Where are the sugar-phosphate backbones located in the DNA double helix?

• A. Inside the helix core
• B. On the outside of the helix
• C. Intercalated between base pairs
• D. In the grooves only

Answer: (B)

DNA's structure places the sugar-phosphate backbones on the outside of the double helix, while the nitrogenous bases pair and stack on the inside. Each strand has a sugar-phosphate backbone made of deoxyribose sugars linked by phosphate groups, forming the outer framework that is exposed to the aqueous environment. The bases sit inward, pairing with their complement and stacking in the core of the helix. This arrangement is favored because the backbone is highly polar and interacts with water, keeping the charged phosphate groups on the exterior, while the relatively hydrophobic bases stay tucked inside the helix. Intercalating between base pairs and having backbones confined to the grooves don’t describe the overall, stable organization of the DNA double helix.