DNA, or deoxyribonucleic acid, is a molecule that contains the biological instructions that make each species unique. It is the hereditary material in humans and almost all other organisms. DNA is made up of chemical building blocks called nucleotides, which are made of three parts: a phosphate group, a sugar group, and one of four types of nitrogen bases (adenine, guanine, cytosine, and thymine). To form a strand of DNA, nucleotides are linked into chains, with the phosphate and sugar groups alternating. The nitrogen bases pair up with each other, A with T and C with G, to form units called base pairs. Each base is also attached to a sugar molecule and a phosphate molecule, and together, a base, sugar, and phosphate are called a nucleotide. Nucleotides are arranged in two long strands that form a spiral called a double helix. The structure of the double helix is somewhat like a ladder, with the base pairs forming the ladder’s rungs and the sugar and phosphate molecules forming the vertical sidepieces of the ladder. An important property of DNA is that it can replicate, or make copies of itself.