DNA, deoxyribonucleic acid is composed of a sugar (deoxyribose), a phosphate and a base either adenine, guanine, cytosine or thymine (A,G,C or T). A single unit of a sugar, phosphate and base is called a nucleotide. The nucleotides are joined in a chain, the sugar of one nucleotide and the phosphate of the next nucleotide are joined by covalent bonds. The base hangs out from the sugar-phosphate backbone. The bases of one DNA chain bind to another matching DNA chain and for a twisting double chain structure called the DNA double helix. The DNA double helix is wound around proteins to form chromosomes. In humans DNA is found in the nucleus of cells and some in the mitochondria.