. Biological structure and function; proceedings. Biochemistry; Cytology. Molecular Configuration of Nucleic Acids M. H. F. WiLKINS ]\IRC Biophysics Research Unit, Physics Department, King's College, London, England Need for certainty in the structure determination of DNA Molecular theory of replication of genetic material and of mutation is based on the structure of DNA. Since the ideas of Watson and Crick concerning DNA are so aesthetically attractive and are now being extended in many ways to create almost a whole subject of nucleic acid biology ( the structures of RNA's with various fu
. Biological structure and function; proceedings. Biochemistry; Cytology. Molecular Configuration of Nucleic Acids M. H. F. WiLKINS ]\IRC Biophysics Research Unit, Physics Department, King's College, London, England Need for certainty in the structure determination of DNA Molecular theory of replication of genetic material and of mutation is based on the structure of DNA. Since the ideas of Watson and Crick concerning DNA are so aesthetically attractive and are now being extended in many ways to create almost a whole subject of nucleic acid biology ( the structures of RNA's with various functions in protein synthesis are being derived by analogy with DNA), it is important that these ideas do not become a dogma and that alternatives are not ignored. It is also desirable that a stage be reached where the structure of DNA can no longer be regarded as hypothetical. It is essential therefore that the structure be placed on a sound basis of experimental fact. It is generallv agreed that DNA consists of two polynucleotide chains linked together bv hvdrogen bonds between adenine and thymine and between guanine and cytosine. It is, however, still a somewhat open question whether the hydrogen bonding scheme in the base pairs is that proposed by Watson and Crick or has some other form. Valuable evidence supporting the Watson Crick scheme is supplied by the studies of enzymic synthesis of DNA ( Josse and Romberg [i]) and evidence in favour is also given by studies of complexes of synthetic polyribonucleotides [2]. I wish to discuss here, however. X-ray diffraction data on DNA itself and the extent to which these data provide an exact structure for DNA and give a unique solution. Difficulties in the X-ray structure analysis of DNA There are two main difficulties in studying DNA by means of X-ray difl'raction. First, DNA, like other chain polymer molecules, does not form single crystals. The advantage of single crystals is that they enable diffraction to be separated in a
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