The History Behind The Double Helix Unraveled. As one of the most important discoveries of the twenty-first century, deoxyribonucleic

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1 The History Behind The Double Helix Unraveled As one of the most important discoveries of the twenty-first century, deoxyribonucleic acid, or DNA, changed the world of science and revolutionized modern genetics (Gibbons, 2012). DNA was first identified in the late 1860s by a Swiss scientist named Friedrich Miescher, who studied leukocytes, or white blood cells, from the bandages of hospital patients; within the nuclei of these cells, he found a new substance known as nucleic acid. Furthermore, a Russian biochemist Phoebus Levene was then able to determine that this nucleic acid was made of nucleotides, which were composed of a nitrogenous base, a sugar molecule, and a phosphate group (Pray, 2008). At the beginning of the twentieth century, because of the simplicity of DNA structure, most biochemists were skeptical that it could do a job as apparently complex as making genes (Ferry, 2008: 151). Genes were the smallest unit of genetic information, and their structure and transfer between generations was initially unknown (Hickel, 2005). Surprisingly, Oswald Avery and his colleagues at Rockefeller University were able to demonstrate that DNA was the main component of genetic material (Pray, 2008). James Watson and Francis Crick then realized that determining the structure of genes was a critical aspect of molecular biology and of understanding heredity and reproduction. The two men did not perform their own DNA experiments, so they made conclusions from the research of their peers (Hickel, 2005). While Watson and Crick are mainly credited for the discovery of the DNA double helix, the independent research and discoveries of many dedicated scientists, such as Rosalind Franklin, Linus Pauling, and Erwin Chargaff, greatly contributed to their overall success and their distinction in the scientific community. Working at King s College in the lab of Maurice Wilkins, a friend of Watson and Crick, Rosalind Franklin was indispensable in the discovery of the DNA double helix. It was evident to 1

2 Franklin that she needed to use x-ray crystallography in order to uncover the structure of DNA (Watson, 1968). The x-ray crystallography used by Franklin passed an x-ray beam through crystallized DNA onto a photographic plate that displayed the structure of the molecule by recording the pattern and indicating the arrangement of the molecule. Franklin was an expert at x-ray crystallography and she was able to create high-quality images by extracting thin fibers of DNA and by using a microcamera; this allowed the x-ray beam to image the crystallized DNA from multiple angles (Gibbons, 2012). Franklin is well known for her image Photograph 51. The DNA depicted in Photograph 51 was called B form (Ferry, 2008), which showed an X and indicated a uniformly twisted helix that continued in both directions. It could also be seen in the image that the sugar-phosphate backbone was on the outside of the helix and the nitrogenous bases were on the inside. Without Franklin s knowledge, Maurice Wilkins showed Watson and Crick some of her work, including the image of Photograph 51, and they were able to use that information in their discovery of the structure of DNA (Gibbons, 2012). There was fear of a debate with the laboratory at King s College about giving Franklin credit for her findings. It was not publicly known that Watson and Crick had seen Photograph 51 until The Double Helix was published in 1968; this continuous debate has yet to be resolved (Ferry, 2008). Crick had a background in x-ray crystallography (Hickel, 2005); he thought that Franklin s Photograph 51 clearly indicated the helical structure of DNA, but Franklin believed that there was not a shred of evidence to indicate that DNA was a helix (Watson, 1968: 68). This image was incredibly important in the validation of the discovery of the structure of DNA because it clearly supported the ideas of Watson and Crick. Overall, Watson and Crick were more quick to make assumptions about information they received, while Franklin wanted to put together the pieces of the puzzle by producing more and more and better x-ray diffraction images instead of 2

3 jumping to rapid, and possibly incorrect, conclusions (Gibbons, 2012: 69). Using the information about Franklin s work from Wilkins, Watson and Crick set out to build their own threedimensional DNA model using her findings as a basis for the appearance of DNA. Their new model had phosphates on the inside and bases on the outside and was based on an erroneous understanding of the water content (Gibbons, 2012: 66); Franklin corrected Watson and Crick on their mistake by explaining that there was an indication to believe that the structure was the reverse, with the sugar-phosphate backbone on the outside and the bases on the inside. Although Franklin would be correct in the end, Watson and Crick were doubtful at the time and maintained their opinion (Watson, 1968). While Rosalind Franklin was instrumental in helping to discover and image the structure of DNA, Erwin Chargaff, an Austrian biochemist working at Columbia University in New York, also contributed to Watson and Crick s discovery; his main focus was trying to calculate the relationships between purine and pyrimidine bases (Watson, 1968). After reading a 1944 paper by Oswald Avery, Chargaff became very interested in knowing more about DNA and recognized that DNA was an important piece of genetic material, unlike many of his peer scientists at the time. Chargaff began his studies by looking at the DNA differences between species. (Pray, 2008). In all his research, he found similar proportions between the bases; although the number of base pairs was incredibly different between species, the proportions between adenine (A) and thymine (T), and guanine (G) and cytosine (C), were always the same (Robinson, 2010). While Chargaff could not explain his findings, he knew that they were significant and should not be ignored (Watson, 1968). Watson and Crick used what was then named Chargaff s rule, A=T and G=C, as a foundation for nucleotide base pairing in their quest to discover the structure of DNA (Gibbons, 2012). Each of the bases could fit inside the double helix model when paired with their 3

4 corresponding nucleotide; A-T was the same width as C-G and allowed the DNA double helix to wind in consistent helical shape (Calladine, Drew, Luisi and Travers, 2004). Beginning with the contributions from Franklin and Chargaff, Watson and Crick also gained important knowledge from the biochemist Linus Pauling about molecular distances and bond angles of the DNA molecule (Pray, 2008). Pauling was interested in the structure of proteins and how polypeptide chains were folded (Bright Hub, 2012). He was thought to be successful in his endeavors because he relied on the facts and laws of structural chemistry, such as atom interactions, to support his findings. While there was initially gossip that Linus Pauling had discovered the structure of DNA, he had in fact made the discovery of the alpha helix, a single polypeptide chain folded into a helical arrangement held together by hydrogen bonds between groups on the same chain (Watson, 1968: 36). Discovering this protein structure was important in the area of biological sciences because the alpha helix was essential to understanding molecular biology and the function of proteins. Although he did not have distinct supporting images or data, in 1953 Pauling also proposed that DNA was triple-stranded. While his suggestion of the triple helix was incorrect, Pauling s mistakes allowed Watson and Crick to discover DNA (Bright Hub, 2012). His mistakes were the basis for interpretation and correction by Watson and Crick, and it also gave them a window of opportunity that allowed them to make the discovery themselves. Pauling would eventually discover his mistake, but by then Watson and Crick had already made the discovery of the structure of DNA (Watson, 1968). Throughout the world, Watson and Crick are the ones accepted for the discovery of DNA, having won the Nobel Prize in 1962, and often the contributions of the other scientists are not recognized. The discovery of DNA was a cooperative process that was possible because of the contributions of many people in the scientific community; Rosalind Franklin, Linus Pauling, 4

5 and Erwin Chargaff each made findings that Watson and Crick were able to use in their explanation of the structure of the DNA double helix. As a collaboration of findings, the discovery of DNA was an exceedingly important accomplishment in science; the building block of life was discovered and paved the way for even greater advancements in modern genetics and molecular biology. 5

6 Reference List Bright Hub, Linus Pauling and the Discovery of DNA. [online]. Available at: < [Accessed 30 October 2012]. Calladine, C.R., Drew, H.R., Luisi, B.F. and Travers, A.A., Understanding DNA: The Molecule and How it Works. [e-book] San Diego: Academic Press. Available at: Google Books.< [Accesssed 30 October 2012]. Ferry, G., Max Perutz and the Secret of Life. London: Pimlico. Gibbons, M. G., Reassessing Discovery: Rosalind Franklin, Scientific Visualization, and the Structure of DNA. Philosophy of Science, 79(1), pp Available through JSTOR < [Accessed 24 October 2012]. Hickel, W., The Francis Crick Papers: The Discovery of the Double Helix , National Library of Medicine s Profiles in Science. [online] Available at: < [Accessed 27 October 2012]. Pray, L.A., Discovery of DNA Structure and Function: Watson and Crick, Nature. [online] Available at: < [Accessed 26 October 2012]. Robinson, T.R., Genetics for Dummies. 2nd ed. Hoboken: Wiley Publishing, Inc. Watson, J.D., The Double Helix: A Personal Account of the Discovery of the Structure of DNA. London: Phoenix. 6

7 Bibliography Hargittai, I., Linus Pauling s quest for the structure of proteins. Structural chemistry, [e- journal], 21(1), 1-7. Available through SpringerLink < [Accessed 24 October 2012]. Long, B.W., The Double Helix : A Bonus for DNA Students. The American Biology Teacher, 41(2), pp Available through JSTOR < > [Accessed 24 October 2012]. Watson, J.D. and Crick, F.H.C., A Structure for Deoxyribose Nucleic Acid [pdf]. Available at: Nature < [Accessed 24 October 2012]. 7