Side-by-side Models of DNA
The lack of a direct solution of the phase problem in fibre diffraction has provided an opportunity for opponents of the double-helix hypothesis to criticise the details of the Watson-Crick model and sometimes to propose alternative conformations. For example, the fibre diffraction evidence for Watson-Crick base-pairs has been a source of considerable debate (Donohue, 1969, 1970; Arnott, 1970; Crick, 1970; Wilkins et al., 1970). Recently attention has been focused on the sugar-phosphate backbone. A number of models of DNA have been described in which the two polynucleotide strands are in side-by-side association rather than entwined in a double-helical structure (Rodley et al., 1976; Sasisekharan and Pattabiraman, 1976, 1978; Bates et al., 1977, 1980a; Sasisekharan et al., 1978; Albiser and Premilat, 1980; Millane and Rodley, 1981; Premilat and Albiser, 1982). In all these side-by-side (SBS) models, the two polynucleotide strands are antiparallel and are linked through complementary base-pairing of the Watson-Crick type. In the SBS models which have been described in most detail, a region of five nucleotide-pairs in a right-handed double-helical conformation similar to that in the Watson-Crick B model is followed by five nucleotide-pairs in a left-handed conformation, which is in turn followed by another right-handed region, and so on throughout the length of the molecule.
KeywordsBragg Reflection Helix Axis Nucleic Acid Structure Fibre Diffraction Crystalline Array
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