What is the DNA base-pairing rule?

• A. A pairs with G and C with T
• B. A pairs with T and C with G
• C. A pairs with U and G with C
• D. A pairs with C and T with G

Answer: (B)

DNA base pairing is the rule that adenine always pairs with thymine and cytosine always pairs with guanine. This pairing keeps the two DNA strands complementary and preserves a uniform width of the double helix, which is essential for accurate replication because each strand can serve as a precise template for the other. The A–T pair forms two hydrogen bonds, while the G–C pair forms three, giving extra stability to regions with many G–C pairs. In RNA, thymine is replaced by uracil, so adenine pairs with uracil there, not in DNA. So the statement that reflects DNA is that adenine pairs with thymine and cytosine with guanine. The other options mix wrong pairings (like two purines together) or refer to RNA instead of DNA.