A student models DNA and notes that uniform helix diameter depends on pairing rules. Which statement best explains how base structure maintains a consistent double-helix width?
- Purines pair with purines, producing a constant width because both have two rings
- Pyrimidines pair with pyrimidines, producing a constant width because both have one ring
- Purines pair with pyrimidines, keeping the distance between backbones relatively constant (correct answer)
- Any base can pair with any other base, and the backbone adjusts to preserve width
- The 3′ hydroxyl groups hydrogen-bond to each other, fixing helix diameter independent of bases
Explanation: This question tests understanding of nucleic acid structure-function relationships regarding DNA helix geometry. The correct answer C explains that purine-pyrimidine pairing maintains uniform helix width because a two-ring purine (A or G) always pairs with a single-ring pyrimidine (T or C), creating consistent base pair dimensions. This complementary pairing ensures that each base pair spans approximately the same distance between the sugar-phosphate backbones, maintaining the regular helical structure. If purines paired with purines or pyrimidines with pyrimidines, the helix would have variable width—too wide or too narrow at different positions. Choice A incorrectly suggests purine-purine pairing, which would create overly wide regions and violate Chargaff's rules. The strategy is to visualize how ring structures (2-ring purines + 1-ring pyrimidines = constant width) maintain helical regularity.