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Preformation Blending Theory… a blend between mom and dad Proteins were the likely suspect 20 different amino acids vs. only 4 different nucleotides found in DNA The march to understanding that DNA is the genetic material T.H. Morgan (1908) Frederick Griffith (1928) Avery, McCarty & MacLeod (1944) Erwin Chargaff (1947) Hershey & Chase (1952) Watson & Crick (1953) Meselson & Stahl (1958) Worked with fruit flies… Associated phenotype with a specific chromosome Conclusion: •Genes are on Chromosomes •But is it protein or the DNA of the chromosomes that are the genes? Army doctor working to develop a vaccine against bacteria that caused pneumonia When grown on agar plates the bacterium had two forms: smooth (S) ○ Contained coating on bacteria that made them immune to immune system…LETHAL rough (R) ○ No coat…The R bacteria were harmless Griffith Conclusion: •This experiment strongly implied that genetic material had been transferred from the dead to the living cell. •Protein or DNA? Griffith’s Experiment Revisited . Conclusion: •First experiment to strongly imply the DNA is the “transforming factor” and not proteins or other materials •Transforming principle-genes are made of DNA Video worked with bacteriophage viruses that infect bacteria grew phage viruses in 2 media, radioactively labeled with either ○ 35S in their proteins ○ 32P in their DNA infected bacteria with labeled phages Video Conclusion: Confirmed DNA as the genetic material!! DNA composition: “Chargaff’s rules” varies from species to species all 4 bases not in equal quantity bases present in characteristic ratio ○ humans: A = 30.9% T = 29.4% G = 19.9% C = 19.8% 1953 | 1962 Watson & Crick Video developed double helix model of DNA ○ other leading scientists working on question: Rosalind Franklin Maurice Wilkins Linus Pauling Franklin Wilkins Pauling Watson Crick DNA is a double helix antiparallel Nucleotides Nitrogenous Bases ○ A, T, C, G Deoxyribose Sugar Phosphate Backbone: Sugar-Phosphate Phosphodiester Linkages Each strand has a 5’ and 3’ end Complementary Base Pairing: Chargaff’s Rules A-T= 2 H bonds C-G = 3 H bonds Replication of DNA base pairing suggests that it will allow each side to serve as a template for a new strand “It has not escaped our notice that the specific pairing we have postulated immediately suggests a possible copying mechanism for the genetic material.” — Watson & Crick conservative P 1 2 semiconservative dispersive Matthew Meselson Franklin Stahl Matthew Meselson Franklin Stahl animation March to understanding that DNA is the genetic material T.H. Morgan (1908) ○ genes are on chromosomes Frederick Griffith (1928) ○ a transforming factor can change phenotype Avery, McCarty & MacLeod (1944) ○ transforming factor is DNA Erwin Chargaff (1947) ○ Chargaff rules: A = T, C = G Hershey & Chase (1952) ○ confirmation that DNA is genetic material Watson & Crick (1953) ○ determined double helix structure of DNA Meselson & Stahl (1958) ○ semi-conservative replication