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Chapter 16
• The Molecular Basis of Inheritance
DNA Structure and function
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
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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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 (encapsulated)- S strain
and a nonpathogenic strain (no capsule to
protect)- R strain
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• Griffith found that when he mixed heat-killed
remains of the pathogenic strain
– With living cells of the nonpathogenic strain,
some of these living cells became pathogenic
EXPERIMENT 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:
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.
Figure 16.2
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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• Griffith called the phenomenon transformation
– Now defined as a change in genotype and
phenotype due to the assimilation 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
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• 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
• Alfred Hershey and Martha Chase
– Performed experiments showing that DNA is
the genetic material of a phage known as T2
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• The Hershey and Chase experiment
EXPERIMENT
In their famous 1952 experiment, Alfred Hershey and Martha Chase used radioactive sulfur
and phosphorus to trace the fates of the protein and DNA, respectively, of T2 phages that infected bacterial cells.
1 Mixed radioactively
labeled phages with
bacteria. The phages
infected the bacterial cells.
Phage
2 Agitated in a blender to 3 Centrifuged the mixture
separate phages outside
so that bacteria formed
the bacteria from the
a pellet at the bottom of
bacterial cells.
the test tube.
Radioactive Empty
protein
protein shell
Radioactivity
(phage protein)
in liquid
Bacterial cell
Batch 1: Phages were
grown with radioactive
sulfur (35S), which was
incorporated into phage
protein (pink).
Batch 2: Phages were
grown with radioactive
phosphorus (32P), which
was incorporated into
phage DNA (blue).
4 Measured the
radioactivity in
the pellet and
the liquid
DNA
Phage
DNA
Centrifuge
Radioactive
DNA
Pellet (bacterial
cells and contents)
Centrifuge
Radioactivity
(phage DNA)
Pellet
in pellet
RESULTS
Phage proteins remained outside the bacterial cells during infection, while phage DNA entered the cells.
When cultured, bacterial cells with radioactive phage DNA released new phages with some radioactive phosphorus.
Figure 16.4
CONCLUSION
Hershey and Chase concluded that DNA, not protein, functions as the T2 phage’s genetic material.
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
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
• Erwin Chargaff analyzed the base composition of DNA
– From a number of different organisms
• In 1947, Chargaff reported
– That DNA composition varies from one species to
the next
• This evidence of molecular diversity among species
– Made DNA a more credible candidate for the genetic
material
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Building a Structural Model of DNA: Scientific Inquiry
• Once most biologists were convinced that DNA
was the genetic material
– The challenge was to determine how the
structure of DNA could account for its role in
inheritance
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• 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
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
(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
• Franklin had concluded that DNA
– Was composed of two antiparallel sugarphosphate 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
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
5 end
O
OH
Hydrogen bond
P
–O
3 end
OH
O
O
A
T
O
O
O
CH2
P
–O
O
H2C
O
–O
P
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–
O
O
O–
O
P
H2C
O
O
A
O
T
O
CH2
OH
3 end
O
O–
P
O
Figure 16.7b
(b) Partial chemical structure
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
O
5 end
• Watson and Crick reasoned that there must be
additional specificity of pairing
– Dictated by the structure of the bases
• Each base pair forms a different number of
hydrogen bonds
– Adenine and thymine form two bonds,
cytosine and guanine form three bonds
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
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
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• In DNA replication
– The parent molecule unwinds, and two new
daughter strands are built based on basepairing 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 is semiconservative
– Each of the two new daughter molecules will
have one old strand, derived from the parent
molecule, and one newly made strand
First
Second
Parent cell replication replication
(a) Conservative
model. The two
parental strands
reassociate
after acting as
templates for
new strands,
thus restoring
the parental
double helix.
(b) Semiconservative
model. The two
strands of the
parental molecule
separate,
and each functions
as a template
for synthesis of
a new, complementary strand.
Figure 16.10 a–c
(c) Dispersive
model. Each
strand of both
daughter molecules contains
a mixture of
old and newly
synthesized
DNA.
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CONCLUSION Meselson and Stahl concluded that DNA replication follows the semiconservative
model by comparing their result to the results predicted by each of the three models in Figure 16.10.
The first replication in the 14N medium produced a band of hybrid (15N–14N) DNA. This result eliminated
the conservative model. A second replication produced both light and hybrid DNA, a result that eliminated
the dispersive model and supported the semiconservative model.
First replication
Conservative
model
Semiconservative
model
Dispersive
model
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Second replication