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16 - 1 Griffith was working with two strains of...

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1. Griffith was working with two strains of Streptococcus pneumonia. When he mixed the remains of heat-killed pathogenic bacteria with harmless bacteria, some bacteria were changed into disease-causing bacteria. These bacteria had incorporated external genetic material in a process called transformation. Although the identity of the substance was not known, some chemical component of the dead pathogenic cells had caused the heritable change. Avery tried to identify the transforming agent by purifying chemicals from heat-killed pathogenic cells. He tried to then transform live nonpathogenic bacteria with each chemical. Avery, McCarty, and Macleod announced that DNA was the molecule that transformed the bacteria. Hershey and Chase showed that DNA was the genetic material of a phage known as T2 that infects the bacterium E. coli. They used different radioactive isotopes to tag phage DNA and protein. First they grew T2 with E. coli in the presence of the radioactive sulfur. The radioactive atoms were incorporated only into the protein of the phage. Separate samples of E. coli were infected with the differently labeled T2 cells, then blended and centrifuged to isolate the bacterial cells from the lighter viral parts. They found that when the bacteria had been
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