Chargaff's rules describe base-pairing proportions in DNA: the amount of adenine equals thymine and the amount of cytosine equals guanine.

• A. Adenine equals Thymine and Cytosine equals Guanine
• B. Adenine equals Cytosine and Thymine equals Guanine
• C. Adenine equals Guanine and Cytosine equals Thymine
• D. No relationship between base amounts

Answer: (A)

Chargaff's rules state that in double-stranded DNA, the amount of adenine equals the amount of thymine and the amount of cytosine equals the amount of guanine. This comes from the specific pairing: A pairs with T and C pairs with G, so every time you have an A on one strand, there's a T opposite it on the other, and similarly for C and G. Because of that complementary pairing, the total numbers of A and T must be equal, and the total numbers of C and G must be equal in the DNA sample. The GC content can vary between species, but within a given DNA molecule the A=T and C=G relationships hold. Options suggesting A pairs with C or with G, or claiming no relationship, don’t fit the actual pairing pattern.