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DNA: The Molecular Basis of
Inheritance
PowerPoint Lectures for
Biology, Seventh Edition
Neil Campbell and Jane Reece
Lectures by Chris Romero
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Overview: Life’s Operating Instructions
• In 1953, James Watson and Francis Crick
shook the world
– With an elegant double-helical model for the
structure of deoxyribonucleic acid, or DNA
Figure 16.1
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DNA, the substance of inheritance
– Is the most celebrated molecule of our time
• Hereditary information
– Is encoded in the chemical language of DNA
and reproduced in all the cells of your body
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
The Search for the Genetic Material: Scientific Inquiry
• The role of DNA in heredity
– Was first worked out by studying bacteria and
the viruses that infect them
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Evidence That DNA Can Transform Bacteria
• Frederick Griffith was studying Streptococcus
pneumoniae
– A bacterium that causes pneumonia in
mammals
• He worked with two strains of the bacterium
– A pathogenic strain and a nonpathogenic
strain
Pathogenic- capable of causing disease
Nonpathogenic- incapable of causing disease
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Results from Griffith’s experiments.
When he mixed heat-killed remains of the
pathogenic strain with living cells of the
nonpathogenic strain, some of these living
cells became pathogenic
Bacteria of the “S” (smooth) strain of Streptococcus pneumoniae are pathogenic because they
have a capsule that protects them from an animal’s defense system. Bacteria of the “R” (rough) strain lack a capsule
and are nonpathogenic. Frederick Griffith injected mice with the two strains as shown below:
EXPERIMENT
Living S
(control) cells
Living R
Heat-killed
(control) cells (control) S cells
Mixture of heat-killed S cells
and living R cells
RESULTS
Mouse dies
Mouse healthy
Mouse healthy
Mouse dies
Living S cells
are found in
blood sample.
CONCLUSION
Griffith concluded that the living R bacteria had been transformed into pathogenic S bacteria by
an unknown, heritable substance from the dead S cells.
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Tranformation
• Griffith called the phenomenon transformation
– Now defined as a change in genotype and
phenotype due to the assimilation or
integration of external DNA by a cell
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Evidence That Viral DNA Can Program Cells
• Additional evidence for DNA as the genetic
material
– Came from studies of a virus that infects
bacteria
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Viruses that infect bacteria, bacteriophages
– Are widely used as tools by researchers in
molecular genetics
Phage
head
Tail
Tail fiber
Figure 16.3
Bacterial
cell
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100 nm
DNA
Additional Evidence That DNA Is the Genetic Material
• Prior to the 1950s, it was already known that DNA
– Is a polymer of nucleotides, each consisting of
three components: a nitrogenous base, a
sugar, and a phosphate group
Sugar-phosphate
backbone
5 end
O– 5
O P O CH2
O 1
O– 4 H
H
H
H
2
3
H
O
O P O CH2 O
O–
H
H
H
H
H
O
O P O CH2 O
O–
H
H
H
H
H
Figure 16.5
O 5
O P O CH2
O 1
O– 4 H
H
PhosphateH
H
3 2
OH H
Sugar (deoxyribose)
3 end
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Nitrogenous
bases
CH3
O
H
N
N
H
O
Thymine (T)
H
N
H
N
N
N
N
H
H
Adenine (A)
H H
H
N H
N
N
O
Cytosine (C)
H
N
N
N
O
N H
N H
H
Guanine (G)
DNA nucleotide
Maurice Wilkins and Rosalind Franklin
– Were using a technique called X-ray
crystallography to study molecular structure
• Rosalind Franklin
– Produced a picture of the DNA molecule using
this technique
Figure 16.6 a, b
(a) Rosalind Franklin
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(b) Franklin’s X-ray diffraction
Photograph of DNA
Watson and Crick deduced that DNA was a double
helix
– Through observations of the X-ray
crystallographic images of DNA
G
C
A
T
T
A
1 nm
C
G
C
A
T
G
C
T
A
T
A
A
T
T
A
G
A
Figure 16.7a, c
3.4 nm
G
C
0.34 nm
T
(a) Key features of DNA structure
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(c) Space-filling model
DNA double- helical structure
• DNA
– composed of two antiparallel sugar-phosphate
backbones, with the nitrogenous bases paired
in the molecule’s interior
• The nitrogenous bases
– Are paired in specific combinations: adenine
with thymine, and cytosine with guanine
– A goes with T, and C goes with G
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Hydrogen Bonding between bases:
5 end
O
OH
Hydrogen bond
P
–O
3 end
OH
O
O
A
T
O
O
O
CH2
P
–O
O
O
H2C
O
–O
O
O
G
O
C
O
O
CH2
P
O
O
O–
P
H2C
O
O
C
O
G
O
O
O
CH2
P
–O
O–
P
O
O
O–
P
H2C
O
O
A
O
T
O
CH2
OH
3 end
O
O–
P
O
(b) Partial chemical structure
Figure 16.7b
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
O
5 end
Purine- A & G
Pyramidine- C & T
H
N
N
N
N
Sugar
O
H
H
CH3
N
N
N
O
Sugar
Thymine (T)
Adenine (A)
H
O
N
N
Sugar
N
H
N
N
N
N
N
Figure 16.8
H
H
Guanine (G)
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H
O
Sugar
Cytosine (C)
The Basic Principle: Base Pairing to a Template Strand
• Since the two strands of DNA are
complementary
– Each strand acts as a template for building a
new strand in replication
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In DNA replication
– The parent molecule unwinds, each strand is
now a template, and two new daughter strands
are built based on base-pairing rules.
T
A
T
A
T
A
C
G
C
G
C
T
A
T
A
T
A
A
T
A
T
A
T
G
C
G
C
G
C
G
A
T
A
T
A
T
C
G
C
G
C
G
T
A
T
A
T
A
T
A
T
A
T
C
G
C
G
C
A
G
(a) The parent molecule has two
complementary strands of DNA.
Each base is paired by hydrogen
bonding with its specific partner,
A with T and G with C.
(b) The first step in replication is
separation of the two DNA
strands.
Figure 16.9 a–d
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(c) Each parental strand now
serves as a template that
determines the order of
nucleotides along a new,
complementary strand.
(d) The nucleotides are connected
to form the sugar-phosphate
backbones of the new strands.
Each “daughter” DNA
molecule consists of one parental
strand and one new strand.
DNA Replication: A Closer Look
• The copying of DNA
– Is remarkable in its speed and accuracy
• More than a dozen enzymes and other proteins
– Participate in DNA replication
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Getting Started: Origins of Replication
• The replication of a DNA molecule
– Begins at special sites called origins of
replication, where the two strands are
separated
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Elongating a New DNA Strand
• Elongation of new DNA at a replication fork
– Is catalyzed by enzymes called DNA
polymerases, which add nucleotides from the
5’ to 3’ direction on the new strand.
New strand
5 end
Sugar
Template strand
3 end
A
Base
Phosphate
T
A
T
C
G
C
G
G
C
G
C
A
T
A
OH
Pyrophosphate 3 end
P
OH
Figure 16.13
3 end
5 end
Nucleoside
triphosphate
C
P
C
2 P
5 end
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5 end
DNA polymerases add nucleotides to complete
strands
DNA Polymerase can
synthesize a
complementary strand
continuously, moving
toward the replication
fork
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The DNA Replication Machine as a Stationary Complex
• The various proteins that participate in DNA
replication
– Form a single large complex, a DNA replication
“machine”
• The DNA replication machine
– Is probably stationary during the replication
process
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Proofreading and Repairing DNA
• DNA polymerases proofread newly made DNA
replacing any incorrect nucleotides
• In mismatch repair of DNA repair enzymes
correct errors in base pairing
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Telomeres:
•
http://learn.genetics.utah.edu/content/begin/traits/telomeres/
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Eukaryotic chromosomal DNA molecules
– Have at their ends nucleotide sequences,
called telomeres, that postpone the erosion of
genes near the ends of DNA molecules
Figure 16.19
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