Nucleic acids such as DNA and RNA are polymers of monomeric nucleotides. Each nucleotide is made up of phosphoric acid, sugar (5-carbon), and nitrogenous base (or nucleobase). There are five nitrogenous bases that serve as fundamental units of the genetic code: (1) adenine, (2) guanine, (3) cytosine, (4) thymine, and (5) uracil. These nitrogenous bases may be classified into purines and pyrimidines.
Adenine and guanine are purine nitrogenous bases. Guanine can be distinguished from adenine by its amine group at position 2 and a carbonyl group at position 6 of its heterocyclic aromatic (pyrimidine) ring. It has a chemical formula of C5H5N5O.
Guanosine and deoxyguanosine are the nucleosides of guanine. The guanosine (i.e. guanine bound to a ribose sugar) that is phosphorylated with three phosphoric acid groups becomes guanosine triphosphate (GTP), which serves as one of the nucleotide monomeric units that build up RNA molecule. It may also act as a secondary messenger in signal transduction. The deoxyguanosine (i.e. guanine bound to a deoxyribose sugar) that is phosphorylated with three phosphoric acid groups becomes deoxyguanosine triphosphate (dGTP), one of the nucleotides that build up DNA molecules.
The name guanine is derived from the Spanish gano, meaning bird or bat droppings, since it was first obtained from guano. Guanine crystals are used in cosmetics as they furnish iridescent effect or shimmering, pearly luster.