Genetic information is carried on a biological substance known as deoxyribonucleic acid (DNA), which is a double-stranded polymer of nucleotides. A DNA nucleotide is composed of deoxyribose sugar; a phosphate residue; and a purine or pyrimidine base.
Purines: adenine (A) and guanine (G)
Each purine contains 2 carbon-nitrogen rings
Pyrimidines: cytosine (C) and thymine (T)
Each pyrimidine contains 1 carbon-nitrogen ring