Deoxyribonucleic acid is a polymer of nucleotides that are composed of deoxyribose, phosphate and optional a purine base (adenine, guanine) or a pyrimidine base (cytosine, thymine). Forming diphosphate ester bonds, each nucleotide is connected to a chain. The DNA-double-strand developes according to the principle of base pairs, so that a purine bases always form hydrogen bonds with pyrimidine bases, thus adenine with thymine and guanin with cytosine. Due to apolar interactions of the bases, the antiparallel double strand twists.
