Final answer:
The consistent diameter of the DNA double helix is due to base pairing of pyrimidines with purines, ensuring a uniform width that gives DNA its characteristic 2 nm diameter.
Step-by-step explanation:
The consistent diameter of the DNA double helix arises because of base pairing of pyrimidines with purines. In the DNA structure, this pairing ensures that a purine (adenine or guanine), with a two-ring structure, always pairs with a pyrimidine (thymine or cytosine), which has a one-ring structure. Due to this, the width formed by the pair remains constant, thus giving the DNA double helix a uniform diameter of approximately 2 nm. The antiparallel nature of the DNA strands also contributes to this uniform diameter, but it is the purine-pyrimidine pairing that specifically accounts for it.