What is DNA made of?

DNA is composed of molecular building components known as nucleotides. These building blocks are composed of three components: a phosphate group, a sugar group, and one of four different forms of nitrogen bases. Nucleotides are linked together in chains, with the phosphate and sugar groups alternating. Adenine (A), thymine (T), guanine (G), and cytosine are the four forms of nitrogen bases present in nucleotides (C).

What biological instructions are present in a strand of DNA are determined by the order, or sequencing, of these bases. For instance, the sequence ATCGTT may educate for blue eyes, but ATCGCT may instruct for brown eyes. A human's whole DNA instruction book, or genome, has around 3 billion bases and approximately 20,000 genes spread over 23 pairs of chromosomes.