DNA usually exists in a supermolecular form consisting of two hydrogen-bonded anti-parallel strands wound in the shape of a double helix.
Each strand is a directional polymer of mononucleotides. A DNA-mononucleotide consists of a phosphorylated 5-carbon sugar (deoxyribose) linked to heterocyclic nitrogenous bases by an N-glycosidic linkage. There are usually four types of bases in DNA:
- Adenine (A)
- Guanine (G)
- Thymine (T)
- Cytosine (C)
They make up four different kinds of mononucleotides. Each mononucleotide is linked to each other through a phosphodiester bond [a mononucleotide is already a phosphoric ester (ester of sugar alcohol group and phosphoric acid). A second ester bond is formed between the 3-OH of a mononuclotide and the 5-phosphate of another nucleotide].
In the DNA supermolecular structure, A in a given strand is hydrogen bonded to T in the other strand. Similarly, G is bound to C. This pattern of hydrogen bonding is called Watson-Crick base pairing, named for the discoverers of the double helix structure.