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Unformatted text preview: DNA Structure Mendel & Morgan: • T.H. Morgan’s group showed that genes are located along the chromosomes • DNA & proteins [& RNA] were the two candidates for genetic material • Mendel & Morgan’s work showed that a key factor in determining the identity of the genetic material was the choice of organism- Role of DNA in heredity was worked out by studying bacteria & viruses that infect them Frederick Griffith (1928): • Studied two strains of Streptococcus pneumoniae : S & R- S = smooth/pathogenic; R = rough/harmless- S has a capsule on it, making it harder to phagocytosize • Griffith killed the S strain with heat and mixed the cell remains with living bacteria of the nonpathogenic strain, some of the living cells became pathogenic- This pathogenicity was inherited by offspring • Some chemical component of the dead pathogenic cells caused the change • Griffith called it transformation [a change in genotype and phenotype due to the assimilation of external DNA by a cell] Oswald Avery (with McCarty & MacLeod), 1944: • Wanted to identify transforming substance (3 candidates: DNA, protein, RNA) • Broke open the heat killed pathogenic bacteria & extracted cellular components • Used specific treatments that inactive each molecule & tested each sample for its ability to transform live nonpathogenic bacteria- Concluded that it was DNA, but met opposition because some biologists were not convinced the genes in bacteria were like those in more complex organisms Alfred Hershey & Martha Chase (1952): • Bacteriophages (aka phages) – viruses that infect bacteria • Virus = DNA (or RNA) enclosed by a protective coat (made of protein) • Hershey & Chase performed experiments showing that DNA is the genetic material of the...
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