Passed to descendants unchanged Individual inherits one such factor from each

Passed to descendants unchanged individual inherits

This preview shows page 4 - 7 out of 98 pages.

Passed to descendants unchanged Individual inherits one such factor from each parent Frederick Miescher (1860s) o Took pus cells from discarded bandages o Isolated “nuclein” from cell n uclei by adding a salt solution then a weak alkaline solution to lyse the cells resulting in precipitated nuclei o Thought nuclein functioned as the storehouse for the phosphorus atom Chemically, nuclein is high in phosphorus Frederick Griffith (1928) o Transformation 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 o Blood culture from dead mice S strain Conclusion: Something transformed the R strain to the S strain o Griffith called this the “transforming principle” o Now known as “Griffith’s transformation experiments” Oswald Avery, Colin MacLeod, Maclyn McCarty (1940s) o Continued Frederick Griffith’s work o Determined transforming substance: DNA o Many scientists still thought protein coded for genetic material DNA too simple Protein more likely because more complex o Results 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 o Conclusion: DNA was the transforming substance
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3 DNA composition determined o Four nitrogenous bases Purines Adenine (A) Guanine (G) Pyrimidines Thymine (T) Cytosine (C) o Deoxyribose (pentose sugar) o Phosphate Erwin Chargaff & his Rules o Base composition of DNA varies with species o DNA from different tissues from same species = same o DNA bases do not change with age, nutrition, environment o In all DNA, A=T and G=C o “Chargaff’s Rules” Rosalind Franklin & Maurice Wilkins (1950s) o X-ray diffraction studies James Watson & Francis Crick (1950s) o Proposed 3D model of DNA (1953) o Awarded Nobel Prize (along with Wilkins) o Key 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
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4 “Lecture” 2: PBS Documentary The Secret of Life Objectives 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 developing the 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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