Passed to descendants unchanged Individual inherits one such factor from each parent Frederick Miescher (1860s) oTook pus cells from discarded bandages oIsolated “nuclein” from cell nuclei by adding a salt solution then a weak alkaline solution to lyse the cells resulting in precipitated nuclei oThought nuclein functioned as the storehouse for the phosphorus atom Chemically, nuclein is high in phosphorus Frederick Griffith (1928) oTransformation experiment Pneumococci S strain: encapsulated; highly pathogenic R strain: unencapsulated; nonpathogenic The experiment: S strain + mice dead mice R strain + mice alive mice Heat-killed S strain + mice alive mice Mix of heat-killed S strain & live R strain + mice dead mice oBlood culture from dead mice S strain Conclusion: Something transformed the R strain to the S strain oGriffith called this the “transforming principle”oNow known as “Griffith’s transformation experiments”Oswald Avery, Colin MacLeod, Maclyn McCarty (1940s) oContinued Frederick Griffith’s workoDetermined transforming substance: DNA oMany scientists still thought protein coded for genetic material DNA too simple Protein more likely because more complex oResults confirmed with a systematic elimination study When DNA was degraded, the mixture was no longer able to transform bacteria All other combinations of enzymes were able to transform the bacteria oConclusion: DNA was the transforming substance
3 DNA composition determined oFour nitrogenous bases Purines Adenine (A) Guanine (G) Pyrimidines Thymine (T) Cytosine (C) oDeoxyribose (pentose sugar) oPhosphate Erwin Chargaff & his Rules oBase composition of DNA varies with species oDNA from different tissues from same species = same oDNA bases do not change with age, nutrition, environment oIn all DNA, A=T and G=C o“Chargaff’s Rules”Rosalind Franklin & Maurice Wilkins (1950s) oX-ray diffraction studies James Watson & Francis Crick (1950s) oProposed 3D model of DNA (1953) oAwarded Nobel Prize (along with Wilkins) oKey aspects of model: Double-stranded Anti-parallel 5’,3’-phosphodiester bonds run in opposite directions Double helix Major groove Minor groove Outside of helix: hydrophilic backbone of deoxyribose & phosphate Purine & pyrimidine bases stacked internally A=T G=C DNA strands not identical but complementary
4 “Lecture” 2: PBS Documentary The Secret of LifeObjectives Following the video “The Secret of Life,” the student will be able to answer the following study questions: 1.Name the scientist who played a key role in developingthe X-ray diffraction technique. 2.Who made the X-ray diffraction images which provided key DNA information? 3.Which three scientists received the Nobel Prize for determining DNA’s structure?4.Why was Rosalind Franklin not awarded the Nobel Prize?
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