On the structure of DNA

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Abstract
Evidence for the structure of double-stranded DNA and related duplexes is considered in relation to the existing double helix model and a possible alternative to it. It is shown by mathematical analysis that an alternative side-by-side (SBS) model is capable of producing the gross features of the X-ray diffraction pattern of moist DNA fibres. This is complemented by model-independent calculations based on X-ray data for paracrystalline B-DNA that suggest the duplex molecule may not have tne circular (axial projection) symmetry expected for a regular double helix structure. In addition, the application of the SBS model to other areas of DNA structure and function are considered. Long-standing problems associated with the highly intertwined feature of the helix model, esrecially the problem of unwinding, do not arise for the SBS model. In particular, electron micrograph evidence for the formation of “bubble” regions during the melting of DNA raises serious questions about earlier attempts to rationalise kinetic data for strand separation.