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Transcript
The Discovery of DNA 1928 – Frederick Griffith Transformation Change in genotype and phenotype because of assimilation of external DNA by a cell Used disease-causing (pathogenic) and nondisease-causing (nonpathogenic) bacteria and mice But what caused the change in phenotype?? He wasn’t sure… 1944 – Avery, MacLeod, McCarty Discovered that DNA is the “transforming agent” Their finding was not believed by the scientific community because proteins were believed to be better candidates for the genetic material 1952 – Alfred Hershey & Martha Chase Is it protein or DNA that is the genetic material? Used bacteriophages (viruses that infect bacteria) to show that since DNA enters the bacterial cells, but protein doesn’t, DNA must be the genetic material Hershey & Chase’s Experiment Early 1950s – Erwin Chargaff Analyzed the base composition of DNA Set the stage for the base-pairing rules # of adenines = # of thymines # of cytosines = # of guanines Known as Chargaff’s Rule Early 1950s – Maurice Wilkins & Rosalind Franklin Rosalind Franklin took X-ray crystallography diffraction photograph of DNA 1953 – James Watson & Francis Crick Constructed model of DNA as a double helix Purine + pyrimidine for consistent width C-G 3 hydrogen bonds A-T 2 hydrogen bonds Late 1950s – Matthew Meselson & Franklin Stahl Semi-conservative replication of DNA Each new molecule of DNA (after DNA replication) contains 1 old and 1 new strand