Chromosomes are made of deoxyribonucleic acid (DNA).

DNA contains only four chemical bases or building blocks: Adenine, Thymine, Cytosine, and Guanine - called A, T, G, and C, for short. There are roughly 3.2 billion chemical bases (A, T, C, G) in the human genome.

Each DNA molecule is made up of two long complementary (related) strands, which are intertwined like a rope. This is the famous "double helix." Since A always pairs with T, and C with G, the order on one strand dictates the order on the other. This is called base pairing and enables the genome to make copies of itself.