Deoxyribonucleic acid, or DNA, is a long molecule essential for and instrumental in the passing on and preservation of life and genetics. It is usually in the form of a double helix formed by pairs of the four nucleic bases: cytosine [C], guanine [G], adenine [A], and thymine [T]. Cytosine can only pair with guanine and adenine can only pair with thymine.


The four bases of deoxyribonucleic acid can be split up into the purines and the pyrimidines. Pyrimidines are pyrimidine derivative, where pyrimidine is a benzene molecule with a carbon and another carbon meta relative to the first carbon replaced with nitrogen atoms. Purines are a bicyclic pyrimidine, where pyrimidine has been fused with imidazole (basically pyrimidine but with a pentagon instead of a hexagon).

The structure of all four molecules can be seen in the gallery.



The double helix of DNA is composed of short and long sections, not equally spaced sections as incorrectly depicted in many models. This is because the sugar-phosphate backbones of the molecule are not 'perfectly' out of phase with eachother.


One of the two resonance structures of pyrimidine.


Cytosine is a pyrimidine with an amino and carbonyl group at the 2,4 carbon positions respectively while thymine has two carbonyl groups at the 2,4 position and a methyl group at 5.


Guanine is adenine with an amino group swapped with carbonyl, a hydrogen swapped with an amino group, and an extra hydrogen. Note the closeness to xanthine, itself only being three methyl groups away from 1,3,7-trimethylxanthine, or caffeine.