DNA is made of chemical building blocks called nucleotides. These nucleotides are linked like chains with alternating phosphate and sugar groups to form a strand of DNA. 

Each nucleotide is composed of a nitrogen base, a phosphate group, and a sugar group. There are four possible nitrogen bases present in DNA - Adenine (A), Thymine (T), Guanine (G) and Cytosine (C). The sequence, or order, of these nitrogen bases determines the biological instructions that are enclosed in a DNA strand.