10.2 Structure and function of dna  (Page 2/11)

Discovering the double helix

By the early 1950s, considerable evidence had accumulated indicating that DNA was the genetic material of cells, and now the race was on to discover its three-dimensional structure. Around this time, Austrian biochemist Erwin Chargaff N. Kresge et al. “Chargaff's Rules: The Work of Erwin Chargaff.” Journal of Biological Chemistry 280 (2005):e21. (1905–2002) examined the content of DNA in different species and discovered that adenine, thymine, guanine, and cytosine were not found in equal quantities, and that it varied from species to species, but not between individuals of the same species. He found that the amount of adenine was very close to equaling the amount of thymine, and the amount of cytosine was very close to equaling the amount of guanine, or A = T and G = C. These relationships are also known as Chargaff’s rules .

Other scientists were also actively exploring this field during the mid-20th century. In 1952, American scientist Linus Pauling (1901–1994) was the world’s leading structural chemist and odds-on favorite to solve the structure of DNA. Pauling had earlier discovered the structure of protein α helices, using X-ray diffraction , and, based upon X-ray diffraction images of DNA made in his laboratory, he proposed a triple-stranded model of DNA. L. Pauling, “A Proposed Structure for the Nucleic Acids.” Proceedings of the National Academy of Science of the United States of America 39 no. 2 (1953):84–97. At the same time, British researchers Rosalind Franklin (1920–1958) and her graduate student R.G. Gosling were also using X-ray diffraction to understand the structure of DNA ( [link] ). It was Franklin’s scientific expertise that resulted in the production of more well-defined X-ray diffraction images of DNA that would clearly show the overall double-helix structure of DNA.

James Watson (1928–), an American scientist, and Francis Crick (1916–2004), a British scientist, were working together in the 1950s to discover DNA’s structure. They used Chargaff’s rules and Franklin and Wilkins ’ X-ray diffraction images of DNA fibers to piece together the purine-pyrimidine pairing of the double helical DNA molecule ( [link] ). In April 1953, Watson and Crick published their model of the DNA double helix in Nature . J.D. Watson, F.H.C. Crick. “A Structure for Deoxyribose Nucleic Acid.” Nature 171 no. 4356 (1953):737–738. The same issue additionally included papers by Wilkins and colleagues, M.H.F. Wilkins et al. “Molecular Structure of Deoxypentose Nucleic Acids.” Nature 171 no. 4356 (1953):738–740. as well as by Franklin and Gosling , R. Franklin, R.G. Gosling. “Molecular Configuration in Sodium Thymonucleate.” Nature 171 no. 4356 (1953):740–741. each describing different aspects of the molecular structure of DNA. In 1962, James Watson, Francis Crick, and Maurice Wilkins were awarded the Nobel Prize in Physiology and Medicine. Unfortunately, by then Franklin had died, and Nobel prizes at the time were not awarded posthumously. Work continued, however, on learning about the structure of DNA. In 1973, Alexander Rich (1924–2015) and colleagues were able to analyze DNA crystals to confirm and further elucidate DNA structure. R.O. Day et al. “A Crystalline Fragment of the Double Helix: The Structure of the Dinucleoside Phosphate Guanylyl-3',5'-Cytidine.” Proceedings of the National Academy of Sciences of the United States of America 70 no. 3 (1973):849–853.