noun, plural: pyrimidines
Nucleic acids such as DNA and RNA molecules contain the genetic information important for all cellular functions and heredity. Each nucleic acid is comprised of a series of nucleotides. The nucleotide, in turn, is made up of phosphoric acid, sugar (5-carbon), and nitrogenous base. The nitrogenous base is a fundamental component of nucleotides and nucleosides and occurs in two major forms: purines and pyrimidines.
Both purines and pyrimidines are heterocyclic aromatic compounds. Both of them have a pyrimidine ring. However, the purines have additional ring (i.e. imidazole ring) fused to the pyrimidine ring. The pyrimidines have only the pyrimidine ring, and therefore have only one carbon ring (as opposed to purines that have two carbon rings). Two nitrogen atoms are located at positions 1 and 3 of the ring of a pyrimidine.
In DNAs, purines complementary base pair with pyrimidines. In particular, guanine (a purine) pairs with cytosine (a pyrimidine) while adenine (another purine) pairs with thymine (another pyrimidine). In RNA, adenine complementary pairs with uracil instead of thymine since thymine is present only in DNA and not in RNA. The term purine bases refer to guanine and adenine whereas the pyrimidine bases pertain to cytosine, thymine, and uracil. This precept is one of the major distinctions used to distinguish DNA from RNA.
Apart from nucleic acids, pyrimidines are also important components of certain proteins and starches. Thus, their functions are not just to serve as structural constituents of DNA and RNA but they are also involved in the regulation of enzymes and cell signaling.