Discovering the Double Helix


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The X-ray diffraction pattern of DNA shows its helical nature. (credit: National Institutes of Health)

OpenStax Microbiology

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 (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.[6] 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. 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 doublehelix 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. In April 1953, Watson and Crick published their model of the DNA double helix in Nature. The same issue additionally included papers by Wilkins and colleagues, as well as by Franklin and Gosling, 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.

In 1953, James Watson and Francis Crick built this model of the structure of DNA, shown here on display at the Science Museum in London.

Source: OpenStax Microbiology


Parker, N., Schneegurt, M., Thi Tu, A.-H., Forster, B. M., & Lister, P. (n.d.). Microbiology. Houston, Texas: OpenStax. Access for free at: