an organic compound belonging to the purine group, a class of compounds with a characteristic two-ringed structure, composed of carbon and nitrogen atoms, and occurring free or combined in such diverse natural sources as guano (the accumulated excrement and dead bodies of birds, bats, and seals), sugar beets, yeast, and fish scales. It is a component of nucleic acids (nucleic acid), the cell constituents that store and transmit hereditary traits.

Guanine was first discovered in guano in 1846 and was isolated in 1891 from nucleic acids from which it is readily prepared.

More complex compounds obtainable from nucleic acids include the nucleosides guanosine and deoxyguanosine, in which guanine is combined with the sugars ribose and deoxyribose and the nucleotides guanylic acid and deoxyguanylic acid, which are phosphoric acid esters of guanosine and deoxyguanosine. Nucleotides are molecules that constitute the fundamental structural units of nucleic acids; nucleosides are smaller subunits lacking in phosphoric acid.

Guanosine triphosphate (GTP) is used by the body to form the guanylic acid units in ribonucleic acids (RNA's).