In 1953, Watson and Crick presented their model for the DNA double helix, revealing the way genetic information is held in a cell. In the years since, DNA has proven to be a lively molecule that is looped, unwound, copied, repaired, edited, chemically modified, and more recently, engineered. A closer look at DNA's atomic structures (and its related cellular partners) has since revealed much more about the complex workings of this amazing molecule. Early work on DNA structure used DNA that had beenisolated from cells. Fibres of this natural DNA diffract x-rays into a characteristic pattern - the classic helical structure proposed by Watson and Crick, known as a B-helix. A truly atomic glimpse at DNA, however, had to wait for several decades. Techniques for the chemical synthesis of small pieces of DNA, with exact nucleotide sequences, made this possible. These well-defined pieces of DNA can be coaxed to form single crystals, which provide much more detailed x-ray diffraction patterns (as shown inFig. 2 overleaf), which can be analysed to determine the location of each atom in the molecule.
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