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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.
He isolated two different strains of pneumonia
bacteria from mice and grew them in his lab.
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Griffith and Transformation
Griffith made two observations:
(1) The disease-causing strain of bacteria grew
into smooth colonies on culture plates.
(2) The harmless strain grew into colonies with
rough edges.
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Griffith and Transformation
Griffith's Experiments
Griffith set up four
individual experiments.
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.
Harmless bacteria
(rough colonies)
Lives
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Griffith and Transformation
Experiment 3: Griffith
heated the diseasecausing bacteria. He then
injected the heat-killed
bacteria into the mice.
The mice survived.
Heat-killed diseasecausing bacteria (smooth
colonies)
Lives
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Griffith and Transformation
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)
Harmless bacteria
(rough colonies)
Live diseasecausing bacteria
(smooth colonies)
Dies of pneumonia
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Griffith and Transformation
Griffith concluded that
the heat-killed bacteria
passed their diseasecausing ability to the
harmless strain.
Heat-killed diseasecausing bacteria
(smooth colonies)
Harmless bacteria
(rough colonies)
Live diseasecausing bacteria
(smooth colonies)
Dies of pneumonia
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Griffith and Transformation
Transformation
Griffith called this process transformation
because one strain of bacteria (the harmless strain)
had changed permanently into another (the
disease-causing strain).
Griffith hypothesized that a factor must contain
information that could change harmless bacteria
into disease-causing ones.
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Avery and DNA
Avery and DNA
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
The enzymes destroyed proteins, lipids,
carbohydrates, and other molecules, including the
nucleic acid RNA.
Transformation still occurred.
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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.
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Avery and DNA
What did scientists discover about the
relationship between genes and DNA?
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Avery and DNA
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.
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The Hershey-Chase Experiment
The Hershey-Chase Experiment
Alfred Hershey and Martha Chase studied
viruses—nonliving particles smaller than a cell that
can infect living organisms.
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The Hershey-Chase Experiment
Bacteriophages
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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The Hershey-Chase Experiment
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.
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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
If 35S was found in the bacteria, it would mean that
the viruses’ protein had been injected into the
bacteria.
Bacteriophage with
sulfur-35 in protein coat
Phage infects
bacterium
No radioactivity
inside 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
Phage infects
bacterium
Radioactivity
inside bacterium
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The Hershey-Chase Experiment
Nearly all the radioactivity in the bacteria
was from phosphorus (32P).
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
What is the overall structure of the DNA
molecule?
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The Components and Structure of DNA
The Components and Structure of DNA
DNA is made up of nucleotides.
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
There are four
kinds of bases in
in DNA:
• adenine
• guanine
• cytosine
• thymine
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The Components and Structure of DNA
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.
• The percentages of adenine [A] and thymine
[T] bases are almost equal in any sample of
DNA.
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The Components and Structure of DNA
X-Ray Evidence
Rosalind Franklin used X-ray
diffraction to get information
about the structure of DNA.
She aimed an X-ray beam at
concentrated DNA samples
and recorded the scattering
pattern of the X-rays on film.
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The Components and Structure of DNA
The Double Helix
Using clues from Franklin’s pattern, James
Watson and Francis Crick built a model that
explained how DNA carried information and
could be copied.
Watson and Crick's model of DNA was a
double helix, in which two strands were
wound around each other.
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The Components and Structure of
DNA
DNA Double Helix
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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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Avery and other scientists discovered that
a. DNA is found in a protein coat.
b. DNA stores and transmits genetic
information from one generation to the next.
c. transformation does not affect bacteria.
d. proteins transmit genetic information from
one generation to the next.
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The Hershey-Chase experiment was based on
the fact that
a. DNA has both sulfur and phosphorus in its
structure.
b. protein has both sulfur and phosphorus in
its structure.
c. both DNA and protein have no phosphorus
or sulfur in their structure.
d. DNA has only phosphorus, while protein
has only sulfur in its structure.
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DNA is a long molecule made of monomers
called
a. nucleotides.
b. purines.
c. pyrimidines.
d. sugars.
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Chargaff's rules state that the number of
guanine nucleotides must equal the number of
a. cytosine nucleotides.
b. adenine nucleotides.
c. thymine nucleotides.
d. thymine plus adenine nucleotides.
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In DNA, the following base pairs occur:
a. A with C, and G with T.
b. A with T, and C with G.
c. A with G, and C with T.
d. A with T, and C with T.
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