Nucleobases are nitrogen-containing organic molecules that serve as major constituents of nucleosides and nucleotides. They are classified into purines, which are heterocyclic, and pyrimidines, which consist of a six-carbon ring.
Five nucleobases are found in naturally-occurring nucleic acids: adenine, guanine, cytosine, thymine and uracil. Of these, the first four are found in DNA, and the fifth, uracil, is found in RNA in lieu of thymine.