文档介绍:plementary structure of deoxyribonucleic acid BY F. H. C. CRICK AND J. D. Wa!csoN*t Medical Research Council Unit for the Study of the Molecular Structure of Biological Systems, Cavendish Laboratory, University of Cambridge (Communicated by Sir Lawrence Bragg, .---Received 24 August 1953) [Plate 21 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 mon 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 plex 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 essive nucleotides is regular and involves 3’-5’-phospho-di-ester bonds. Information about the three-dimensional shape is much plete than that about its chemical formula. Physical-chemical studies, involving sedimentation, diffusion and light-scattering measurements, have suggested that the DNA chains exist in the form of thin rather rigid fibres approximately 208 in diameter and many thousand of angstroms in length (Jordan 1951; Sadron 1953). Very recently these indirect inferences have been directly confirmed by the electron micrographs of Williams (1952) and of Kahler & Lloyd (1953). Both sets of investigators have presented very good evidence for the presence in preparations of DNA of very long thin fibres with a diameter of 15 to 20 A, and so there now appears little doubt about the general asymmetrical shape of DNA. The only source of detailed i