Pyrimidines are a class of nitrogenous bases that are one of the two primary categories of nucleobases found in nucleic acids, the other being purines. They are characterized by a single six-membered ring structure containing nitrogen atoms. The three main pyrimidines in DNA and RNA are cytosine (C), thymine (T), and uracil (U). Pyrimidines play essential roles in the genetic coding and are critical for the synthesis of nucleotides.