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The Chemical Structure of DNA
Deoxyribonucleic acid (DNA) is a molecule that contains genetic information, and is stored within the cells of organisms. This information is transmitted from parent to offspring, and down through generations. Traits such as height, eye color, fat type and predisposition to disease are all transmitted. Such transmissions are referred to as heredity.
The DNA molecule is large, with a double helix structure that is often compared to a “twisted ladder”. Along the twisted strands are nucleotides – essentially the ladder’s rungs – each made up of a phosphate group, a sugar and a chemical base. There are four types of base: adenine (A), thymine (T), guanine (G) and cytosine (C). These bases are the letters that make up the DNA alphabet.
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Who was Rosalind Franklin?
Rosalind Franklin (July 1920 to April 1958) was born in London, England. Her interest in science led her to study a degree at Newnham College Cambridge in 1938. She received her doctorate from Cambridge in 1945, where she studied carbons. Franklin then worked in France, where she developed her skills in X-ray crystallography.
Franklin was appointed by King’s College London in 1951, where she used her expertise in X-ray diffraction techniques to photograph DNA. At the time, DNA’s role in heredity was only a hypothesis, and its structure was unknown. Other scientists working on DNA at King’s included James Watson, Francis Crick and Maurice Wilkins. Franklin was also the academic supervisor to research student Raymond Gosling.
The working atmosphere at King’s was reported to have been unpleasant, and Franklin was not happy there. In 1953, she left King’s College and went to work at Birkbeck College, where she studied the structure of viruses. She continued to work following a diagnosis of cancer up until her death in 1958.
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The Discovery of the Double Helix
Crick and Watson were working on models of DNA, whereas Franklin was working with images. The X-ray diffraction technique Franklin used involved shooting X-rays into DNA crystals, which were then photographed and analyzed. This may sound straightforward but is actually rather difficult, and the resulting images wouldn’t have made much sense to the untrained eye.
Wilkins apparently showed one of Franklin’s photographs to Watson, who recognized the photo’s indication of a helical structure. Although Wilkins probably had no malicious intent, Franklin had not granted permission for this photograph to be shown. This image, nicknamed Photo 51, was a significant piece of evidence in determining DNA’s structure.
Franklin was briefly mentioned in Crick and Watson’s proposal of the double helix structure in the journal Nature in April 1953. She published her own work in the same issue. In 1962, Watson, Crick and Wilkins shared the Nobel Prize in Physiology or Medicine for their work with nucleic acids. Since the award cannot be given after death, Franklin could not be acknowledged. The significance of her accomplishments was not realized until the publication of Watson’s book The Double Helix in 1968.
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- Molecular Structure of Nucleic Acids – A Structure for Deoxyribose Nucleic Acids, published in Nature (no. 4356, pp.737-738) April 25th in 1953, authors: J.D. Watson and F.H. C. Crick.
- Light on a Dark Lady, published in Trends in Biochemical Sciences (no. 23, pp.151-154) in 1998, author: Anne Piper.
- Rosalind Franklin and the Double Helix, published in Physics Today (p.42) March 2003, author: Lynne Osman Elkin.
- Rosalind Franklin, Spartacus Educational blog, published on 12th December 2008, accessed: May 31st 2009, at: http://www.spartacus.schoolnet.co.uk/SCfranklinR.htm.