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• Chapter 12~ The
Molecular Basis of Inheritance
12.1 The Genetic Material
• Frederick Griffith investigated virulence of Streptococcus pneumoniae Concluded that virulence could be passed
from a dead strain to a nonvirulent living strain Transformation
• Further research by Avery et al. Discovered that DNA is the transforming
substance DNA from dead cells was being incorporated
into the genome of living cells2
The Genetic Material
• Griffith’s Transformation Experiment Mice were injected with two strains of
pneumococcus: an encapsulated (S) strain and a non-encapsulated (R) strain.
• The S strain is virulent (the mice died); it has a mucous capsule and forms “shiny” colonies.
• The R strain is not virulent (the mice lived); it has no capsule and forms “dull” colonies.
3
Griffith’s Transformation Experiment
4
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
capsule
Injected live S strain hascapsule and
causes mice to die.
a. b.
Injected live R strain hasno capsule
and mice do not die.
c.
Injected heat-killed S strain does not cause
mice to die.
d.
Injected heat-killed S strain plus liveR strain causes
mice to die. Live S strain iswithdrawn from
dead mice.
Searching for Genetic Material
• Hershey and Chase (1952)• bacteriophages (phages)• DNA, not protein, is the hereditary material• Experiment: sulfur(S) is in protein,
phosphorus (P) is in DNA; only P was found in host cell
The Genetic Material
• Transformation of organisms today: Result is the so-called genetically modified
organisms (GMOs) • Invaluable tool in modern biotechnology today• Commercial products that are currently much used • Green fluorescent protein (GFP) can be used as a
marker – A jellyfish gene codes for GFP
– The jellyfish gene is isolated and then transferred to a bacterium, or the embryo of a plant, pig, or mouse.
– When this gene is transferred to another organism, the organism glows in the dark
8
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Animation
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D:\ImageLibrary1-17\16-MolecularBasisInheritance\16-02-PhageT2Reproduction.mov
The Genetic Material
• DNA contains: Two Nucleotides with purine bases
• Adenine (A)• Guanine (G)
Two Nucleotides with pyrimidine bases• Thymine (T)• Cytosine (C)
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The Genetic Material• Chargaff’s Rules:
The amounts of A, T, G, and C in DNA:• Are constant among members of the same species• Vary from species to species
In each species, there are equal amounts of:• A and T• G and C
All this suggests that DNA uses complementary base pairing to store genetic information
Each human chromosome contains, on average, about 140 million base pairs
The number of possible nucleotide sequences is 4140,000,000
12
Nucleotide Composition of DNA
13
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O
N
N
CH
CH
C
C
NH2
cytosine(C)
3C C2
C1
OHO P O
O
H
HH
HH
OH
CH3
O
HN
N
C
CH
C
C
OHO P O
O
H
HH
HH
OH
HN
N
N
CCH
O
C
CC
NH2N
C2
C2
C1
C1
OHO P O
O
guanine(G)
phosphate
H
HH
HH
OH
N
N
N
HCCH
NH2
C
CC
N
4
3C
2
C1
5 O
O
O
O
O
O
H
HH
HH
OH
c. Chargaff’s data
DNA Composition in Various Species (%)
Species
Homo sapiens (human)
Drosophila melanogaster (fruit fly)
Zea mays (corn)
Neurospora crassa (fungus)
Escherichia coli (bacterium)
Bacillus subtilis (bacterium)
31.0
27.3
25.6
23.0
24.6
28.4
31.5
27.6
25.3
23.3
24.3
29.0
19.1
22.5
24.5
27.1
25.5
21.0
18.4
22.5
24.6
26.6
25.6
21.6
A T G C
a. Purine nucleotides b. Pyrimidine nucleotides
nitrogen-containingbase
sugar = deoxyribose
thymine(T)
adenine(A)
HO P O CH2
5CH2
5CH2
5CH2
C
4C
4C
4C
C
3C
3C
O
The Genetic Material
• X-Ray diffraction: Rosalind Franklin studied the structure of DNA using
X-rays. She found that if a concentrated, viscous solution of
DNA is made, it can be separated into fibers. Under the right conditions, the fibers can produce an
X-ray diffraction pattern• She produced X-ray diffraction photographs.• This provided evidence that DNA had the following features:
– DNA is a helix.– Some portion of the helix is repeated.
14
X-Ray Diffraction of DNA
15
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© Photo Researchers, Inc.; c: © Science Source/Photo Researchers, Inc.
X-ray beam
b.c.
Rosalind Franklin
diffraction pattern
CrystallineDNA
diffractedX-rays
a.
The Genetic Material
• The Watson and Crick Model (1953)
Double helix model is similar to a twisted ladder
• Sugar-phosphate backbones make up the sides
• Hydrogen-bonded bases make up the rungs
Complementary base pairing ensures that a purine is always bonded to a pyrimidine (A with T, G with C)
Received a Nobel Prize in 1962
16
Watson and Crick Model of DNA
17
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
P
P
P
P
c.
P
S SCG
PP
S
SAT
G
C
C
G
T
T
A
A
C
G
P
0.34 nm3.4 nm
sugar-phosphatebackbone
a.
b.
d.
5′ end 3′ end
3′ end 5′ endcomplementarybase pairing
hydrogen bondssugar
2 nm
a: © Photodisk Red/Getty RF; d: © A. Barrington Brown/Photo Researchers
18
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Animation
12.2 Replication of DNA
• DNA replication is the process of copying a DNA molecule.
• Semiconservative replication - each strand of the original double helix (parental molecule) serves as a template (mold or model) for a new strand in a daughter molecule.
19
Semiconservative Replication
20
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oldstrand
newstrand
A
A
A
A
A
A
A
A
AA
A
A
AA
A
A
A
A
A
A
T
T
T
T
T
TT
T
T
T
TT
T
G
G
G
G
GG
GG
G
G
G
G
GG
G
G
C
C
C
CC
C
C
C
C
C C
C
C
A
T
TG
C
A
T
T
GC
C
region of parentalDNA double helix
region ofreplication:new nucleotidesare pairingwith those ofparental strands
region ofcompletedreplication
oldstrand
newstrand
daughter DNA double helix
daughter DNA double helix
DNA polymeraseenzyme
5′ 3′
3′
5′
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Animation
22
Replication of DNA
• Replication requires the following steps:
Unwinding, or separation of the two strands of the parental DNA molecule
Complementary base pairing between a new nucleotide and a nucleotide on the template strand
Joining of nucleotides to form the new strand
• Each daughter DNA molecule contains one old strand and one new strand
DNA Replication: a closer look
• Origin of replication (“bubbles”): beginning of replication• Replication fork: ‘Y’-shaped region where new strands of
DNA are elongating• Helicase:catalyzes the untwisting of the DNA at the
replication fork• DNA polymerase:catalyzes the elongation of new DNA
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Animation
Replication of DNA• Eukaryotic Replication
DNA replication begins at numerous points along each linear chromosome
DNA unwinds and unzips into two strands
Each old strand of DNA serves as a template for a new strand
Complementary base-pairing forms a new strand paired with each old strand
• Requires enzyme DNA polymerase
26
Replication of DNA• Eukaryotic Replication
Replication bubbles spread bidirectionally until they meet
The complementary nucleotides are joined to form new strands. Each daughter DNA molecule contains an old strand and a new strand.
Replication is semiconservative:
• One original strand is conserved in each daughter molecule, i.e., each daughter double helix has one parental strand and one new strand.
27
Aspects of DNA Replication
28
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GC
AT
T
GC
G C
AP
P
P
P
P
P
P
P
P
P
P
P
5′
4′C
3′ C C 2′
C1′
H H
H
H H
O
5′ end
3′ end
Deoxyribose molecule
3′
3′
3′
5′
5′
3′
5′
3
5
2
4
6
7
1
OH Is attached herebase is attached here
helicase at replication fork
DNA polymerase
templatestrand
Okazaki fragmenttemplatestrandlagging
strand
Direction of replication
template strand
parental DNA helix
DNA polymerase
Replication fork introduces complications
DNA ligase
RNA primer
3 ′end
OHCH2
OH
5′ end
new strand
leadingnew strand
DNA polymeraseattaches a newnucleotide to the3 ′ carbon of theprevious nucleotide.
Replication of DNA
• Accuracy of Replication
DNA polymerase is very accurate, yet makes a mistake about once per 100,000 base pairs.
• Capable of identifying and correcting errors
29
DNA Replication, II
• Antiparallel nature: sugar/phosphate backbone runs in opposite directions (Crick);
• one strand runs 5’ to 3’, while the other runs 3’ to 5’;
• DNA polymerase only adds nucleotides at the free 3’ end, forming new DNA
strands in the 5’ to 3’ direction only
Figure 16.12 The two strands of DNA are antiparallel
DNA Replication, III• Leading strand: synthesis toward the
replication fork (only in a 5’ to 3’ direction from the 3’ to 5’ master strand)
• Lagging strand: synthesis away from the replication fork (Okazaki fragments); joined by DNA ligase (must wait for 3’ end to open; again in a 5’ to 3’ direction)
• Initiation: Primer (short RNA sequence~w/primase enzyme), begins the replication process
DNA Replication: the leading and lagging strand
D:\ImageLibrary1-17\16-MolecularBasisInheritance\16-13-LeadingStndNarrAnim_S.mov
D:\ImageLibrary1-17\16-MolecularBasisInheritance\16-13-LaggingStrandAnim_B.mov
Figure 16.15 The main proteins of DNA replication and their functions
DNA Repair
• Mismatch repair: DNA polymerase
• Excision repair:Nuclease
• Telomere ends:telomerase
Figure 16.17 Nucleotide excision repair of DNA damage
Figure 16.18 The end-replication problem
Figure 16.17 Nucleotide excision repair of DNA damage
Figure 16.19b Telomeres and telomerase