What feature distinguishes purines from pyrimidines?

Purines are characterized by their two-ring structure, which consists of a fused pyrimidine and imidazole ring. This distinct structural feature differentiates purines from pyrimidines, which only have a single-ring structure. The two major purines found in nucleic acids are adenine and guanine. Their larger size and more complex structure allow them to pair with pyrimidines (cytosine, thymine, and uracil) in the formation of the double helix in DNA and RNA, thus playing a vital role in the molecular architecture of genetic material.

The other options do not accurately describe purines: they do not contain a single ring structure, nor are they smaller than pyrimidines, and the concept of including or excluding specific bases like thymine refers to the differences between various nucleotides rather than a defining structural characteristic of purines.