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The theory of deconvolving the microdiffraction data-set available in a scanning transmission electron microscope or, equivalently, the set of all bright- and dark-field images available in a conventional transmission electron microscope to obtain super- resolution micrographs (which are not limited by the transfer function of the objective lens) is developed and described with reference to holography and other phase-retrieval schemes. By the use of a Wigner distribution, influences of the instrument function can be entirely separated from the information pertaining to the specimen. The final solution yields an unambiguous estimate of the complex value of the specimen function at a resolution which in theory is only limited by the electron wavelength. The faithfulness of the image processing is shown to be not seriously affected by specimen thickness or partial coherence in the illuminating beam. The inversion procedure is remarkably noise insensitive, implying that it should result in a robust and practicable experimental technique, though one that will require very large computing facilities.

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A model is presented for double-stranded polynucleotides which involves side-by-side meshing of the two strands rather than double helical intertwining. The sugarphosphate backbone has a twisted strip-like character, yet base-pairing of the Watson-Crick type is still possible. Structural features of the basic model are described and coordinates are presented for a representative example. The structure has, on the whole, reasonable stereochemical contacts, and can be shown to produce a fiber diffraction pattern with x-rays not unlike that of the B form of DNA.The Watson-Crick model (1) has been outstandingly successful in providing a framework for understanding a wide range of observations of duplex structures. Initial concern about its intertwined nature (2) has now diminished. Nonetheless, as this feature remains something of a puzzle, we recently attempted to construct a base-paired model where the strands do not intertwine. We have found that such a model, having a sideby-side association of single strands, can be built without gross stereochemical difficulty. While this structure may not be generally applicable to duplexes it does have sufficiently attractive features for us to consider it appropriate to present the model at this time for appraisal as an additional or alternative conformation to the double helix. We outline broad structural details of the initial model together with a preliminary analysis of it in relation to various physical properties of duplexes. One key feature of the side-by-side (SBS) model is that it can be mathematically shown to be capable of producing the characteristic x-ray diffraction pattern of the B form of DNA. Structural basis of the model Using the Watson-Crick mode of base pairing and antiparallel strands we have constructed a model that does not involve gross intertwining of the phosphate backbones. This was achieved by constructing approximately half of the repeat unit in a basically Watson-Crick manner for a right-handed double helix with acceptable structural parameters (3), and then continuing the model in the form of a left-handed double helix. The model retains the essential features of base stacking and repeat distance of the Watson-Crick model that are required by x-ray data. However, by alternating the helical sense every half-repeat distance a net winding of the phosphate strands is minimized and the structure becomes a side-by-side intermeshing of strands rather than an intertwining of them. Various views of the side-by-side (SBS) structure are shown in Figs. 1-4. One view (Figs. 2b and 3a) resembles that of the Watson-Crick model. Other views (Figs. 1-4) however reveal marked differences. The SBS structure has a twisted strip-like character that contrasts with the rod-like appearance of a double helix. The structure approximates to that of a strip of corrugated sheet cut on the diagonal with the cut edges representing the phosphate strands. In constructing the model, two kinds of bending of the backbone (p and q) were employed. The p bend involves a ch...

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