Which type of nitrogenous base is classified as a purine?

Adenine is classified as a purine because it has a double-ring structure, which is characteristic of purines. Purines are one of the two classes of nitrogenous bases found in nucleotides, the building blocks of nucleic acids like DNA and RNA. The other class is pyrimidines, which include bases such as cytosine, thymine, and uracil, all of which have a single-ring structure. This structural difference is fundamental in the formation of DNA and RNA, as purines pair with pyrimidines during base pairing, contributing to the overall stability and functionality of nucleic acid structures. Thus, adenine's classification as a purine is crucial for understanding its role in genetic encoding and protein synthesis.