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Chapter 16
Molecular Basis of Inheritance
PowerPoint Lectures for
Campbell Biology: Concepts & Connections, Seventh Edition
Reece, Taylor, Simon, and Dickey
© 2012 Pearson Education, Inc.
Lecture by Edward J. Zalisko
Figure 10.0_1
Chapter 16: Big Ideas
The Structure of the
Genetic Material
DNA Replication
The Flow of Genetic
Information from DNA to
RNA to Protein
The Genetics of Viruses
and Bacteria
THE STRUCTURE OF THE
GENETIC MATERIAL
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SCIENTIFIC DISCOVERY: Experiments that
showed that DNA is the genetic material
 In 1928, Frederick Griffith discovered that a
“transforming factor” could be transferred into a
bacterial cell. He found that
– when he exposed heat-killed pathogenic bacteria to
harmless bacteria, some harmless bacteria were
converted to disease-causing bacteria and
– the disease-causing characteristic was inherited by
descendants of the transformed cells.
– Used two strains of Strepcoccus pneumoniae Rough
(R) coat = harmless and Smooth (S) coat = harmful
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SCIENTIFIC DISCOVERY: Experiments that showed
that DNA is the genetic material
Conclusion: The rough strain was transformed to
smooth strain in the mouse body. How??
SCIENTIFIC DISCOVERY: Experiments that showed
that DNA is the genetic material
 Avery, McCarty and
MacLeod
 Separated and purified
the S cell contents
 Mixed with live R cells
and injected into mice
 Only the mice with DNA
extract died
SCIENTIFIC DISCOVERY: Experiments that
showed that DNA is the genetic material
 Until the 1940s, the case for proteins serving as
the genetic material was stronger than the case
for DNA.
– Proteins are made from 20 different amino acids.
– DNA was known to be made from just four kinds of
nucleotides.
 Scientists were still skeptical about these results
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SCIENTIFIC DISCOVERY: Experiments
showed that DNA is the genetic material
 In 1952, Alfred Hershey and Martha Chase used
bacteriophages to show that DNA is the genetic
material of T2, a virus that infects the bacterium
Escherichia coli (E. coli).
– Bacteriophages (or phages for short) are viruses that
infect bacterial cells.
– Phages were labeled with radioactive sulfur to detect
proteins or radioactive phosphorus to detect DNA.
– Bacteria were infected with either type of labeled phage to
determine which substance was injected into cells and
which remained outside the infected cell.
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Figure 10.1A
Life cycle of the bacteriophage
Head
Tail
Tail fiber
DNA
Figure 10.1C
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.
4 The cell lyses
and releases
the new phages.
10.1 SCIENTIFIC DISCOVERY: Experiments
showed that DNA is the genetic material
– The sulfur-labeled protein stayed with the phages outside
the bacterial cell, while the phosphorus-labeled DNA was
detected inside cells.
– Cells with phosphorus-labeled DNA produced new
bacteriophages with radioactivity in DNA but not in protein.
Animation: Hershey-Chase Experiment
Animation: Phage T2 Reproductive Cycle
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Figure 10.1B_1
Phage
Empty
protein shell
Radioactive
protein
Bacterium
DNA
Batch 1:
Radioactive
protein
labeled in
yellow
2
1
Batch 2:
Radioactive
DNA labeled
in green
Phage
DNA
Radioactive
DNA
Figure 10.1B_2
Empty
protein shell
The radioactivity
is in the liquid.
Phage
DNA
Centrifuge
Pellet
3
4
Centrifuge
Pellet
The radioactivity
is in the pellet.
Figure 10.1B
Phage
Empty
protein shell
Radioactive
protein
Bacterium
Phage
DNA
DNA
Batch 1:
Radioactive
protein
labeled in
yellow
The radioactivity
is in the liquid.
Centrifuge
Pellet
1
Batch 2:
Radioactive
DNA labeled
in green
2
3
4
Radioactive
DNA
Centrifuge
Pellet
The radioactivity
is in the pellet.
Figure 10.0_2
DNA is the genetic material
What does DNA look like and how does it work as
genetics material?
 DNA and RNA are nucleic acids.
 One of the two strands of DNA is a DNA polynucleotide,
a nucleotide polymer (chain).
 A nucleotide is composed of a
– nitrogenous base,
– five-carbon sugar, and
– phosphate group.
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Figure 10.2A_3
Nitrogenous base
(can be A, G, C, or T)
Thymine (T)
Phosphate
group
Sugar
(deoxyribose)
DNA nucleotide
Figure 10.2B
Thymine (T)
Cytosine (C)
Pyrimidines
Guanine (G)
Adenine (A)
Purines
DNA are polymers of nucleotides
 Each type of DNA
nucleotide has a
different nitrogencontaining base:
 Chargaff isolated and
quantified the amount of
A, T, G, C from cells
and found
– adenine (A),
 Quantity of A=T
– cytosine (C),
 Quantity of G=C
– thymine (T), and
– guanine (G).
 Known as Chargaff’s
rules
Animation: DNA and RNA Structure
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DNA are polymers of nucleotides
 More evidence for DNA as the genetic material
– DNA doubles in cells getting ready for cell division
– A haploid cell has half as much DNA as a diploid cell
 Race to discover the DNA structure was on!
– Contenders:
– Wilkins and Franklin of England
– Linus Pauling of USA
– Watson and Crick of ……
Discovery of DNA Structure
 World renowned X- ray
crystallography expert
 X-rays of DNA
 Information can be used
to find out the width of
molecule
 Spacing between bases

10.3 SCIENTIFIC DISCOVERY: DNA is a
double-stranded helix
 In 1953, James D. Watson and Francis Crick
deduced the secondary structure of DNA, using
– X-ray crystallography data of DNA from the work of
Rosalind Franklin and Maurice Wilkins and
– Chargaff’s observation that in DNA,
– the amount of adenine was equal to the amount of thymine
and
– the amount of guanine was equal to that of cytosine.
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10.3 SCIENTIFIC DISCOVERY: DNA is a
double-stranded helix
 Watson and Crick reported 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.
– The two strands are anti-parallel
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Figure 10.3B
Watson and …..
Circa 2004
Figure 10.2A
T
A
C
T
G
Sugar-phosphate
backbone
A
C
G
T
A
C
G
A
G
T
Covalent
bond
joining
nucleotides
T
C
A
C
A
C
A
A
G
T
Phosphate
group
Nitrogenous
base
Nitrogenous base
(can be A, G, C, or T)
Sugar
C
G
T
A
A DNA
double helix
DNA
nucleotide
T
Thymine (T)
T
Phosphate
group
G
G
G
G
Sugar
(deoxyribose)
DNA nucleotide
Two representations
of a DNA polynucleotide
Figure 10.2A_2
Sugar-phosphate
backbone
A
A
Covalent
bond
joining
nucleotides
C
DNA
nucleotide
T
Nitrogenous
base
Sugar
C
T
G
G
G
G
Two representations
of a DNA polynucleotide
Phosphate
group
Figure 10.3D_2
Hydrogen bond
G
T
C
A
A
C
T
G
Partial chemical
structure
Figure 10.3C
Twist
10.3 SCIENTIFIC DISCOVERY: DNA is a
double-stranded helix
 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.
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DNA REPLICATION
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10.4 DNA replication depends on specific base
pairing
 In their description of the structure of DNA,
Watson and Crick noted that the structure of DNA
suggests a possible copying mechanism.
 DNA replication follows a semiconservative
model.
– The two DNA strands separate.
– Each strand is used as a pattern to produce a
complementary strand, using specific base pairing.
– Each new DNA helix has one old strand with one new
strand.
Animation: DNA Replication Overview
© 2012 Pearson Education, Inc.
How does the DNA make a copy of itself - Replication
A. Conservative: The
parent DNA strands
remain together after
replication and the
daughter DNA consists of
new strands
B. Semi-conservative:
Each new DNA consists of
a parent and a new strand
C. Dispersive: Each
daughter DNA consists
of a mixture of parent
and daughter DNA
Meselson and Stahl Experiment with Light and
Heavy Nitrogen
Grew E. coli in heavy
nitrogen N15 till all DNA
showed as a heavy band
when centrifuged
Grew for second
generation in N14
 Transferred to N14 and
grew for one generation
Grew for the third
generation in N14
 Prediction?
Prediction for each type of  Prediction?
replication?
Meselson and Stahl Experiment with Light and
Heavy Nitrogen
Figure 10.4A_s1
A
T
C
G
G
C
A
T
T
A
A parental
molecule
of DNA
Figure 10.4A_s2
T
A
T
A
C
G
C
G
C
G
C
A
T
A
T
T
A
T
A parental
molecule
of DNA
G
A
C
Free
nucleotides
The parental strands
separate and serve
as templates
T
G
A
Figure 10.4A_s3
A
T
A
C
G
C
G
C
A
T
T
A
T
A
T
A
T
G
C
G
C
G
G
C
G
C
G
C
T
A
T
A
T
A
T
A
T
A
T
A
T
A
A parental
molecule
of DNA
G
C
Free
nucleotides
The parental strands
separate and serve
as templates
Two identical
daughter molecules
of DNA are formed
Figure 10.4B
A
T
G
A
A
T
C
T
T
A
Parental DNA
molecule
Daughter
strand
Parental
strand
Daughter DNA
molecules
10.5 DNA replication proceeds in two directions
at many sites simultaneously
 DNA replication begins at the origins of replication
where
– DNA unwinds at the origin to produce a “bubble,”
– replication proceeds in both directions from the origin,
and
– replication ends when products from the bubbles
merge with each other.
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Figure 10.5A
Parental
DNA
molecule
Origin of
replication
“Bubble”
Two
daughter
DNA
molecules
Parental strand
Daughter strand
10.5 DNA replication proceeds in two directions
at many sites simultaneously
 DNA replication occurs in the 5 to 3 direction.
– Replication is continuous on the 3 to 5 template.
– Replication is discontinuous on the 5 to 3 template,
forming short segments.
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Figure 10.5B
3 end
5 end
P
HO
5
4
3
2
1
2
A
T
5
P
C
P
G
C
P
P
T
3 end
P
G
P
OH
3
4
1
A
P
5 end
10.5 DNA replication proceeds in two directions
at many sites simultaneously
 Proteins are involved in DNA replication.
1. DNA ligase joins small fragments into a continuous
chain.
2. DNA polymerase I and III
– adds nucleotides to a growing chain and
– proofreads and corrects improper base pairings.
3. Helicase – unwinds DNA at replication fork
4. Single stranded binding protein – stabilizes
unwound DNA
5. Topoisomerase – corrects overwinding ahead of DNA
fork by breaking and joining DNA
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10.5 DNA replication proceeds in two directions
at many sites simultaneously
 DNA polymerases and DNA ligase also repair
DNA damaged by harmful radiation and toxic
chemicals.
 DNA replication ensures that all the somatic cells
in a multicellular organism carry the same genetic
information.
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DNA Replication Process
 Helicase unwinds the
DNA double helix
 Single stranded
binding proteins
stabilize the unwound
DNA
DNA Replication
 Primase synthesizes a
small section of RNA on
each 5’ end
 Nucleotides pair up
 DNA Pol III joins the
backbone together
DNA Replication
Figure 10.5C
DNA polymerase
molecule
5
3
Parental DNA
Replication fork
5
3
DNA ligase
Overall direction of replication
3
5
This daughter
strand is
synthesized
continuously
This daughter
strand is
3 synthesized
5 in pieces
DNA Replication
 DNA Pol I removes the
primer and replaces with
DNA nucleotides
A gap formed is sealed
with DNA Ligase
Molecular process of DNA replication
You should now be able to
1. Describe the experiments of Griffith, Hershey,
and Chase, which supported the idea that DNA
was life’s genetic material.
2. Explain the structure of DNA.
3. Explain how the structure of DNA facilitates its
replication.
4. Describe the process of DNA replication.
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