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Frederick Miescher - is the man who discovered DNA. The Swiss physician isolated a high phosphorous-containing substance from white blood cell nuclei in 1869. It was DNA, which he called 'nuclein' because it had come from the nucleus. He did not know its true nature.
Phoebus Levene - the Russian-American biochemist discovered the order of components of nucleic acids; phosphate-sugar-base. He coined the term 'nucleotide.' He also discovered the ribose sugar in RNA, the deoxyribose sugar in DNA, as well as identifying the way the nucleic acids - RNA and DNA - are put together.
Oswald Avery - in 1944 he discovered that DNA transmitted hereditary information. This was a revolutionary concept, because the scientific consensus at the time was that DNA was too simple for this task, and that proteins were more likely to be the candidates.
Erwin Chargaff - in 1950 he demonstrated that the nucleotide composition varies amongst species. This was different from Levene's view that the same nucleotides repeat in the same order. He also found that in any given species the ratio of adenine to thymine was roughly equal, and the ratio of cytosine and guanine was also roughly equal. This is known as Chargaff's rule and it helped to pave the way for Crick and Watson's studies.
Rosalind Franklin - her X-ray diffraction image of DNA structure, named 'Photo 51' was a significant piece of evidence in determining DNA structure. She was briefly mentioned in Crick and Watson’s proposal of the double helix structure in the journal Nature in April 1953. Watson, Crick, and Wilkins shared the Nobel Prize in Physiology or Medicine in 1962 for their work. Franklin was not acknowledged because she had died by this time and the award cannot be given posthumously. The significance of her role in DNA structure discovery was only realized after the publication of Watson’s book The Double Helix in 1968.
Linus Pauling - the methods he used to work out the structure of proteins were adopted by Crick and Watson. They were a combination of model building, chemistry, and physics.
Maurice Wilkins - obtained the first X-ray image of DNA. He taught Francis Crick about DNA, and his images of DNA inspired James Watson. He shared the 1962 Nobel Prize in Physiology or Medicine with Crick and Watson.