What is Chargaff's rule?

• A. A pairs with T and G pairs with C in DNA.
• B. A pairs with G and C pairs with T.
• C. A = T and G = C in DNA.
• D. All bases are present in equal amounts.

Answer: (C)

Chargaff's rule states that in double-stranded DNA, the amount of adenine equals thymine and the amount of guanine equals cytosine. This balance arises because A pairs with T and G pairs with C across the two strands, so their numbers must match to maintain the base-paired structure. Therefore, expressing the rule as A = T and G = C in DNA captures the essential proportional relationship across the molecule. It’s not just about which bases pair; it’s about the actual quantities in the DNA sample. In contrast, statements claiming all bases are present in equal amounts or misdescribing the pairing don’t reflect this observed balance in base composition.