Deoxyribonucleic acid is the chemical inside the nucleus of a cell that carries the genetic instructions for making living organisms. DNA is composed of two anti-parallel strands, each a linear polymer of nucleotides. Each nucleotide has a phosphate group linked by a phosphoester bond to a pentose (a five-carbon sugar molecule, deoxyribose), that in turn is linked to one of four organic bases, adenine, guanine, cytosine, or thymine, abbreviated A, G, C, and T, respectively. The bases are of two types: purines, which have two rings and are slightly larger (A and G); and pyrimidines, which have only one ring (C and T). Each nucleotide is joined to the next nucleotide in the chain by a covalent phosphodiester bond between the 5′ carbon of one deoxyribose group and the 3′ carbon of the next. DNA is a helical molecule with the sugar–phosphate backbone on the outside and the nucleotides extending toward the central axis. There is specific base-pairing between the bases on opposite strands in such a way that A always pairs with T and G always pairs with C.