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Biochemistry 3107: Week 4 Tutorial. Phil Williams [email protected] 777-6878 HSC4333. Hershey-Delbruck Experiment. E. coli was infected with two strains of bacteriophage T2. One strain is h-r+ ; One strain is h+r-. Hershey-Delbruck Experiment.

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Biochemistry 3107:Week 4 Tutorial

Phil Williams

[email protected]



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Hershey-Delbruck Experiment

  • E. coli was infected with two strains of bacteriophage T2.

    • One strain is h-r+;

    • One strain is h+r-

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Hershey-Delbruck Experiment

  • h locus determines whether the phage can grow on a particular strain of E. coli

    • phage that are h- can infect the strain

    • phage that are h+ cannot infect the strain

  • r locus is a gene that determines whether the phage will lyse the host cells rapidly or slowly

    • phage that are r- will lyse the host cells rapidly

    • phage that are r+ will lyse the host cell slowly

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Hershey-Delbruck Experiment

  • two strains of E. coli were used in the experiment

    • strain 1 supports growth of h- phage but not h+ phage

    • strain 2 supports the growth of both phage

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Hershey-Delbruck Experiment

  • Bacteriophage are plated on a lawn of bacteria that consists of a mixture of both strains of E. coli

  • Expect two different phenotypes:

    • Both strains infected: Clear plaques

    • Infect only one of the two strains: Turbid plaques

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Hershey-Delbruck Experiment Proves:

  • If an E. coli cell was infected at the same time with two genetically different bacteriophage, the resulting phage population included recombinant phage types as well as the original parental phage types

  • The Nobel Prize in Physiology or Medicine 1969

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Meselson-Weigle Experiment

  • First demonstrated that both strands of each molecule must be broken and then rejoined in 1961

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Meselson-Weigle Experiment

  • Infected E. coli cells at the same time with phage from two different stocks of bacteriophage lambda.

    • Bacteriophage lambda c- mi-

      • Heavy isotopes of carbon 13C and nitrogen 15N

    • Bacteriophage lambda c+ mi+

      • Light isotopes of carbon and nitrogen

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Meselson-Weigle Experiment

  • Phenotypes of the intermediate density phage were recombinant phages

    • c- mi+ found near the band of "heavy“ non- recombinant phage

    • c+ mi- found near the band of "light" non- recombinant phage.

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Meselson-Weigle Experiment

  • These results can only be explained if recombination between the two parental phage involves breakage and rejoining of both DNA strands

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Holliday Model

  • Holliday Model