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DDNA - DNA Molecular Genetics What is the molecular basis...

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DNA & Molecular Genetics Heyer 1 Molecular Genetics Feb 16, 2001 Early 20th century, most scientists assumed proteins. 1928—Frederick Griffith discovered that something from heat-killed pathogenic strain of Streptococcus pneumoniae could “transform” a nonpathogenic strain to become pathogenic. This pathogenicity was inherited by all subcultures. What is the molecular basis of inheritance? Bacteria of the “S” (smooth) strain of Streptococcus pneumoniae are pathogenic because they have a capsule that protects them from an animal’s defense system. Bacteria of the “R” (rough) strain lack a capsule and are nonpathogenic. Frederick Griffith injected mice with the two strains as shown below: Griffith concluded that the living R bacteria had been transformed into pathogenic S bacteria by an unknown, heritable substance from the dead S cells. EXPERIMENT RESULTS CONCLUSION Living S (control) cells Living R (control) cells Heat-killed (control) S cells Mixture of heat-killed S cells and living R cells Mouse dies Mouse healthy Mouse healthy Mouse dies Living S cells are found in blood sample. Figure 16.2 In 1940s, it was found that the fraction containing DNA extracted from the pathogenic strain was causing the transformation. 1952—The Alfred Hershey and Martha Chase experiment What is the molecular basis of inheritance? In their famous 1952 experiment, Alfred Hershey and Martha Chase used radioactive sulfur and phosphorus to trace the fates of the protein and DNA, respectively, of T2 phages that infected bacterial cells. Radioactivity (phage protein) in liquid Phage Bacterial cell Radioactive protein Empty protein shell Phage DNA DNA Centrifuge Pellet (bacterial cells and contents) Radioactive DNA Centrifuge Pellet Batch 1: Phages were grown with radioactive sulfur ( 35 S ), which was incorporated into phage protein (pink). Batch 2: Phages were grown with radioactive phosphorus ( 32 P ), which was incorporated into phage DNA (blue). 1 2 3 4 Agitated in a blender to separate phages outside the bacteria from the bacterial cells. Mixed radioactively labeled phages with bacteria. The phages infected the bacterial cells. Centrifuged the mixture so that bacteria formed a pellet at the bottom of the test tube. Measured the radioactivity in the pellet and the liquid Phage proteins remained outside the bacterial cells during infection, while phage DNA entered the cells. When cultured, bacterial cells with radioactive phage DNA released new phages with some radioactive phosphorus. Hershey and Chase concluded that DNA, not protein, functions as the T2 phage’s genetic material. RESULTS CONCLUSION EXPERIMENT Radioactivity (phage DNA) in pellet Figure 16.4 OK, maybe for viruses and bacteria. But what about in “higher” organisms?
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