Transcript Section 12-1: Identifying The Substance of Genes

Chapter 12
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SECTION 12-1: IDENTIFYING THE
SUBSTANCE OF GENES
How do genes work?
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 First step was to identify what molecule or molecules
go into making a gene
Identifying the Molecule of Heredity
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 Scientists had to discover the chemical nature of the
gene
 Identifying the molecule that carries genetic
information would help us understand how genes
control the inherited characteristics of living things
 Began in 1928 with British scientist Frederick
Griffith, who was trying to figure out how certain
types of bacteria produce pneumonia
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Griffith’s Experiments
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 Griffith isolated two different strains of the same bacterial
species - both strains grew very well in culture, but only one
of the strains caused pneumonia
 The disease-causing bacteria (S strain) grew into smooth
colonies on culture plates, whereas the harmless bacteria
(R strain) produced colonies with rough edges
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Griffith’s Experiments
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 Griffith injected mice with disease-causing bacteria
 mice developed pneumonia and died
 Injected mice with harmless bacteria  mice stayed
healthy
 Perhaps the S-strain bacteria produced a toxin that
made the mice sick?
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Griffith’s Experiments
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 Griffith took a culture of the S strain, heated the cells to kill
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them, and then injected the heat-killed bacteria into
laboratory mice
Mice survived - cause of pneumonia was not a toxin from
these disease-causing bacteria
He mixed the heat-killed, S-strain bacteria with live,
harmless bacteria from the R strain and injected the
mixture into mice
Mice developed pneumonia, and many died
Lungs of these mice were filled with the disease-causing
bacteria
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Transformation
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 Some chemical factor transferred from the heat-killed cells of
the S strain into the live cells of the R strain changed harmless
bacteria into disease-causing bacteria
 Griffith called this process transformation, because one type
of bacteria had been changed permanently into another
 Ability to cause disease was inherited by the offspring of the
transformed bacteria  concluded that the transforming factor
had to be a gene
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Summary of Griffith’s Experiments
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The Molecular Cause of Transformation
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 Group of scientists at the Rockefeller Institute in
New York, led by the Canadian biologist Oswald
Avery, researched which molecule in the heat-killed
bacteria was most important for transformation
 Avery and his team treated heat-killed bacteria with
enzymes that destroyed proteins, lipids,
carbohydrates, and some other molecules, including
the nucleic acid RNA
 Transformation still occurred
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The Molecular Cause of Transformation
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 Repeated the experiment using enzymes that would
break down DNA
 When they destroyed the DNA in the mixture,
transformation did not occur
 Therefore, DNA was the transforming factor
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Bacteriophages
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 When a bacteriophage enters a bacterium, it attaches
to the surface of the bacterial cell and injects its
genetic information into it
 Viral genes produce many new bacteriophages,
which gradually destroy the bacterium
 When the cell splits open, hundreds of new viruses
burst out
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Bacterial Viruses
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 Alfred Hershey and Martha Chase performed the most important
of the experiments relating to Avery’s discovery using viruses—
nonliving particles that can infect living cells
 The kind of virus that infects bacteria is known as a
bacteriophage, which means “bacteria eater”
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The Hershey-Chase Experiment
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 They studied a bacteriophage that is composed of a DNA core
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and a protein coat
Wanted to determine which part of the virus entered the
bacterial cell – attempting to prove or disprove Avery
Hershey and Chase grew viruses in cultures containing
radioactive isotopes of phosphorus-32 (P-32) and sulfur-35
(S-35)
Proteins contain almost no phosphorus and DNA contains no
sulfur
Radioactive isotopes used as markers – could trace which
molecule entered cell
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The Hershey-Chase Experiment
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The Hershey-Chase Experiment
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 Mixed the marked viruses with bacterial cells
 Tested the bacteria for radioactivity
 Bacterial radioactivity was from phosphorus P-32 , the marker
found in DNA
 Concluded that the genetic material of the bacteriophage was
DNA, not protein
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The Role of DNA
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 Storing information: Genes control patterns of
development – each cell has instructions for that
organism written in its DNA
 Copying information: Before a cell divides, it makes
a complete copy of its genes – once DNA structure was
figured out, its copying method became clear
 Transmitting information: Each daughter cell
must get a complete set of genes during cell division,
and sorting of genes is important in meiosis – nothing
can be lost without consequences
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