In DNA base pairing, how many hydrogen bonds stabilize A-T pairs and C-G pairs?

• A. A-T One Hydrogen Bond; C-G Two
• B. A-T Three Hydrogen Bonds; C-G Two
• C. A-T Two Hydrogen Bonds; C-G Three
• D. A-T Zero Hydrogen Bonds; C-G One

Answer: (C)

Hydrogen bonding between bases determines how strong each base pair is. Adenine pairs with thymine using two hydrogen bonds, forming a precise and stable connection. Cytosine pairs with guanine using three hydrogen bonds, which adds extra stability to that pairing. Because of the extra bond, GC-rich regions are more stable and melt at higher temperatures than AT-rich regions. So the pairing has two bonds for A–T and three bonds for C–G, explaining the difference in stability.