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3/23/15 CHAPTER 12: DNA 12.1 – Identifying the Substance of Genes A. Bacterial Transformation B. Bacterial Viruses Mr. M. Varco ‘07 Saint Joseph High School Bacterial TransformaIon • In 1928, British scientist Frederick Griffith was searching for why bacteria produced lung infections • Griffith isolated two strains of the same bacteria, noting that only one caused illness (S-strain), while the other did not (R-strain) 1 3/23/15 Bacterial TransformaIon Griffith’s Experiment • When Griffith injected mice with disease-causing bacteria, the mice developed pneumonia and died. • When he injected mice with harmless bacteria, the mice stayed healthy. • Perhaps the S-strain bacteria produced a toxin that caused illness? Bacterial TransformaIon Griffith’s Experiment • First, Griffith took a culture of S-strain, heated the cells to kill them, and then injected the heat-killed bacteria into mice • The mice survived, indicating that the cause of pneumonia was not a toxin from the disease-causing bacteria 2 3/23/15 Bacterial TransformaIon Griffith’s Experiment • Next, Griffith mixed heat-killed, S-strain bacteria with live, harmless R-strain bacteria. • Surprisingly, the injected mice developed pneumonia and many died • The lungs of these mice were filled with the disease-causing bacteria Bacterial TransformaIon 3 3/23/15 Bacterial TransformaIon Griffith’s Experiment • Some chemical factor had transferred from the heat-killed, disease causing cells of the S-strain to live cells of the R-strain • He called this process transformation, because one type of bacteria had been changed into another Bacterial TransformaIon Molecular Cause of Transformation • In 1944, a group of scientists led by Oswald Avery wanted to learn which molecule in the heat-killed bacteria resulted in transformation • Despite extracting various molecules from the heat-killed bacteria, transformation still occurred. 4 3/23/15 Bacterial TransformaIon Molecular Cause of Transformation • Avery’s team repeated the experiment, using enzymes to break down a different nucleic acid – DNA • When DNA was destroyed, transformation did not occur, indicating that DNA was the transforming factor. • Avery concluded that DNA stores and transmits genetic information from one generation of bacteria to another Bacterial Viruses • What role did viruses have in the identification of genetic material? • A bacteriophage is a kind of virus that infects bacteria by attaching to the surface of the bacterial cell and injecting its genetic material 5 3/23/15 Bacterial Viruses • The viral genes act to produce many new bacteriophages, which gradually destroy the bacterium. • When the cell splits open, hundreds of new viruses burst out. Bacterial Viruses Hershey-Chase Experiment • Hershey and Chase studied a bacteriophage composed of a DNA core and a protein coat. • The pair grew viruses containing the isotopes phosphorus-32 and sulfur-35, as proteins contain almost no phosphorus, and DNA contains almost no sulfur 6 3/23/15 Bacterial Viruses Hershey-Chase Experiment • The two scientists mixed the marked viruses in with bacterial cells, waiting for the virus to inject its genetic material • Next, they separated the viruses from the bacteria and tested the bacteria for radioactivity Bacterial Viruses Hershey-Chase Experiment • Results indicated that all of the radioactivity found in the cell contained phosphorous, the marker found in DNA • The experiment confirmed Avery’s findings that DNA was the genetic material found in genes, whether a virus, bacteria, or other cell 7 3/23/15 Role of DNA What is the role of DNA in heredity? Storing Information • The DNA that makes up genes must be capable of storing genetic information of a cell • This information is accessed and acts as instructions for particular traits and functions Role of DNA Copying Information • Before a cell divides, it must make a complete copy of every one of its genes Transmitting Information • DNA molecules must be carefully sorted and passed along during cell division, as the loss of DNA = the loss of genetic information 8