Which base pairs with adenine in DNA?

• A. Adenine pairs with cytosine
• B. Adenine pairs with thymine
• C. Adenine pairs with guanine
• D. Adenine pairs with uracil

Answer: (B)

DNA base pairing follows a simple rule: purines pair with pyrimidines to keep the helix width uniform. Adenine, a purine, pairs with thymine, a pyrimidine, through two hydrogen bonds in DNA. This pairing is essential for faithful replication because the two strands are complementary. Pairing adenine with cytosine or with guanine would place two purines together or two pyrimidines together, which would distort the helix and disrupt normal DNA structure. Uracil is used in RNA instead of thymine, so adenine pairs with uracil only in RNA, not in DNA.