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Biology
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12–1 DNA
Griffith and Transformation
In 1928, British
scientist Fredrick
Griffith was
trying to learn
how certain
types of bacteria
caused
pneumonia.
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He isolated
two
different
strains of
pneumonia
bacteria
from mice
and grew
them in his
lab.
12–1 DNA
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
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Griffith and Transformation
Experiment 2:
Harmless bacteria
(rough colonies)
Lives
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Griffith and Transformation
Experiment 3:
Heat-killed diseasecausing bacteria (smooth
colonies)
Lives
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Griffith and Transformation
Experiment 4:
Dies of pneumonia
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Griffith and Transformation
Heat-killed diseasecausing bacteria
(smooth colonies)
Griffith concluded
that the heat-killed
bacteria passed
their diseasecausing ability to
the harmless
strain.
Live disease-
Harmless bacteria
(rough colonies)
causing bacteria
(smooth colonies)
Dies of pneumonia
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Griffith and Transformation
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.
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Avery and DNA
Oswald Avery repeated
Griffith’s work to determine
which molecule was most
important for transformation.
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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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What did scientists discover about the
relationship between genes and DNA?
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12–1 DNA
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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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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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
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No radioactivity
inside bacterium
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
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Radioactivity
inside
bacterium
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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What is the overall structure of the DNA
molecule?
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12–1 DNA
The Components and Structure of DNA
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
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
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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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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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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How do the 3 molecules join to form a
nucleotide?
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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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Adenine Nucleotide
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Thymine Nucleotide
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Guanine Nucleotide
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Cytosine Nucleotide
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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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Paper Lab: Joining Nucleotides of DNA
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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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