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
DNA supercoil points for replication or gene expression. This implies that it is a very dynamic process in which both DNA and proteins each influences how the other acts and reacts. In nature, circular DNA is always isolated as a higher-order helix-upon-a-helix, known as a "superhelix." In discussions of this subject, the Watson-Crick twist is referred to as a "secondary" winding, and the superhelices as a "tertiary" winding. The sketch at right indicates a "relaxed", or "open circular" Watson-Crick double-helix, and, next to it, a right-handed superhelix. The "relaxed" structure on the left is not found unless the chromosome is nicked; the