Download Ch 12.1

12–1 DNA
Biology
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12–1 DNA
Essential Question
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Griffith and Transformation
Griffith and Transformation
In 1928, British scientist Fredrick Griffith was trying
to learn how certain types of bacteria caused
pneumonia.
What is the overall
structure of DNA?
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He isolated two different strains of pneumonia
bacteria from mice and grew them in his lab.
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Griffith and Transformation
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Griffith and Transformation
Griffith's Experiments
Griffith made two observations:
(1) The disease-causing strain of bacteria grew
into smooth colonies on culture plates.
Griffith set up four
individual experiments.
(2) The harmless strain grew into colonies with
rough edges.
Experiment 1: Mice
were injected with the
disease-causing strain
of bacteria. The mice
developed pneumonia
and died.
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Griffith and Transformation
Experiment 2: Mice were
injected with the harmless
strain of bacteria. These
mice didn’t get sick.
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Experiment 3: Griffith
heated the diseasecausing bacteria. He then
injected the heat-killed
bacteria into the mice.
The mice survived.
Harmless bacteria
(rough colonies)
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Griffith and Transformation
Harmless bacteria
(rough colonies)
Griffith and Transformation
Griffith concluded that
the heat-killed bacteria
passed their diseasecausing ability to the
harmless strain.
Live diseasecausing bacteria
(smooth colonies)
Heat-killed diseasecausing bacteria
(smooth colonies)
Harmless bacteria
(rough colonies)
Live diseasecausing bacteria
(smooth colonies)
Dies of pneumonia
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Heat-killed diseasecausing bacteria
(smooth colonies)
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Lives
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Experiment 4: Griffith
mixed his heat-killed,
disease-causing bacteria
with live, harmless
bacteria and injected the
mixture into the mice.
The mice developed
pneumonia and died.
Heat-killed diseasecausing bacteria (smooth
colonies)
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Lives
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Griffith and Transformation
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Dies of pneumonia
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Griffith and Transformation
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Transformation
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Avery and DNA
Avery and DNA
Griffith called this process transformation because
one strain of bacteria (the harmless strain) had
changed permanently into another (the diseasecausing strain).
Griffith hypothesized that a factor must contain
information that could change harmless bacteria
into disease-causing ones.
Oswald Avery repeated Griffith’s work to determine
which molecule was most important for
transformation.
Avery and his colleagues made an extract from the
heat-killed bacteria that they treated with enzymes.
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Avery and DNA
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The enzymes destroyed proteins, lipids,
carbohydrates, and other molecules, including the
nucleic acid RNA.
Avery and DNA
Avery and other scientists repeated the experiment
using enzymes that would break down DNA.
When DNA was destroyed, transformation did not
occur. Therefore, they concluded that DNA was the
transforming factor.
Transformation still occurred.
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Avery and DNA
12–1 DNA
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12–1 DNA
Avery and DNA
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The Hershey-Chase Experiment
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The Hershey-Chase Experiment
Bacteriophages
The Hershey-Chase Experiment
Alfred Hershey and Martha Chase studied viruses
—nonliving particles smaller than a cell that can
infect living organisms.
A virus that infects bacteria is known as a
bacteriophage.
Bacteriophages are composed of a DNA or RNA
core and a protein coat.
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Avery and other scientists discovered
that the nucleic acid DNA stores and
transmits the genetic information from
one generation of an organism to the
next.
What did scientists discover about the
relationship between genes and DNA?
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The Hershey-Chase Experiment
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When a bacteriophage enters a bacterium, the
virus attaches to the surface of the cell and injects
its genetic information into it.
The viral genes produce many new
bacteriophages, which eventually destroy the
bacterium.
When the cell splits open, hundreds of new viruses
burst out.
The Hershey-Chase Experiment
If Hershey and Chase could determine which part
of the virus entered an infected cell, they would
learn whether genes were made of protein or DNA.
They grew viruses in cultures containing
radioactive isotopes of phosphorus-32 (32P) and
sulfur-35 (35S).
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The Hershey-Chase Experiment
Phage infects
bacterium
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The Hershey-Chase Experiment
If 32P was found in the bacteria, then it was the DNA
that had been injected.
Bacteriophage with
phosphorus-32 in DNA
No radioactivity
inside bacterium
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12–1 DNA
If 35S was found in the bacteria, it would mean that
the viruses’ protein had been injected into the
bacteria.
Bacteriophage with
suffur-35 in protein coat
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Phage infects
bacterium
Radioactivity
inside bacterium
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The Hershey-Chase Experiment
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The Components and Structure of DNA
Nearly all the radioactivity in the bacteria
was from phosphorus (32P).
What is the overall structure of the DNA
molecule?
Hershey and Chase concluded that the
genetic material of the bacteriophage
was DNA, not protein.
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The Components and Structure of DNA
12–1 DNA
The Components and Structure of DNA
There are four
kinds of bases in
in DNA:
The Components and Structure of DNA
DNA is made up of nucleotides.
•  adenine
•  guanine
•  cytosine
•  thymine
A nucleotide is a monomer of nucleic acids made
up of a five-carbon sugar called deoxyribose, a
phosphate group, and a nitrogenous base.
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The Components and Structure of DNA
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The backbone of a DNA chain is formed by sugar and
phosphate groups of each nucleotide.
The nucleotides can be joined together in any order.
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The Components and Structure of DNA
Chargaff's Rules
Erwin Chargaff discovered that:
•  The percentages of guanine [G] and cytosine
[C] bases are almost equal in any sample of
DNA. [G] = [C]
•  The percentages of adenine [A] and thymine
[T] bases are almost equal in any sample of
DNA. [A] = [T]
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The Components and Structure of DNA
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X-Ray Evidence
The Components and Structure of DNA
The Double Helix
Rosalind Franklin used X-ray
diffraction to get information
about the structure of DNA.
Using clues from Franklin’s pattern, James
Watson and Francis Crick built a model that
explained how DNA carried information and
could be copied.
She aimed an X-ray beam at
concentrated DNA samples
and recorded the scattering
pattern of the X-rays on film.
Watson and Crick's model of DNA was a
double helix, in which two strands were
wound around each other.
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12–1 DNA
The Components and Structure of
DNA
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DNA Double Helix
The Components and Structure of
DNA
Watson and Crick discovered that hydrogen bonds
can form only between certain base pairs—adenine
and thymine, and guanine and cytosine.
This principle is called base pairing.
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