The Double Helix and the Warped Zipper — An Exemplary Tale

Watson and Crick's celebrated double helix is a model problem-solution in the study of the conformation of biological macromolecules. It is, in Kuhn's terms, an `exemplar' for that `disciplinary matrix'. In 1976, however, two groups independently suggested essentially the same alternative, non-helical structure for DNA. No disciplinary `crisis' of confidence in the double-helical model preceded this proposal and, although Kuhn allows for such occurrences, he does not explain them. A Kuhnian account of the genesis and reception of the `side-by-side' or `warped zipper' model of DNA is provided. It invokes both explicit methodological norms at variance with the implicit presumptions of the scientific community and the freedom from conceptual constraint of amateurs and scientists outside, or on the fringe of, the disciplinary matrix. On this account the advent of the novel structure is seen as extrinsic to that matrix. But, as Francis Crick acknowledged, the competitive challenge `served a useful purpose'. It demonstrated the need for a `more careful scrutiny' of the empirical evidence for the double helix — with surprising results. This indicates that the effects of an exemplar are not necessarily advantageous.