Q.The fact that a purine base always pairs through hydrogen bonds with a pyrimidine base in the DNA double helix leads to:

Complementary base pairing is the rule that adenine (a purine) pairs with thymine (a pyrimidine), and guanine (a purine) pairs with cytosine (a pyrimidine). Because a purine is a larger two-ring structure and a pyrimidine is a smaller single-ring structure, pairing a purine with a pyrimidine on opposite strands creates a constant combined width across the helix.

If two purines paired, the width would bulge outward; if two pyrimidines paired, it would narrow. The purine–pyrimidine pairing ensures that the distance between the two sugar-phosphate backbones remains the same at every rung of the ladder, giving the DNA molecule its characteristic uniform diameter of about 2 nm throughout its length. …

The architecture of DNA is one of the most elegant solutions in molecular biology. When Watson and Crick proposed the double helix model in 1953, they recognized that the structure had to account for Chargaff's rules—the observation that the amount of adenine always equals thymine, and guanine always equals cytosine. The answer lay in how these bases physically fit together.

Purines are the larger nitrogenous bases, with a two-ring structure. Adenine and guanine belong to this category. Pyrimidines, on the other hand, are smaller, single-ring bases—cytosine and thymine (and uracil in RNA). If two purines tried to pair across the helix, they would be too bulky and push the sugar-phosphate backbones apart. If two pyrimidines paired, they would be too small, leaving a gap and causing the backbones to collapse inward.

Nature's solution is beautifully simple: a purine always pairs with a pyrimidine. Adenine pairs with thymine through two hydrogen bonds, and guanine pairs with cytosine through three. This complementary pairing means that every "rung" of the DNA ladder consists of one large base and one small base, which together span exactly the same distance.

Reason from approximate physical size instead of the qualitative 'large base + small base' description: a purine's double-ring structure spans roughly 1.08 nm and a pyrimidine's single ring spans roughly 0.88 nm map roughly to a combined ~2 nm span for any purine-pyrimidine pair, whether it's A-T or G-C. Because that combined span is nearly constant regardle …