Final answer:
Nitrogen bases that serve as building blocks of nucleic acids are classified into purines (adenine and guanine) with double-ring structures and pyrimidines (cytosine, thymine in DNA, and uracil in RNA) with single-ring structures.
Step-by-step explanation:
The nitrogen bases that serve as building blocks for nucleic acids include two categories: purines and pyrimidines. Purines have a double-ring structure and include adenine (A) and guanine (G). Pyrimidines have a single-ring structure and in the context of DNA, include cytosine (C) and thymine (T); whereas in RNA, thymine is replaced by uracil (U). Each nucleotide in a DNA or RNA molecule includes one of these nitrogenous bases, along with a 5-carbon sugar (deoxyribose in DNA and ribose in RNA) and a phosphate group.