Bases in nucleic acids fall into two categories. Cytosine, thymine and uracil are:

1. Nitrogenous bases are divided into two categories, purines and pyrimidines. 2. The division is by ring structure. 3. Cytosine, thymine and uracil each have a single carbon-nitrogen ring, which classifies them as pyrimidines. 4. Pentose sugars and phosphate groups are the other two nucleotide components, so neither is a base category. _Source: OpenStax Biology for AP(R) Courses (CC BY 4.0), Ch 3 'Biological Macromolecules', sections 3.1-3.5_