Guanine is one of the four common nucleobases of the nucleic acids deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). In double stranded DNA, guanine forms base pairs with cytosine on the opposite strand, while RNA strands can also fold into secondary structure such as hairpin loops stabilized by the same guanine/cytosine base pairing. Binding occurs through three hydrogen bonds. Guanine is synthesized in cells by guanosine phosphorylase from guanosine and phosphate.
Guanine plays a number of biological roles including camouflage, display, and vision. In addition some amphibians, spiders, and scorpions convert ammonia to guanine, the latter of which can be excreted with minimal loss of water. Interestingly, it has been reported based on NASA studies of meteorites containing guanine and other nucleobases that guanine can be formed extraterrestrially.