Which statements describe the stabilization of base pairs in DNA and the role of hydrogen bonding and stacking?

• A. Hydrophobic stacking interactions alone stabilize the helix; hydrogen bonds do not contribute.
• B. Hydrogen bonds between bases and hydrophobic stacking interactions stabilize the helix; hydrogen bonding involves functional groups on the Watson–Crick faces.
• C. Covalent bonds between bases stabilize the pairs.
• D. Ionic interactions with the phosphate backbone stabilize base pairs.

Answer: (B)

Base-pair stabilization in DNA comes from two complementary forces: hydrogen bonding between paired bases and base stacking along the helix. Hydrogen bonds form between specific functional groups on the Watson–Crick faces of the bases, giving the A–T pairing two bonds and the G–C pairing three bonds, which provides sequence-specific recognition and contributes to stability. But the major stabilizing factor is base stacking—the aromatic bases lie in a flat, stacked array, and hydrophobic interactions plus van der Waals forces between adjacent bases help hold the two strands together and exclude water from the interior of the helix. This combination, hydrogen bonding for correct pairing and stacking for overall stability, best explains the structure. Covalent bonds do not form between bases to stabilize the pairs, and ionic interactions with the phosphate backbone don’t stabilize the base pairs themselves.