What type of nitrogenous base has a double-ring structure and includes adenine and guanine?

The correct choice is purine, which refers to a class of nitrogenous bases that have a double-ring structure. This structure is significant because it affects how these bases pair with one another in the formation of nucleic acids (DNA and RNA) and their roles in cellular processes. Purines include adenine and guanine, which are essential components of DNA and RNA. Their double-ring configuration makes them larger compared to pyrimidines, the other category of nitrogenous bases, which have a single-ring structure.

Understanding the structure of purines is crucial because it helps explain interactions within nucleotides and the overall structure of nucleic acids. Each nucleotide consists of a nitrogenous base (which can be either purine or pyrimidine), a five-carbon sugar, and a phosphate group. While amino acids and nucleotides are related to proteins and nucleic acids respectively, they are not nitrogenous bases themselves.