Evidence for DNA as Genetic Material

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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

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Griffiths Transformation Experiments

• Diplococcus pneumoniae
• two strains IIR IIIS
• two differences in genotype
• R rough, avirulent S smooth, virulent
• II and III indicate different serotypes

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Griffiths Transformation Experiments

• Griffith injected live mice with the two strains
• alone and in combination
• living and heat-killed

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Griffiths Transformation Experiments
Live IIR
Live IIIS
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Heat-killed IIIS
Heat-killed IIIS live IIR
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Heat-killed IIIS
Live IIR
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Heat-killed IIIS
Live IIR
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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

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Avery, MacLeod and McCarty
heat-killed IIIS IIR
removed polysaccharide coat
heat-killed IIIS IIR
removed protein
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Avery, MacLeod and McCarty
heat-killed IIIS IIR
removed RNA
heat-killed IIIS IIR
removed DNA
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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

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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?

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32P labeled DNA
35S labeled protein
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Hershey and Chase

• Mix phage and bacteria
• Allow infection
• Shear off whats on outside
• Centrifuge
• Look for radioactivity in pellet (bacteria)

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blender
infection
centrifuge
Supernatant w/ protein coats
E. coli w/ hot DNA
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blender
infection
centrifuge
Supernatant w/ hot protein coats
E. coli w/ DNA
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Guthrie and Sinsheimer

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

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Naked DNA could code for entire viruses!