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Evidence for DNA as Genetic Material

Evidence for DNA as Genetic Material. Griffith - 1928 transformation Avery, MacLeod, and McCarty - 1944 DNA = transforming principle Hershey & Chase -1952 Viral DNA enters cells Guthrie and Sinsheimer - 1960 Viral DNA codes for complete virus. Griffith’s Transformation Experiments.

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Evidence for DNA as Genetic Material

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  1. Evidence for DNA as Genetic Material • Griffith - 1928 • transformation • Avery, MacLeod, and McCarty - 1944 • DNA = transforming principle • Hershey & Chase -1952 • Viral DNA enters cells • Guthrie and Sinsheimer - 1960 • Viral DNA codes for complete virus

  2. Griffith’s Transformation Experiments • Diplococcus pneumoniae • two strains: IIR & IIIS • two differences in genotype • R = rough, avirulent; S = smooth, virulent • II and III indicate different serotypes

  3. Griffith’s Transformation Experiments • Griffith injected live mice with the two strains • alone and in combination • living and heat-killed

  4. Griffith’s Transformation Experiments Live IIR Live IIIS

  5. Heat-killed IIIS Heat-killed IIIS + live IIR

  6. Heat-killed IIIS Live IIR

  7. Heat-killed IIIS Live IIR

  8. Explanations? • IIR cells mutated to S? • Serotype was changing • IIR were changing into IIIS • 2 mutations? • Only happened when IIR and dead IIIS mixed • Non-living component of IIIS cells changing genetics of IIR cells

  9. Avery, MacLeod and McCarty heat-killed IIIS + IIR removed polysaccharide coat heat-killed IIIS + IIR removed protein

  10. Avery, MacLeod and McCarty heat-killed IIIS + IIR removed RNA heat-killed IIIS + IIR removed DNA

  11. Avery, MacLeod and McCarty • DNA is the transforming principle • DNA could change IIR bacteria into IIIS • Now know bacteria may pick up pieces of exogenous DNA and incorporate them into their own chromosomes = transformation

  12. Hershey and Chase • Studied bacteriophage - T2 • virus which attacks E. coli • Ran two side-by-side studies • labeled DNA of phage w/ 32P in one study • labeled protein of phage w/ 35S in second • In which study does radioactivity end up in E. coli?

  13. 32P labeled DNA 35S labeled protein

  14. Hershey and Chase • Mix phage and bacteria • Allow infection • Shear off what’s on outside • Centrifuge • Look for radioactivity in pellet (bacteria)

  15. blender infection centrifuge Supernatant w/ protein coats E. coli w/ hot DNA

  16. blender infection centrifuge Supernatant w/ hot protein coats E. coli w/ DNA

  17. Guthrie and Sinsheimer • Refined the Hershey and Chase study • E. coli with ruptured outer membrane • protoplasts • mixed with T2 DNA alone

  18. “Naked” DNA could code for entire viruses!

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