Download Chapter 12 Section 12_1 DNA

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)
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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
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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
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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.
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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
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•  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
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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
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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
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