In response to Donohue's argument that Fourier analysis of X-ray diffraction data collected by Maurice Wilkins and his colleagues did not rule out models of DNA with other base pairs than those posited by Watson and Crick, Crick here reviewed key concepts of crystallography and helical diffraction theory on which the debate hinged. He also cited the biochemical evidence for his and Watson's model of DNA then available. Crick came to the conclusion that Donohue's "hypothetical models of base-pairs are theoretically not to be trusted without further justification." As he had in his response to Donohue in the pages of Science, Crick in his letter once again challenged him to build models of his proposed structures, which Crick insisted was the only way by which the validity of Donohue's claims could be decided.
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