The Complementary Structure of Deoxyribonucleic Acid

This paper describes a possible structure for the pamcrystalline form of the sodium salt of deoxyribonucleic acid. The structure consists of two DNA chains wound helically round a common axis, and held together by hydrogen bonds between specific pairs of bases. The assumptions made in deriving the structure are described, and co-ordinates are given for the principal atoms. The structure of the crystalline form is discussed briefly. I~TR~DUOTI~N The basic chemical formula of DNA is now fairly well established. It is a very long chain molecule formed by,the joining together of complex monomeric units called nucleotides. Four main types of nucleotides are found in DNA, and it is probable that their sequence along a given chain is irregular. The relative amounts of the four nucleotides vary from species to species. The linkage between successive nucleotides is regular and involves 3’-5’-phospho-di-ester bonds. Information about the three-dimensional shape is much less complete than that about its chemical formula. Physical-chemical studies, involving sedimentation, diffusion and light-scattering measurements, have suggested that the DNA chain