Purines differ from pyrimidines structurally because purines have:

1. The two base categories differ in the size and shape of their ring systems. 2. The purines have a double ring structure with a six-membered ring fused to a five-membered ring. 3. Pyrimidines are smaller in size and have a single six-membered ring structure. 4. Pairing a large purine with a small pyrimidine is what keeps the helix a constant width. 5. Three fused rings appear in no DNA base at all. _Source: OpenStax Biology for AP(R) Courses (CC BY 4.0), Ch 14 'DNA Structure and Function', sections 14.1-14.6_