Lecture14.pdf - Lecture 14 DNA All life on Earth uses DNA...

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All life on Earth uses DNA to store and transmit an organism’s cellular “operating instructions”. Lecture 14: DNA
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DNA = double-helix (twisted ladder) polymer formed of a sugar and phosphate backbone + 4 base-pair molecules .
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DNA Molecule James Watson & Francis Crick (1953) showed how DNA can store and replicate information. Discovery of DNA structure frequently attributed to Watson and Crick. - Real story is more nuanced; they relied on many clues discovered by others. - Generally true of scientific advances - problems are too big, too complex to be solved by individuals. - Science is ultimately collaborative, people build off of the work of others. - The race for the structure of DNA is a good example of how true breakthroughs happen through the work of a diverse community.
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(X-ray diffraction expert) Rosalind Franklin (Physicist) Maurice Wilkins
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Towards understanding the role of DNA 1870’s : Scientists concluded that hereditary factors reside in cell nucleus. - e.g. Oskar Hertwig’s observations of sperm & egg fusion in the sea urchins. 1880-1900 : Understood that chromosomes - thread- like structures in nuclei - may play a role in heredity - e.g. Walther Flemming showed chromosomes divide, separate, split in cell division 1910 : Knew that chromosomes are vectors of heredity - sites of hereditary factors or “genes” - e.g. Morgans’ experiments with fruit flies
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Rosalind Franklin By 1930s most biologists understood that there was a giant “hereditary molecule” making up chromosomes. They had even isolated the makeup of this molecule “DNA”. The challenge was to determine how its structure accounts for its role in heredity. 1952 Franklin’s x-ray diffraction pattern of DNA Rosalind Franklin, Raymond Gosling (PhD student), & Maurice Wilkins , used a technique called X-ray crystallography to image the DNA molecule.
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Why X-rays? Atoms are too small (0.1 nm between them) to be revealed using visible light (500 nm). X-ray radiation fits the appropriate wavelength to be diffracted by a DNA molecule and produce observable results.
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When light bounces off atoms in a lattice that has a grid size comparable to the light wavelength, we see a “diffraction pattern.”
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Type of pattern you see in the image tells you something about the structure of the crystal.
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Same kind of patterns emerge when visible light passes through slits
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Human chromosomes magnified 1000 times - Knew DNA was a chain of phosphates and sugars, - in some way attached to a set of 4 ring-shaped molecules called nitrogenous bases that came in four "flavors": adenine (A), thymine (T), cytosine (C), and guanine (G). But how could DNA carry all this information? - Maybe it could be revealed by the molecule's three- dimensional structure? 1940s: How does DNA work?
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Met a biochemist giving away DNA samples. A Ph.D. student, Raymond Gosling, in Wilkins' lab, suggested looking at the DNA with the new technique of X-ray diffraction.
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