In this article we will discuss about the X-ray crystallographic studies of DNA.
In 1920, the basic chemical composition of nucleic acid was elucidated through the efforts of P.A. Levene. In 1940, for the first time W.T. Astbury gave first three dimensional structure of DNA model studied through X-ray crystallography. He concluded that its flat nucleotides get stacked to form polynucleotide.
Each nucleotide was oriented perpendicularly to the axis of molecule present at every 3.4 Å along the stack. M.H.F Wilkins, Rosalind Franklin and coworkers continued to study the work of Astbury and prepared the highly oriented DNA fibres which facilitated for getting photographs of X-ray diffraction. His female associate, R. Franklin prepared a super X-ray diffraction photograph of DNA.
This photograph supported Astbury’s intermediate distance 3.4 Å and also suggested for helical configuration of DNA.
Some of the points consistently noticed through X-ray diffraction photographic studies made by Wilkins and Franklin were that:
(i) the DNA from different species gave the identical X-ray diffraction besides variation in their base composition,
(ii) the DNA molecules are about 20 Å thick and more than 30,000 Å long,
(iii) the DNA molecules have repeating structure at every 34 Å.
However, the pertinent experimental facts were achieved through the study of detailed chemical structure of DNA, Chargaff s base pairing rule and Wilkins-Franklin’s X- ray diffraction photographic studies. There was an urgent need to get three dimensional structure of DNA molecule.
In February 1953, Pauling and R.B. Corey gave a triple helix model of DNA molecule. However, they could not explain the process of DNA replication. It was April 2, 1953 when a young American biologist James D.Watson and an English physicist Francis H. Crick of the Cavendish Laboratory, Cambridge University sent a brief letter to the Journal Nature entitled “a structure for deoxyribose nucleic acid” suggesting the three dimensional double helical structure of DNA molecule.
For this ground – breaking work, Watson and Crick were honoured with the Nobel Prize in 1962. They shared the prize with Maurice Wilkins of the King’s College, London who investigated the X-ray diffraction photographs of the DNA molecule. Unfortunately, Rosalind Franklin who also contributed a lot was deprived, by cruel death in 1958, of possible sharing of the prize.