Guanine is a purine base found in DNA and RNA. In nucleosides and nucleotides, guanine is linked to the sugar moiety via a covalent bond between nitrogen #9 of guanine and carbon #1 of the sugar.

The ribonucleoside containing guanine is called guanosine. The deoxyribonucleoside containing guanine is called deoxyguanosine.