Download Chapter 10: Intro to DNA

5/2/16 Chapter 10
Molecular Biology of the Gene
PowerPoint Lectures
Campbell Biology: Concepts & Connections, Eighth Edition
Biol
REECE • TAYLOR • SIMON • DICKEY • HOGAN
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1408
Dr.
Doumen
Lecture by Edward J. Zalisko
Introduction
•  The 2009 H1N1 influenza (flu) virus
•  spread so quickly that it was declared a pandemic,
•  reached 207 countries,
•  infected more than 600,000 people, and
•  killed an estimated 20,000 people.
•  Viruses share some of the characteristics of living
organisms, but are generally not considered alive
because they are not cellular and cannot
reproduce on their own.
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Introduction
•  So what makes up a virus ?
•  In general a virus is made from a
•  Capsid : this is the protein shell of a virus
•  Internal genome : usually a DNA or RNA sequence.
•  Some viruses have an additional viral envelope.
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1 5/2/16 Introduction
Examples of common viruses ( look at the size and
also see next slide for comparison)
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Introduction
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Introduction
•  Viruses can be dangerous and combating any
virus requires a detailed understanding of
•  molecular biology,
•  the study of DNA/RNA, and
•  its mode of replication, and how DNA serves as the
basis of heredity.
•  The study on how viruses work has helped
scientists in the discovery of the genetic material
and how it works in most living cells.
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2 5/2/16 SCIENTIFIC THINKING: Experiments showed
that DNA is the genetic material
•  Early in the 20th century, the molecular basis for
inheritance was a mystery.
•  Biologists did know that genes were located on
chromosomes. But it was unknown if the genetic
material was a protein based system or a nucleic
acid based system.
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SCIENTIFIC THINKING: Experiments showed
that DNA is the genetic material
•  Biologists finally established the role of DNA in
heredity through experiments with bacteria and the
viruses that infect them.
•  This breakthrough ushered in the field of
molecular biology, the study of heredity at the
molecular level.
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Frederick Griffith Experiment
•  Pneumonia was a serious cause
of death following Spanish
Influenza Pandemic (1918)
•  Frederick Griffith (1879–1941), a
British bacteriologist, wanted to
create a vaccine against
pneumonia and started working
with pneumococcus bacteria,
using mice as his ‘patients’
Streptococcus pneumoniae.
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3 5/2/16 Frederick Griffith Experiment
•  He used two strains of bacteria
•  S-strain (virulent):
•  this bacteria covers itself with a smooth capsule,
protecting itself against the host’s immune system
•  It will kill the host
•  R-strain (non-virulent):
•  Does not have a protective capsule and gets killed
by the host’s immune system.
•  The host will not die
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Frederick Griffith Experiment
•  At that time, they believed that these strains were
fixed and un-changeable: the R-strain could not
become the S-strain and visa versa
•  They also had a test to see which form was
growing in a petri-dish
•  His experiment was to inject mice with the different
strains and see what happened.
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4 5/2/16 Frederick Griffith Experiment
•  Inject mice with living S-strain bacteria
•  Result: mice die
•  Inject mice with living R-strain bacteria
•  Result : mice stay alive
•  Inject mice with heat killed S-strain
•  Result : mice stay alive
•  Inject mice with a mix of living R-strain and heat
killed S-strain
•  Result : mice die
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Frederick Griffith Experiment
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5 5/2/16 Frederick Griffith Experiment
•  When they examined the blood of the dead mice
from the last experiment, they found live S-strain
bacteria that could be culture
•  All of the descendants of the transformed bacteria
inherited the newly acquired ability to cause
disease
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Frederick Griffith Experiment
•  Griffith concluded that some transforming factor
( he called it a “transforming principle”) present in
the dead S-strain, had transformed the living Rstrain
•  So, the harmless strain was transformed into a
deadly strain because something from the heatkilled, dead bacteria strain slipped into the
harmless strain and made them virulent.
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Frederick Griffith Experiment
•  Today, we know that the "transforming principle"
Griffith observed was the DNA of the S-strain
bacteria
•  That DNA survived the heating process and was
taken up by the R-strain, providing the genes to
make the protective capsule
•  The exact nature of the transforming principle (the
DNA) was verified by later experiments (eg. The
Hershey Chase experiment)
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6 5/2/16 Hershey – Chase Experiments
•  In 1952, Alfred Hershey and Martha Chase used
bacteriophages to show that DNA is the genetic
material
•  Bacteriophages (or phages for short) are viruses that
infect bacterial cells.
•  They used a bacteriophage called of T2, a virus that
infects the bacterium Escherichia coli (E. coli).
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A bacteriophage : a virus that infects bacteria
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Reproductive Cycle of a Bacteriophage
•  The phage attaches to the cell wall of a bacterium
•  It injects its DNA into the bacterial cell
•  The viral DNA directs the destruction of the host DNA
•  Viral genes instruct the making of replicate viral DNA 1
A phage
attaches itself
to a bacterial
cell.
2
The phage
injects its
DNA into the
bacterium.
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7 5/2/16 Reproductive Cycle of a Bacteriophage
•  The viral DNA takes charge of the protein production line of
the bacteria (eg. Ribosomes).
•  Viral genes direct the protein machinery to make new viral
proteins and assemble new viruses
•  Bacterium lyses and new viruses spill out 3 The phage DNA directs the
host cell to make more
phage DNA and proteins;
new phages assemble.
4 The cell lyses and
releases the new phages.
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Figure 10.1c-0
Reproductive Cycle of a Bacteriophage
1 A phage
attaches itself
to a bacterial
cell.
2 The phage
injects its
DNA into the
bacterium.
3 The phage DNA
directs the host cell
to make more phage
DNA and proteins;
new phages assemble.
The cell lyses
and releases
the new phages.
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Hershey-Chase Experiments
•  Hershey and Chase knew their biology and
biochemistry very well to realize that sulphur is an
element found in certain amino acids
•  On the other hand, phosphorus is an element
mainly found in nucleic acids such as RNA and
DNA
•  They came up with a brilliant idea to figure out
weather protein or nucleic acids were the
biomolecule that carries the genetic information.
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8 5/2/16 Hershey-Chase Experiments
Grow viruses in a media with
radioactive sulfur
Sulfur will become
incorporated into protein
Thus the coat/shell of the
viruses will be radioactive
Grow viruses in a media with
radioactive phosphorus
Phosphorus will become
incorporated into DNA/RNA
Radioactive material will
now be inside the virus
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Hershey-Chase Experiments
•  They infected bacteria
•  with radioactive sulfur labeled viruses ( so the
radioactive element is in the protein coat)
Or
•  with radioactive phosphorus labeled viruses ( so the
radioactive element is inside in the DNA)
•  After a while, they used a simple blender to shake
off all the viruses from the bacteria and centrifuged
it all down ( bacteria would pellet down – viruses
remain in upper suspension liquid)
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•  In bacteria infected with radioactive sulfur, most radioactivity
remained in suspension liquid,
none in the bacterial pellet.
•  The new viruses that eventually
came out of the bacterial pellet
had no radioactive sulfur
•  In bacteria infected with radioactive phosphorus, most
radioactivity was found within
the bacterial pellet
•  The new viruses that eventually
came out of the bacterial pellet
contained radioactive
phosphorus
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9 5/2/16 Figure 10.1b-0
Phage
Bacterium
Radioactive
protein
Empty
protein shell
The radioactivity
is in the liquid.
Phage
DNA
DNA
Centrifuge
Pellet
Batch 1: Radioactive protein labeled in yellow
Radioactive
DNA
Centrifuge
Pellet
The radioactivity
is in the pellet.
Batch 2: Radioactive DNA labeled in green
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Hershey-Chase Experiments
The Hershey and Chase experiment strongly supported DNA as the
hereditary material while it also showed protein was NOT the hereditary
material.
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The search for the DNA structure
•  After the 1952 Hershey-Chase experiment
convinced most biologists that DNA was the
material that stored genetic information, a race was
on to determine how the structure of this molecule
could account for its role in heredity.
•  Researchers focused on discovering the threedimensional shape of DNA.
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10 5/2/16 Watson and Crick
•  American James D. Watson journeyed to
Cambridge University in England, where the more
senior Francis Crick was studying protein structure
with a technique called X-ray crystallography.
•  While visiting the laboratory of Maurice Wilkins at
King’s College in London, Watson saw an X-ray
image of DNA produced by Wilkins’s colleague,
Rosalind Franklin.
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Watson and Crick
Rosalind Franklin and her
important X-ray diffraction
analysis of DNA
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DNA is a double-stranded helix
•  Watson deduced the basic shape of DNA to be a
helix (spiral) with a uniform diameter and the
nitrogenous bases located above one another like
a stack of dinner plates.
•  The thickness of the helix suggested that it was
made up of two polynucleotide strands.
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11 5/2/16 DNA is a double-stranded helix
•  Watson and Crick realized that DNA consisted of
two polynucleotide strands wrapped into a double
helix.
•  The sugar-phosphate backbone is on the outside.
•  The nitrogenous bases are perpendicular to the
backbone in the interior.
•  Specific pairs of bases give the helix a uniform
shape.
•  A pairs with T, forming two hydrogen bonds, and
•  G pairs with C, forming three hydrogen bonds.
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Hydrogen bond
(dotted lines)
C
G
T
Sugarphosphate
backbone
A
T
A
C
Sugarphosphate
backbone
G
Pairs of nitrogenous
bases linked with
hydrogen bonds
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Figure 10.3c
Pairs of nitrogenous bases
linked with hydrogen bonds
Sugar-phosphate
backbone
Twist
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12 5/2/16 Watson and Crick
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Figure 10.3d-0
C
C
G
G
G
Hydrogen bond
C
G
C
G
A
C
Base pair
A
T
T
T
C
A
G
A
T
A
T
C
G
C
C
G
C
A
A
T
A
G
G
T
T
T
A
Ribbon model
Partial chemical structure
Computer model
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Watson and Crick
•  In 1962, the Nobel Prize was awarded to James D.
Watson, Francis Crick, and Maurice Wilkins.
•  Rosalind Franklin probably would have received the
prize as well but for her death from cancer in 1958.
•  Nobel Prizes are never awarded posthumously.
•  The Watson-Crick model gave new meaning to the
words genes and chromosomes. The genetic
information in a chromosome is encoded in the
nucleotide sequence of DNA.
•  It still wasn’t clear how it was encoded….
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13 5/2/16 DNA and RNA are polymers of nucleotides
•  DNA and RNA are nucleic acids consisting of long
chains (polymers) of chemical units (monomers) called
nucleotides.
•  A DNA nucleotide is composed of a
•  nitrogenous base,
•  five-carbon sugar, called deoxyribose
•  phosphate group.
•  The nucleotides are joined to one another by a sugarphosphate backbone.
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Figure 10.2a-3
A DNA nucleotide
Nitrogenous base
(can be A, G, C, or T)
Thymine
(T)
Phosphate
group
Sugar
(deoxyribose)
DNA nucleotide
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Sugar-phosphate
backbone
A DNA poly-nucleotide
A
A
•  A DNA nucleotide can
have 4 different nitrogencontaining base:
•  adenine (A),
•  cytosine (C),
•  thymine (T), and
•  guanine (G).
•  A DNA polynucleotide
will have those 4 bases
represented in a certain
order
Covalent
bond
joining
nucleotides
C
C
DNA
nucleotide
T
Phosphate
group
Nitrogenous
base
Sugar
T
G
G
G
G
Two representations
of a DNA polynucleotide
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14 5/2/16 The nitrogen-containing bases in DNA
Thymine (T)
Cytosine (C)
Adenine (A)
Guanine (G)
Purines
Pyrimidines
• 
• 
• 
• 
Pyrimidines have a 6 ring structure and Purines are a combination of a 5
ring with a 6 ring Each base is part of a nucleotide. They are important in the DNA structure
as they form the “spokes” of the ladder, the connections what holds the
double helix together. T on one polynucleotide strand backbone always pairs with an A on the
other strand
G on one polynucleotide strand backbone always pairs with an C on the
other strand
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A complete DNA Is a double helix
A
T
•  A complete DNA is a double
helix of two polynucleotides,
twisted around each other as
shown earlier
T
G
C
A
C
G
T
A
C
G
G
T
A
•  Notice the base pairing
between the two strands
C
G
T
T
A
A
C
G
T
A
A DNA
double helix
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10.2 DNA and RNA are polymers of
nucleotides
•  The full name for DNA is deoxyribonucleic acid,
•  RNA (ribonucleic acid) is unlike DNA in that it
•  Has nucleotides made from the the sugar ribose
(instead of deoxyribose in DNA) and
•  The nitrogenous bases are similar except that
uracil (U) is used instead of thymine (T).
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15 5/2/16 An RNA nucleotide
Nitrogenous base
(can be A, G, C, or U)
Phosphate
group
Uracil (U)
Sugar
(ribose)
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16