Diameter of DNA double helix is uniform through out the structure and it is 2 nm. This uniformity is due to the base pairing between a purine and a pyramidine. Purine base is made of two rings and pyramidine base has a single ring. All the bases are either derived from purine or from pyramidine structure and a purine always pairs with pyramidine. Two purines or two pyramidines can not pair with each other. So at any point along the DNA molecule, the sugar phosphate backbones always have three ring structure between them, hence the uniform diameter.
