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12–1 DNA
12–1 DNA
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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Station 1 Puzzle:
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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.
*liquefied cells that allow all of the molecules to be
exposed instead of protected by the cell membrane.
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Avery and DNA
The enzymes destroyed proteins, lipids,
carbohydrates, and other molecules, including the
nucleic acid RNA.
Transformation still occurred.
This shocked many scientists, because many thought
proteins would be the material for heredity since it is
complex and widely found in the bodies of all living
things.
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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
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.
12–1 DNA
Station 2 Puzzle:
12–1 DNA
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
suffur-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.
12–1 DNA
Station 3 Puzzle:
In a one-story pink house, there was a pink
person, a pink cat, a pink fish, a pink
computer, a pink chair, a pink table, a pink
telephone, a pink shower– everything was
pink!
What color were the stairs?
12–1 DNA
The Components and Structure of DNA
Chargaff's Rules
Erwin Chargaff discovered that:
• the number of adenine (A) bases always equals the
number of thymine (T) bases;
• the number of guanine (G) bases always equals the
number of cytosine (C) bases;
• the number of purines (A+G) always equals the number of
pyrimidines (T+C) — this rule is an obvious consequence
of rules 1 and 2.
Rumor has it that he met with Watson and Crick at the bar on
Cambridge University’s campus, where after a few drinks he
explained the above to them.
Watson and Crick did not credit him for the help.
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Station 5 Puzzle:
If a strip of DNA is made up of 30% Adenine,
how much Guanine is there?
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The Components and Structure of DNA
X-Ray Evidence
King’s college scientists
Maurice Wilkins and Rosalind
Franklin were using X-ray
Crystalography to study DNA
and to determine the structure
of DNA.
Rosalind Franklin took the
photo on the top right. The X
shape means that DNA is
helical.
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Watson managed to view Franklin’s photo; some
say through nefarious means, others say that it
was freely available information.
It is safe to assume that without the photo,
Watson and Crick would not have discovered
the structure of DNA.
All involved in the search for the structure were
awarded the nobel prize, except Rosalind
Franklin, because she had passed away prior
to award, and thus was ineligible due to being
dead.
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Station 4 Puzzle:
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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.
12–1 DNA
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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The Components and Structure of DNA
The Components and Structure of DNA
DNA is made up of nucleotides.
nucleotide = a five-carbon sugar [deoxyribose]
a phosphate group
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.
Sugar Phosphate
Backbone
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The Components and Structure of
DNA
DNA Double Helix
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Station 6 Puzzle: How Many triangles are there?
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