Final answer:
The uniform diameter of the DNA double helix is maintained by the pairing of purine bases with pyrimidine bases, which ensures consistent base pair size and shape, stabilized by hydrogen bonds and hydrophobic interactions.
Step-by-step explanation:
The consistent diameter of the DNA double helix is due to the pairing between purine bases and pyrimidine bases. Purines, which include Adenine (A) and Guanine (G), pair with the smaller pyrimidines, Thymine (T) and Cytosine (C), respectively. This results in base pairs that have a consistent shape and size, allowing the double helix to have a uniform diameter of 2 nm.
The hydrogen bonds between these complementary bases, along with the structural formation of the sugar-phosphate backbone, give DNA its strong and stable double helix shape, with the hydrophobic interactions of the bases contributing to the stabilization of the helix.