Chargaff's rule states that which pairing relation holds in DNA?

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

Answer: (A)

The concept being tested is that DNA base pairing is complementary, which makes the amounts of the paired bases equal across the molecule. In a double-stranded DNA molecule, adenine always pairs with thymine and cytosine always pairs with guanine. Because each A on one strand is matched with a T on the other, the total number of adenine bases equals thymine bases. Similarly, cytosine equals guanine. This pairing sets up the consistent width of the DNA helix and explains why Chargaff observed equal amounts of A and T, and equal amounts of C and G in most organisms. This rule specifically applies to DNA; RNA uses uracil instead of thymine, so the pairing pattern differs there.