

Chargaff's rule describes base composition in DNA. In any double-stranded DNA, the amount of adenine equals thymine and the amount of guanine equals cytosine. This arises because A pairs with T and G pairs with C across the two strands, so their numbers balance. As a result, A and T are equal, and G and C are equal, and the total purines (A and G) balance the total pyrimidines (T and C).

This is why the correct statement is that A equals T and G equals C. The idea that A would pair with C or that all bases are present in equal amounts goes against the pairing rules and observed composition. Likewise, purines pairing only with purines would disrupt the strand structure, which actually relies on purine–pyrimidine pairing. Chargaff's rule helps explain why the two DNA strands are complementary and how replication can proceed with accurate base-pairing.