Chargaff's rule states which base-pairing is correct?

• A. Adenine with cytosine; thymine with guanine
• B. Adenine with guanine; cytosine with thymine
• C. Adenine with thymine; cytosine with guanine
• D. Thymine with cytosine; adenine with guanine

Answer: (C)

Base-pairing rules in DNA state that adenine pairs with thymine and cytosine pairs with guanine. This pattern, described by Chargaff, means the amounts of A and T are equal and the amounts of C and G are equal in double-stranded DNA. Adenine forms two hydrogen bonds with thymine, while cytosine pairs with guanine through three hydrogen bonds, providing the right balance of stability for the double helix and the ability to separate strands during replication. Pairings like adenine with cytosine or adenine with guanine wouldn’t form the proper hydrogen-bonding pattern or fit the DNA geometry, so they aren’t used in standard DNA. So, adenine with thymine and cytosine with guanine is the correct pairing.