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Copyright Pearson Prentice Hall
12–1 DNA
12–1 DNA
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Copyright Pearson Prentice Hall
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.
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 disease-
causing bacteria. He then
injected the heat-killed
bacteria into the mice.
The mice survived.
Heat-killed disease-
causing 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.
Live disease-
causing bacteria
(smooth colonies)
Dies of pneumonia
Heat-killed disease-
causing bacteria
(smooth colonies)
Harmless bacteria
(rough colonies)
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Griffith and Transformation
Griffith concluded that the heat-killed bacteria passed their disease-causing ability to the harmless strain.
Live disease-
causing bacteria
(smooth colonies)
Heat-killed disease-
causing bacteria
(smooth colonies)
Harmless bacteria
(rough 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.
Transformers!
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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.
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.
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.
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 viruses burst out.
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.
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 joined
together in any order.
Chargaff's Rules
• The percentages of guanine [G] and cytosine
[C] bases are almost equal in any sample of
DNA. So G and C pair up!
• The percentages of adenine [A] and thymine
[T] bases are almost equal in any sample of
DNA. So A and T pair up!
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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.
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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The Components and Structure of
DNA
DNA Double Helix
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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.