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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.