Chapter 16 Frederick Griffith- studied 2 strains of strep; heat-killed pathogenic and mixed remains with nonpathogenic; some became pathogenic Transformation- change in genotype and phenotype due to assimilation of external DNA by cell (not cancerous change) Oswald Avery- purified molecules from heat-killed bacteria (Griffith’s) and tried transforming nonpathogenic; only DNA worked McCarty & MacLeod- along with Avery announced that DNA was the transforming agent; still doubted b/c little known about DNA Bacteriophage- bacteria-eating viruses that Hershey and Chase studied since because of its dual composition of DNA and protein Hershey & Chase- tagged T2 phage in 2 experiments, one with radioactive S (common in protein); radioactive P (common in DNA) a. Once tagged phages infected host cell ( E. Coli ) agitated host in blender and centrifuged mixture to form pellet b. 1 st experiment with S isotope that tested for protein floated at top, not mixed with bacterium protein doesn’t affect c. 2 nd experiment with P isotope sunk to bottom, mixed with the bacterium, which confirmed Avery’s, McCarty’s and MacLeod’s theory that DNA, not protein, transforms and is genetic material Erwin Chargaff- discovered similar ratios between nucleotide bases (i.e. humans have A=30.3%, T=30.3%, G=19.5%, C=19.5%)
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