Chapter 9 Biology

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9-1 Identifying the Genetic Material

Griffith's experiments
Frederick Griffith, a bacteriologist, was trying to prepare a vaccine against the pneumonia-causing bacterium, S. pneumoniae. A vaccine is a substance that is prepared from killed or weakened microorganisms and is introduced into the body to protect the body against future infections by the microorganisms. Griffin worked with 2 types, or strains of S. Pneumonia. The first strain is enclosed in a capsule made of polysaccharides. The capsule protects the bacterium from the body's defense systems; this helps make the microorganisms virulent, or able to cause disease. Because of the capsule, this strain of S. Pneumonia grows as smooth-edged (S) colonies when grown in a Petri dish. The second strain of S. Pneumonia lacks the polysaccharide capsule and does not cause disease. When grown in a Petri dish, the second strain forms rough-edged R colonies Griffith knew that mice infected with the S bacteria grew sick, and died, while mice infected with the R bacteria were not harmed. To determine whether the capsule on the S bacteria were causing the mice to die, Griffith injected the mice with dead S bacteria. The mice remained healthy. Griffith then prepared a vaccine of weakened S bacteria by raising their temperature to a point at which the bacteria were “heat-killed” meaning that they could no longer reproduce ( the capsule remained on the bacteria). When Griffith injected the mice with heat-killed S bacteria, the mice still lived. Thus, Griffith knew it was not the capsule on the S bacteria that killed the mice. He then mixed the harmless live R bacteria with the harmless heat killed S bacteria. Mice injected with this mixture of previously harmless preparations died. When Griffith examined the blood of the dead mice, he found that the live R bacteria had required polysaccharide capsules. Somehow, the harmless R bacteria had changed and became virulent S...
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