What is the structure of DNA?

• A. Double helix with a sugar-phosphate backbone and nitrogenous bases on the inside.
• B. Single-stranded molecule with bases on the outside.
• C. A triple helix with a protein core.
• D. Double helix with no backbone.

Answer: (A)

DNA's structure is a double helix formed by two long polymer strands wound around each other. Each strand has a sugar-phosphate backbone on the outside, with deoxyrose sugars linked by phosphate groups. The nitrogenous bases extend toward the interior and pair specifically (adenine with thymine, guanine with cytosine) via hydrogen bonds, so the two strands are complementary and run in opposite directions (antiparallel). This arrangement places the bases inside the helix and the backbone outside, exactly as described by a double helix with a sugar-phosphate backbone and bases on the inside. The other ideas—single-stranded with bases on the outside, a triple helix with a protein core, or a double helix with no backbone—do not fit the actual architecture.