history of dna - chandler unified school district / … of dna copyright cmassengale . 2 history of...
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History of DNA
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History of DNA
• Early scientists thought genes were made up of protein rather than DNA because proteins are more complex than DNA
• Proteins are composed of 20 different amino acids in long polypeptide chains
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Griffith (1928)
• Experiment suggested DNA was probably cell’s genetic material.
• Worked with virulent S and nonvirulent R strain Pneumoccocus bacteria
• He found that R strain could become virulent when it took in DNA from heat-killed S strain
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Griffith’s Experiment
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Hershey & Chase (1952)
• Experiment proved DNA is cell’s genetic material.
• Viruses contain both DNA & protein.
• “Labeled” DNA and protein with different radioactive isotopes to track DNA & protein.
Radioactive 32P was injected into bacteria! copyright cmassengale
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Chargaff (1953)
• Experiment showed the amounts of the four organic, nitrogen-containing bases in DNA (A,T,C,G)
• In a somatic (body) cell: – amt. Adenine = amt. Thymine – amt. Guanine = amt. Cytosine *All add up to 100% copyright cmassengale
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Chargaff’s Rule
• Adenine pairs with Thymine
• Guanine pairs with Cytosine
• Purines (larger) = Adenine & Guanine
• Pyrimidines (smaller) = Thymine & Cytosine
G C T A
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Practice Chargaff’s Rule
If 20% Adenine…how much of each of the other bases? Remember pairing and all add up to 100%!
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Answer Adenine = 20%
Thymine (same as Adenine since pairs with it) = 20%
**100% - 40% = 60% left… split between the other two bases.
Guanine = 30%
Cytosine = 30%
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Franklin (1953)
• Took X-ray diffraction photographs of DNA suggesting DNA had the shape of a coiled spring.
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Watson & Crick (1953)
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• Built the first model of DNA as a double helix of two strands whose bases face one another.
• DNA maintains a constant thickness.
Summary
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Draw out timeline indicating all important
parties and a short summary of what each
scientific group contributed to the discovery of
the structure of DNA.