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Chapter 12: The Molecular Basis of Inheritance

Chapter 12: The Molecular Basis of Inheritance. Griffith Bacterial Transformation. Frederick Griffith, British M.D. 1928 Studying pneumonia How do bacteria make people sick? Isolated 2 strains of pneumonia from mice 1 pathogenic Smooth colonies 1 non-pathogenic Rough colonies.

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Chapter 12: The Molecular Basis of Inheritance

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  1. Chapter 12:The Molecular Basis of Inheritance

  2. Griffith Bacterial Transformation • Frederick Griffith, British M.D. • 1928 • Studying pneumonia • How do bacteria make people sick? • Isolated 2 strains of pneumonia from mice • 1 pathogenic • Smooth colonies • 1 non-pathogenic • Rough colonies

  3. Bacteria of the “S” (smooth) strain of Streptococcus pneumoniae are pathogenic because they have a capsule that protects them from an animal’s defense system. Bacteria of the “R” (rough) strain lack a capsule and are nonpathogenic. Frederick Griffith injected mice with the two strains as shown below: CONCLUSION EXPERIMENT RESULTS Living S (control) cells Living R (control) cells Heat-killed (control) S cells Mixture of heat-killed S cells and living R cells Mouse dies Mouse healthy Mouse healthy Mouse dies Living S cells are found in blood sample. Griffith concluded that the living R bacteria had been transformed into pathogenic S bacteria by an unknown, heritable substance from the dead S cells.

  4. Griffith’s Conclusions: • Transformation • “Something” chemical was being passed • Change in genotype & phenotype • Had NO idea what it was!

  5. Avery • Oswald Avery • 1944 • American Microbiologist • Question: What molecule is being passed in Griffith’s experiment? • Experiment: • Extract various molecules from Smooth strain • See which one causes transformation • Conclusion: DNA is the molecule • Most researchers rejected the idea!

  6. Hershey & Chase • Alfred Hershey & Martha Chase • 1952 • Studied bacteriophages • Bacteria killing viruses • Bacteriophage • “Bacteria-eater”--Virus • DNA/RNA core • Protein “coat” • Attaches to bacteria  injects DNA/RNA

  7. Phage head Tail Tail fiber DNA 100 nm Bacterial cell T2 Bacteriophage attacking E. coli

  8. EXPERIMENT In their famous 1952 experiment, Alfred Hershey and Martha Chase used radioactive sulfur and phosphorus to trace the fates of the protein and DNA, respectively, of T2 phages that infected bacterial cells. Agitated in a blender to separate phages outside the bacteria from the bacterial cells. Centrifuged the mixture so that bacteria formed a pellet at the bottom of the test tube. 3 Measured the radioactivity in the pellet and the liquid Mixed radioactively labeled phages with bacteria. The phages infected the bacterial cells. 2 4 1 Radioactivity (phage protein) in liquid Radioactive protein Empty protein shell Phage Bacterial cell Batch 1: Phages were grown with radioactive sulfur (35S), which was incorporated into phage protein (pink). DNA Phage DNA Centrifuge Radioactive DNA Pellet (bacterial cells and contents) Batch 2: Phages were grown with radioactive phosphorus (32P), which was incorporated into phage DNA (blue). Centrifuge Radioactivity (phage DNA) in pellet Pellet Hershey & Chase Experiment: Is DNA or protein the genetic material of phage T2?

  9. Results/Conclusions • Results: • Phage proteins remained outside E. coli • Phage DNA entered bacteria E. coli. • Conclusions: Hershey and Chase concluded that DNA, not protein, functions as the T2 phage’s genetic material.

  10. Chargaff • Erwin Chargaff • 1947 • American biochemist • Analyzed amounts of Adenine, cytosine, guanine & thymine • Amount of A = T • Amount of C = G • “Chargaff’s Rule”

  11. Linus Pauling • Won 2 Nobel Prizes in his lifetime • 1951 work with DNA structure • American working at Caltech • Along with Robert Corey, proposed the structure of DNA was a triple helix • This team got very close.

  12. Wilkins & Franklin • Maurice Wilkins & Rosalind Franklin • Early 1950’s • X-ray crystallographers (X-ray crystalline form of molecules & deduce structure) • Hints: • Maybe a“twisted” structure • Maybe a helix • Maybe 2 strands • Maybe nitrogen bases in middle

  13. (b) (a) Rosalind Franklin Franklin’s X-ray diffraction Photograph of DNA

  14. Watson & Crick • James Watson • American biologist • Francis Crick • British physicist • X-ray crystallography of proteins • Cambridge University • 1953  put all the clues together • Built wire/tin model • Published theory in April, 1953 2 page article in “Nature” • “Double Helix” • Nobel Prize 1962 • Watson, Crick, Wilkins

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