Purine-pyrimidine base pairing maintains DNA helix width

Steps:

• DNA consists of two antiparallel sugar-phosphate strands forming a double helix.
• Nitrogenous bases project inward and pair via hydrogen bonds.
• Purines (adenine, guanine) are larger with two rings; pyrimidines (thymine, cytosine) have one ring.
• Pairing one purine to one pyrimidine creates uniform base pair dimensions.

Why C is correct:

• Purine-pyrimidine linkages ensure each base pair spans the same width (about 2 nm), keeping sugar-phosphate backbones a constant distance apart, as defined by Watson-Crick base pairing rules.

Why the others are wrong:

• A: A-T pairs have 2 hydrogen bonds, while C-G have 3, affecting stability but not distance.
• B: Incomplete phrasing; lacks specificity on pairing type that uniformizes width.
• D: Strands are joined by hydrogen bonds between bases, not sulfur bridges (which occur in proteins).

Final answer: C