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13-18 review - 13.1 The Genetic Material Early researchers...

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13.1 The Genetic Material Early researchers knew that the genetic material must be: o able to store information used to control both the development and the metabolic activities of cells; o stable so it can be replicated accurately during cell division and be transmitted for generations; and, o able to undergo mutations providing the genetic variability required for evolution. Previous Knowledge About DNA o Understanding the chemistry of DNA was essential to the discovery that DNA is genetic material. o Friedrich Miescher (1869) removed nuclei from pus cells and isolated DNA "nuclein"; it was rich in phosphorus and lacked sulfur. o Subsequent analysis of nuclein found that it contained an acidic substance: named it nucleic acid. o Two types of nucleic acids were soon discovered: DNA (deoxyribonucleic acid) and RNA (ribonucleic acid). o In the early twentieth century, it was shown that nucleic acids contain four types of nucleotides. DNA was composed of repeating units, each of which always had just one of each of four different nucleotides (a nitrogenous base, a phosphate, and a pentose). In this model, DNA could not vary between species and therefore could not be the genetic material; therefore some other protein component was expected to be the genetic material. Transformation of Bacteria o Bacteriologist Frederick Griffith (1931) experimented with Streptococcus pneumoniae (a pneumococcus that causes pneumonia in mammals). o Mice were injected with two strains of pneumococcus: an encapsulated (S) strain and a non-encapsulated (R) strain. The S strain is virulent (the mice died); it has a mucous capsule and forms "shiny" colonies. The R strain is not virulent (the mice lived); it has no capsule and forms "dull" colonies. o In an effort to determine if the capsule alone was responsible for the virulence of the S strain, he injected mice with heat-killed S strain bacteria; the mice lived. o Finally, he injected mice with a mixture of heat-killed S strain and live R strain bacteria. The mice died; living S strain pneumococcus were recovered from their bodies. Griffith concluded that some substance necessary for synthesis of the capsule--and therefore for virulence--must pass from dead S strain bacteria to living R strain bacteria so the R strain were transformed.
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This change in phenotype of the R strain must be due to a change in the bacterial genotype, suggesting that the transforming substance passed from S strain to R strain. DNA: The Transforming Substance o Oswald Avery et al. (1944) reported that the transforming substance was DNA. o Purified DNA is capable of bringing about the transformation. Evidence: DNA from S strain pneumococcus causes R strain bacteria to be transformed. Enzymes that degrade proteins cannot prevent transformation, nor can enzymes that digest RNA.
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