History of molecular biology Crick and Watson built physical models using metal rods and balls, in which they incorporated the known chemical structures of the nucleotides, as well as the known position of the linkages joining one nucleotide to the next along the polymer. At King's College Maurice Wilkins and Rosalind Franklin examined X-ray diffraction patterns of DNA fibers. Of the three groups, only the London group was able to produce good quality diffraction patterns and thus produce sufficient quantitative data about the structure. In 1948 Pauling discovered that many proteins included helical (see alpha helix) shapes. Pauling had deduced this structure from X-ray
Non-helical models of DNA structure have about 400,000 helical twists in the Watson–Crick model, each of which would have to be removed in as little as 20 minutes, if the strands are to separate during cell replication. This leads to the apparent need of the chromosome - or at least some part of it - to be spinning at speeds up to 6900–9000 rpm throughout the life of the bacterial cell. In 1976, Gordon Rodley and his co-workers published the first non-helical DNA structure, which they named the "side-by-side" (SBS) structure. A similar model was published a few months later by Sasisekharan and his associates.