Final answer:
The consistent diameter of the DNA double helix is due to the base pairing of purines with pyrimidines, forming uniform base pair lengths across the helix.
Step-by-step explanation:
The consistent diameter of the DNA double helix arises because of the base pairing of pyrimidines with purines. In DNA structure, a purine, which has two rings, will always pair with a pyrimidine, which has one ring, resulting in uniform structure and length across all base pairs. This complementarity ensures that adenine (a purine) pairs with thymine (a pyrimidine), and cytosine (a pyrimidine) pairs with guanine (a purine), with adenine and thymine connected by two hydrogen bonds, whereas cytosine and guanine are connected by three hydrogen bonds.
The two strands of the DNA are anti-parallel in nature, where the 3' end of one strand faces the 5' end of the other strand, contributing to the helical structure but not specifically to the uniform diameter of the DNA helix. The uniformity in the DNA double helix's diameter is primarily due to the consistent pairing of purines with pyrimidines.