Deoxyribonucleic acid, or DNA, is composed of a 5-carbon sugar, a phosphate group and a nitrogenous base. The four nitrogenous bases found in DNA are adenine, guanine, thymine and cytosine.
The nitrogenous bases present in DNA can be made of a two-ring structure, purine, or a single-ring structure, pyramidine. Two complementary strands of DNA align in an antiparallel fashion to form base pairs. In the base pair system, a purine will always pair with a pyrimidine. Specifically, guanine pairs with cytosine and adenine pairs with thymine.
In a DNA double helix, the sugar and phosphate group are referred to as the backbone. The bases fit stacked on top of each other within the two strands. The nitrogenous bases can be abbreviated by their first letter. A sequence of DNA can be given using just the one letter abbreviations of the bases present.