Which type of nitrogenous base contains one six-membered ring structure?

Pyrimidines are nitrogenous bases that contain a single six-membered ring structure. This structure is characterized by a ring composed of five carbon atoms and one nitrogen atom, which is typical of the pyrimidine family. The main pyrimidines include cytosine, thymine, and uracil, all of which play essential roles in nucleic acid structure and function.

In contrast, purines, which are also nitrogenous bases, have a more complex structure consisting of a two-ring configuration, featuring a six-membered ring fused to a five-membered ring. Amino acids, on the other hand, are building blocks of proteins and do not have a ring structure characteristic of nitrogenous bases. Nucleotides are the overall units that make up nucleic acids and consist of a nitrogenous base, a sugar, and a phosphate group, but they are not classified specifically by their ring structures. Therefore, pyrimidines are correctly identified as containing the specific six-membered ring structure that distinguishes them from these other categories.