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Friedrich Miescher
Meischer studied the composition of white blood cells which he obtained from bandages he acquired from hospitals. He isolated a new molecule from the nuclei of the white blood cells by soaking the cells in alcohol to remove the lipids and releasing enzymes to digest the proteins. What was left, called nuclein. Nuclein (what we call nucleic acid) consisted of hydrogen, oxygen, nitrogen, and phosphorus. He determined that there was a unique ratio of phosphorus to nitrogen. -
Friedrich Miescher
Meischer's discovery of nucleic acid, or DNA, was momentous, but he believed that proteins contained the unit of heredity, not the nuclein he discovered. He added to the cell theory by discovering that another macromolecule existed inside the cell and that the nucleus was unique from other parts of the cell because it contained this macromolecule when scientists at the time didn't think the nucleus was any different from the rest of the cell. (Photograph of Meischer's isolated DNA) -
Erwin Chargaff
Chargaff was a chemist who was influenced by Erwin Schrodinger's proposal that DNA was the principal unit in heredity. He sought to prove this, by using partition chromatography (separation of a molecule by the different solubilities of its components) and ultraviolet spectrophotometry (absorption of light by different substances). DNA of all species did not contain the same proportion of bases across the board as was believed at the time. -
Erwin Chargaff (Cont.)
His discovery on the differences between DNA helped support the theory that DNA was the unit of heredity. He also discovered that adenine and thymine occurred in about the same amounts, as did cytosine and guanine. He did not; however, suggest that A and T or C and G were connected to each other as we know today. He thought that DNA looked something like the picture, and that it was able to separate along the red line and each half would inherit the topology of the parent strip. -
Rosalind Franklin
Rosalind Franklin discovered the double helix structure of DNA by using X-Ray Diffraction. X-Ray Diffraction is the process by which X-Rays (Beta Radiation or free electrons) are shot at a substance which gives an understanding of the molecular structure of the substance. By using this method, Franklin and her assistant grad student Ray Gosling were able to take this famous picture called Photograph 51, the first picture of DNA. -
Rosalind Franklin (Cont.)
The photo was taken after 100 hours of exposure from an X-Ray diffraction machine that Franklin had worked on herself. Her discoveries helped to prove that DNA was a double helix and not a triple helix, as was a popular theory of the day. Watson and Crick took credit for her discoveries when they published her findings in Nature, along with Photograph 51. Franklin died in 1958 of ovarian cancer, likely due to work with X-Rays. -
Hershey and Chase
Hershey and Chase helped to prove that DNA is the unit of heredity through experiments with viruses and bacteria. Viruses are made of DNA and protein. In the experiment, viruses were grown in an isotope of sulfur and an isotope of phosphorus. The sulfur would be used in the creation of the proteins and the phosphorus would be used in the creation of DNA. Each strand of virus was inserted into a bacteria where it was allowed to infect E. coli bacteria. -
Hershey and Chase (Cont.)
The cells that were infected with the phosphorus isotope virus became radioactive (because the phosphorus was radioactive) and the cells infected with the sulfur isotope virus did not become radioactive. So, the protein of the virus stayed outside of the cell while the DNA of the virus was inserted into the cell. Should protein have been the unit of heredity, the virus would have died. The experiment supports that DNA is the unit of heredity, and not proteins as was another theory. -
Watson and Crick
James Watson and Francis Crick are credited with the discovery of the double helix structure of DNA. They used Rosalind Franklin's research of DNA with X-Ray Diffraction was evidence for their work, along with Photograph 51, without Rosalind's permission. They built a model of DNA and hypothesized that the two sides of DNA could be separated and identical DNA strands could be made from the two sides of the separated DNA. This solved the conundrum of how DNA could replicate itself. -
Watson and Crick (Cont.)
Watson and Crick were awarded the Nobel Prize in 1962, four years after their contributor, Rosalind Franklin, died of ovarian cancer. -
Meselson and Stahl
Meselson and Stahl proved Watson and Crick's hypothesis of semiconservative DNA replication (where one the DNA splits into two sides and each side is used as a template to create a new DNA strand). They proved this using different isotopes of Nitrogen and E. coli bacteria. A set of E. coli was was grown in Nitrogen-15 and then placed in Nitrogen-14 and allowed to divide. Nitrogen-15 is heavier than Nitrogen-14. The resulting density of DNA was consistent with the semiconservative model. -
Meselson and Stahl (Cont.)
After one replication in the Nitrogen-14, there was intermediate density of the DNA. After two replications, there was more Nitrogen-14 than there was Nitrogen-15 which supported the Semiconservative Model of DNA replication. -
Bibliography
Biography.com Editors. "Rosalind Franklin." Biography.com. A&E Networks Television, 07 July 2016. Web. 19 Jan. 2017. Walsh, Fergus. "The Most Important Photo Ever Taken?" BBC News. BBC, 16 May 2012. Web. 19 Jan. 2017. "The Rosalind Franklin Papers: The DNA Riddle: King's College, London, 1951-1953." U.S. National Library of Medicine. National Institutes of Health, n.d. Web. 19 Jan. 2017. -
Biobliography
Landau, Elizabeth. "Watson: 'DNA Was My Only Gold Rush'." CNN. Cable News Network, 23 June 2013. Web. 20 Jan. 2017. "1962 - Francis Crick & James Watson." MRC Laboratory of Molecular Biology. Medical Research Council, n.d. Web. 20 Jan. 2017. Biography.com Editors. "James D. Watson." Biography.com. A&E Networks Television, 07 July 2016. Web. 20 Jan. 2017. Biography.com Editors. "Francis Crick." Biography.com. A&E Networks Television, 07 July 2016. Web. 20 Jan. 2017. -
Bibliography
"Erwin Chargaff." Famous Scientists. N.p., 9 Aug. 2016. Web. 21 Jan. 2017. McCarthy, Eugene M. "Erwin Chargaff." Macroevolution. N.p, n.d. Web. 21 Jan. 2017. "Partition Chromatography." Farlex Partner Medical Dictionary. Farlex, N.d. Web. 21 Jan. 2017. Reusch, William. "UV-Visible Spectroscopy." UV-Visible Spectroscopy. N.p, 5 May 2013. Web. 21 Jan. 2017. "DNA and Proteins are Key Molecules of the Cell Nucleus." DNA From the Beginning, Josiah Macey, Jr. Foundation, n.d. Web. 21 Jan. 2017. -
Bibliography
Dahm, Ralf. "From Discovering to Understanding." EMBO Reports. European Molecular Biology Association, 19 Feb. 2010. Web. 21 Jan. 2017. "Meselson-Stahl Experiment." WIkipedia. Wikimedia Foundation, 3 Jan. 2017. Web. 22 Jan. 2017. "A Half DNA Ladder IS a Template for Copying the Whole." DNA from the Beginning. Josiah Macey, Jr. Foundation, n.d. Web. 23 Jan. 2017. "Hershey and Chase Experiments." Biology Pages. N.p., 19 April 2014. Web. 23 Jan. 2017. -
Bibliography
"The Hershey-Chase Blander Experiments." PaulingBlog. N.p. 29 Oct. 2015. Web. 23 Jan. 2017. "Hershey-Chase Experiment." Wikipedia. Wikemedia Foundation, 18 Jan. 2017. Web. 23 Jan. 2017.
