The DNA X-ray diffraction technique

What is x-ray diffraction?

DNA, unlike proteins, is an exceedingly large molecule which does not lend itself to crystallisation. It is usually extracted in the form of the sodium salt (to neutralise the negatively charged phosphates) to yield a highly viscous suspension of molecular fibers. These fibers are disorderly compared with single crystals and produce smears rather than spots. It was in Maurice Wilkins lab that techniques were developed to form thin fibers of DNA. These almost invisible, spiderweb-like filaments in which the DNA molecules were deduced to be neatly aligned alongside one another. This technique formed the basis for studying the X-ray patterns formed by DNA fibers kept at high humidity. Such fibers produce diffraction patterns that hint at the clarity obtainable from a perfect single crystal. This patterns is visualised of many pictures of a crystal taken at different angles, but with sorted out the overlapping aspects. This was a very hard proces. Rosalind Franklin joined Wilkins lab and further developed the technique and procedures to produce diffraction pictures of outstanding quality. One of those pictures is called Photo 51. The data she obtained enabled Watson and Crick to piece together the puzzle of DNA structure.

Confimation of te model of Watson and Crick

Although the next 20 years of research in molecular biology confirmed indirectly the DNA structure, it wasn't until the 1980's that a detailed X-ray diffraction picture of a crystal DNA structure was obtained. This was produced by a synthetically made double-stranded polynucleotide of defined sequence; the result confirmed the basic deductions of Watson and Crick but also showed that the DNA molecule is rich in structural variation at the level of the local sequence.