Deoxyribonucleic acid (DNA) is a long molecule that contains genetic information for the development and functioning of an organism. It is a polymer composed of two polynucleotide chains that coil around each other to form a double helix. DNA is made up of four basic building blocks or "bases": adenine (A), cytosine (C), guanine (G), and thymine (T) . The order, or sequence, of these bases determines the information available for building and maintaining an organism, similar to the way in which letters of the alphabet appear in a certain order to form words and sentences. The two strands of DNA are connected by chemical bonds between the bases: adenine bonds with thymine, and cytosine bonds with guanine. Each strand has a backbone made of alternating sugar (deoxyribose) and phosphate groups. DNA is located in the cell nucleus, where it is called nuclear DNA, but a small amount of DNA can also be found in the mitochondria, where it is called mitochondrial DNA or mtDNA. DNAs instructions are used to make proteins in a two-step process. First, enzymes read the information in a DNA molecule and transcribe it into an intermediary molecule called messenger ribonucleic acid, or mRNA.