The diameter of the DNA double helix is:

1. A purine always pairs with a pyrimidine, so every rung is the same width. 2. That is why the helix does not bulge or pinch along its length. 3. The diameter of the DNA double helix is 2 nanometres, and it is uniform throughout. 4. Ten nanometres is the width of a nucleosome bead, not of bare DNA. _Source: OpenStax Biology for AP(R) Courses (CC BY 4.0), Ch 14 'DNA Structure and Function', sections 14.1-14.6_