The genome is made up of a molecule called deoxyribonucleic acid, or DNA, which is constructed of just four different building blocks. Known as nucleotides, these are the familiar letters of DNA: A, G, C, and T, shorthand for the chemical groups (also known as bases) of adenine, guanine, cytosine, and thymine that distinguish the four compounds. The letters of these molecules are connected in long single strands. Two of these strands come together to form the famous double-helix structure of DNA.
