guanine

Organic compound of the purine family, often called a base, consisting of two rings, each containing both nitrogen and carbon atoms, and an amino group. It occurs in combined form in many important biological molecules, particularly nucleic acids, and free or combined in various natural sources, including guano, sugar beets, yeast, and fish scales. In DNA its complementary base is cytosine. It or its corresponding nucleoside or nucleotide may be prepared from nucleic acids by selective techniques of hydrolysis.