Download DNA is the genetic material

CAMPBELL
BIOLOGY
TENTH
EDITION
Reece • Urry • Cain • Wasserman • Minorsky • Jackson
16
The Molecular
Basis of
Inheritance
Lecture Presentation by
Nicole Tunbridge and
Kathleen Fitzpatrick
© 2014 Pearson Education, Inc.
Figure 16.1 What is the structure of DNA?
Life’s Operating Instructions
• In 1953, James Watson and Francis Crick introduced a doublehelical model for the structure of deoxyribonucleic acid (DNA), nine
years later in 1962 they shared Nobel Prize in Physiology and
Medicine with Maurice Wilkins, for solving one of the most important
of all biological riddles "for their discoveries concerning the
molecular structure of nucleic acid and its significance for
information transfer in living material". .
• Hereditary information is encoded in DNA and reproduced in all cells of
the body of on our 46 chromosomes.
– Nucleic acids are unique in their ability to direct their own
replication.
– The resemblance of offspring to their parents depends on the
precise replication of DNA and its transmission from one generation
to the next.
• This DNA program directs the development of biochemical, anatomical,
physiological, and (to some extent) behavioral traits.
• DNA, the substance of inheritance, is the most celebrated molecule of
our time
© 2014 Pearson Education, Inc.
The Search for the Genetic Material:
Scientific Inquiry
• When T. H. Morgan’s group showed that genes
are located on chromosomes, the two components
of chromosomes—DNA and protein—became
candidates for the genetic material
• The key factor in determining the genetic material
was choosing appropriate experimental organisms
• The role of DNA in heredity was first discovered by
studying bacteria and the viruses that infect them
© 2014 Pearson Education, Inc.
DNA is the genetic material
• The identification of the molecules of inheritance loomed as a major
challenge to biologists
• The discovery of the genetic role of DNA began with research
conducted research by Frederick Griffith in 1928 on Streptococcus
pneumoniae
 One strain was harmless- R cell.
 The other strain was pathogenic (disease-causing) –S cell.
 Griffith mixed a heat-killed, pathogenic strain S cell with a,
harmless strain of live R-cell bacteria and injected this into a
mouse.
 The mouse died, and Griffith recovered the pathogenic strain from
the mouse’s blood was S cell.
 He called this phenomenon transformation, now defined as a
change in genotype and phenotype due to assimilation of foreign
DNA
What substance was transferred from the heat-killed harmful
strain to the non-harmful strain?
Ans- Some how heat-killed S cell transform live R cell into live
S cell , which cause death of mouse.
© 2014 Pearson Education, Inc.
Figure 16-2
Mixture of
heat-killed
Living S cells Living R cells Heat-killed
S cells and
(control)
(control)
S cells (control) living R cells
EXPERIMENT
RESULTS
Mouse dies Mouse healthy Mouse healthy Mouse dies
Living S cells
• In 1944, Oswald Avery, Maclyn McCarty, and
Colin MacLeod announced that the
transforming substance was DNA
• Their conclusion was based on experimental
evidence that only DNA worked in transforming
harmless bacteria into pathogenic bacteria
• Many biologists remained skeptical, mainly
because little was known about DNA
© 2014 Pearson Education, Inc.
Evidence That Viral DNA Can Program
Cells
Figure 16.3
• More evidence for DNA as the genetic material
came from studies of viruses that infect bacteria
• Such viruses, called bacteriophages (or phages),
are widely used in molecular genetics research
© 2014 Pearson Education, Inc.
• In 1952, Alfred Hershey and Martha Chase performed
experiments showing that DNA is the genetic material of a
phage known as T2
• To determine the source of genetic material in the phage,
they designed an experiment showing that only one of the
two components of T2 (DNA or protein) enters an E. coli cell
during infection
• They concluded that the injected DNA of the phage provides
the genetic information
© 2014 Pearson Education, Inc.
Figure 16-4
EXPERIMENT
Phage
Empty
protein
Radioactive shell
protein
Radioactivity
(phage
protein)
in liquid
Bacterial cell
Batch 1:
radioactive
sulfur (35S)
DNA
Phage
DNA
Centrifuge
Pellet (bacterial
cells and contents)
Radioactive
DNA
Batch 2:
radioactive
phosphorus (32P)
Centrifuge
Pellet
Radioactivity
(phage DNA)
in pellet
Hershey/Chase Experiments• They grew one batch of T2 phage in the presence of radioactive
sulfur, marking the proteins (but not the DNA) radioactive.
• They grew another batch in the presence of radioactive phosphorus,
marking the DNA (but not the proteins) radioactive.
• They allowed each batch to infect separate E. coli cultures.
• Shortly after the onset of infection, Hershey and Chase spun the
cultured infected cells in a blender, shaking loose any parts of the
phage that remained outside the bacteria.
• The mixtures were spun in a centrifuge, which separated the heavier
bacterial cells in the pellet from the lighter free phages and parts of
phages in the liquid supernatant.
• They then tested the pellet and supernatant of the separate
treatments for the presence of radioactivity.
– The bacteria infected with T2 phages containing radiolabeled
proteins: most of the radioactivity was in the supernatant that
contained phage particles, not in the pellet with the bacteria.
– The bacteria infected with T2 phages containing radiolabeled
DNA: most of the radioactivity was in the pellet with the bacteria.
They concluded that the injected DNA of the phage provides the
genetic information that makes the infected cells produce new
viral DNA and proteins to assemble into new viruses.
Additional Evidence That DNA Is the
Genetic Material
•
It was known that DNA is a polymer of nucleotides,
each consisting of a nitrogenous base, a sugar, and
a phosphate group
•
In 1950, Erwin Chargaff reported that DNA
•
•
•
•
composition varies from one species to the next
This evidence of diversity made DNA a more
credible candidate for the genetic material
Chargaff’s rules state that in any species there
is an equal number of A and T bases, and an
equal number of G and C bases
Two findings became known as Chargaff’s rules
–
The base composition of DNA varies
between species
–
In any species the number of A and T
bases are equal and the number of G and
C bases are equal
The basis for these rules was not understood until
the discovery of the double helix
DNA is a Polymer of Nucleotides
•
•
•
•
•
•
•
DNA is a polymer of nucleotides
consisting of a nitrogenous base,
deoxyribose, and a phosphate group.
The bases could be adenine (A), thymine
(T), guanine (G), or cytosine (C).
Chargaff noted that the DNA composition
varies from species to species.
In any one species, the four bases are found
in characteristic, but not necessarily equal,
ratios.
He also found a peculiar regularity in the
ratios of nucleotide bases, known as
Chargaff’s rules.
– In all organisms, the number of adenines
is approximately equal to the number of
thymines (%T = %A).
– The number of guanines is
approximately equal to the number of
cytosines (%G = %C).
Human DNA is 30.3% adenine, 30.3%
thymine, 19.5% guanine, and 19.9%
cytosine.
The basis for these rules remained
unexplained until the discovery of the double
helix by Watson & Crick.
Figure 16.5
Building a Structural Model of DNA: Scientific Inquiry
• After DNA was accepted as the genetic material,
the challenge was to determine how its structure
accounts for its role in heredity
• Maurice Wilkins and Rosalind Franklin were
using a technique called X-ray crystallography
to study molecular structure.
• Franklin produced a picture of the DNA molecule
using this technique
• Franklin’s X-ray images also enabled Watson to
deduce the width of the helix and the spacing of
the nitrogenous bases and DNA was helical.
• The pattern in the photo which was width
suggested that the DNA molecule was made up of
two strands, forming a double helix.
© 2014 Pearson Education, Inc.
Figure 16-6
(a) Rosalind Franklin
(b) Franklin’s X-ray diffraction
photograph of DNA
Rosalind Franklin major work was the basis of DNA discovery but she died in
1958 so did not received Nobel Prize because not awarded posthumously.
The Double Helix
Figure 16.7
•
•
•
•
The helix is “right handed”-curving up to the right.
The sugar-phosphate chains are on the outside and the nitrogenous bases
are on the inside of the double helix.
– This arrangement puts the relatively hydrophobic nitrogenous bases in
the molecule’s interior, away from the surrounding aqueous solution.
The nitrogenous bases are paired in specific combinations: adenine with
thymine and guanine with cytosine (purine:pyrimidine)
The chemical side groups of the nitrogenous bases form hydrogen bonds,
connecting the two strands.
© 2014 Pearson Education, Inc.
The Double Helix
• Watson and Crick built models of a
double helix to conform to the X-rays and
chemistry of DNA
• The DNA “ladder” forms a twist every ten
bases and size 3.4 nm, it means every
base pair has 0.34 nm in distance.
• The strands are antiparallel-they are
oriented in opposite directions.
• X-ray data indicated a 2-nm diameter.
• Watson and Crick built models of a
double helix to conform to the X-rays and
chemistry of DNA
• Franklin had concluded that there were
two antiparallel sugar-phosphate
backbones, with the nitrogenous bases
paired in the molecule’s interior
Figure 16.7-Space-filling model
© 2014 Pearson Education, Inc.
Base Pairing in DNA
• Only a purine-pyrimidine pair
produces the 2-nm diameter
indicated by X-ray data.
• So:
– Adenine pairs with
Thymine with 2 hydrogen
bond
– Guanine pairs with
Cytosine
with 3 hydrogen bond
The Watson-Crick model
explains Chargaff’s rules: in
any organism the amount
of
A = T, and the amount of G
=C
Human DNA is 30.3%
adenine, 30.3% thymine,
19.5% guanine, and 19.9%
cytosine.
Figure 16.8
The Double Helix
Fig. 16.7
• The base pairing rules do
not restrict the sequence
of nucleotides along each
DNA strand.
• The linear sequence of
the four bases can be
varied in countless ways.
• Each gene has a specific
sequence of nucleotide
bases.
Space-filling model
© 2014 Pearson Education, Inc.
Many proteins work together in DNA
replication and repair
• The relationship between structure and function is
manifest in the double helix
• Watson and Crick noted that the specific base
pairing suggested a possible copying mechanism
for genetic material
• Since the two strands of DNA are complementary,
each strand acts as a template for building a new
strand in replication
• In DNA replication, the parent molecule unwinds,
and two new daughter strands are built based on
base-pairing rules
© 2014 Pearson Education, Inc.
Base pairing allows parental DNA
strands to serve as templates
• During DNA replication, base pairing enables
existing DNA strands to serve as templates for new
complementary strands.
– Because each strand is complementary to the other, each can
form a template when separated.
– The order of bases on one strand can be used to add
complementary bases and therefore duplicate the pairs of bases
exactly.
– One at a time, nucleotides line up along the template strand
according to the base-pairing rules.
– The nucleotides are linked to form new strands.
• Watson and Crick’s model— semiconservative
replication, predicts that when a double helix
replicates, each of the daughter molecules has one
old strand and one newly made strand.
© 2014 Pearson Education, Inc.
Complementary Strands and DNA
Replication
Figure 16.9
• The parent DNA molecule has 2 complementary strands of DNA.
The bases are held together by hydrogen bonding.
• The first step in replication is separation of the 2 DNA strands. Each
parental strand now serves as a template that will determine the
order of nucleotides along the new complementary strand.
• The complementary nucleotides line up and are connected to form
the sugar-phosphate backbones of the new strands. Each daughter
strand consists of one parental and one new (complementary)
strand.
© 2014 Pearson Education, Inc.
• Watson and Crick’s semiconservative
model of replication predicts that when a
double helix replicates, each daughter
molecule will have one old strand
(derived or “conserved” from the parent
molecule) and one newly made strand
• Competing models were the conservative
model (the two parent strands rejoin) and
the dispersive model (each strand is a
mix of old and new)
© 2014 Pearson Education, Inc.
Figure 16-10
Parent cell
(a) Conservative
model
(b) Semiconservative model
(c) Dispersive
model
First
replication
Second
replication
• Experiments by Matthew Meselson and Franklin
Stahl supported the semiconservative model
• They labeled the nucleotides of the old strands
with a heavy isotope of nitrogen, while any new
nucleotides were labeled with a lighter isotope The
first replication produced a band of hybrid DNA,
eliminating the conservative model
• A second replication produced both light and
hybrid DNA, eliminating the dispersive model and
supporting the semiconservative model
© 2014 Pearson Education, Inc.
Figure 16-11
EXPERIMENT
1 Bacteria
cultured in
medium
containing
15N
2 Bacteria
transferred to
medium
containing 14N
RESULTS
3 DNA sample
centrifuged
after 20 min
(after first
application)
4 DNA sample
centrifuged
after 40 min
(after second
replication)
CONCLUSION
First replication
Conservative
model
Semiconservative
model
Dispersive
model
Second replication
Less
dense
More
dense
A large team of enzymes and other
proteins carries out DNA replication
• It takes E. coli less than an hour to copy each of
the 4.6 million base pairs in its single
chromosome and divide to form two identical
daughter cells.
• A human cell can copy its 6 billion base pairs
and divide into daughter cells in only a few
hours.
• This process is remarkably accurate, with only
one error per 10 billion nucleotides.
• More than a dozen enzymes and other proteins
participate in DNA replication.
© 2014 Pearson Education, Inc.
Origins of Replication
• The replication of a DNA molecule begins at special sites
called origins of replication.
– In bacteria, this site is a specific sequence of nucleotides that is
recognized by the replication enzymes.
• Enzymes separate the strands, forming a replication
“bubble.”
• Replication proceeds in both directions until the entire
molecule is copied.
• In eukaryotes, there may be hundreds or thousands of
origin sites per chromosome.
• At the origin sites, the DNA strands separate, forming a
replication “bubble” with replication forks at each end,
where the parental strands of DNA are being unwound.
• The replication bubbles elongate as the DNA is
replicated, and eventually fuse.
© 2014 Pearson Education, Inc.
Origin of Replication in Prokaryote
Figure 16.12
• Many bacteria have a circular chromosomes with only one origin of
replication.
• The parental strands separate at the origin and form a replication
bubble with two forks.
• Replication proceeds in both directions until the forks meet.
• This results in 2 daughter DNA molecules.
© 2014 Pearson Education, Inc.
Origins of Replication in Eukaryotes
Figure 16.12
• Eukaryotes have linear chromosomes.
• DNA replication begins when replication bubbles form at many sites
along the DNA molecule.
• The bubbles expand as replication proceeds in both directions.
• Eventually the bubbles fuse and synthesis of the daughter DNA
strands is complete.
© 2014 Pearson Education, Inc.
Proteins involved in DNA unwinding
Replication
fork
Figure 16.13
•
•
•
•
•
At the end of each replication bubble is a replication fork, a Y-shaped
region where new DNA strands are elongating
Helicase untwists the parental double helix and separates the template
DNA strands at the replication fork.
This untwisting causes tighter twisting ahead of the replication fork, and
topoisomerase helps relieve this strain.
Single-strand binding proteins keep the unpaired template strands apart
during replication.
The parental DNA strands are now available to serve as templates for the
synthesis of new complementary DNA strands.
© 2014 Pearson Education, Inc.
The initial DNA nucleotide chain made is
a short RNA chain
Figure 16.13
•
•
•
•
•
•
DNA polymerases cannot initiate synthesis of a polynucleotide; they can
only add nucleotides to the 3′ end
The initial nucleotide chain is a short stretch of RNA called a primer,
synthesized by the enzyme primase.
Primase starts an RNA chain from a single RNA nucleotide, adding RNA
nucleotides one at a time, using the parental DNA strand as a template.
The completed primer is short generally 5 to 10 nucleotides long.
The new DNA strand starts to form from the 3′ end of the RNA primer.
As nucleotides align with complementary bases along the template strand,
they are added to the growing end of the new strand by DNA polymerase.
Synthesizing a New DNA Strand
• DNA polymerase synthesizes a new DNA
strand by adding nucleotides to a
preexisting chain
• Most DNA polymerases require a primer
(RNA) and a DNA template strand
• The rate of elongation is about 500
nucleotides per second in bacteria and 50
per second in human cells
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• Each nucleotide that is added to a growing
DNA strand is a nucleoside triphosphate
• dATP supplies adenine, dGTP supplies
guanine, dCTP supplies cytosine, and dTTP
supplies thymine to the DNA strand.
Nucleosides are similar to ATP
The difference is in their sugars: nucleosides
contain deoxyribose while ATP has ribose
As each monomer of dATP joins the DNA strand,
it loses two phosphate groups as a molecule of
pyrophosphate
© 2014 Pearson Education, Inc.
Adding nucleotides to the DNA strand
Figure 16.14
•
•
•
The nucleosides used for DNA synthesis are chemically reactive,
partly because their triphosphate tails have an unstable cluster of
negative charge (like ATP)
As DNA polymerase adds a d(A,C,G,or T)TP to the growing 3’ end of
the DNA strand, the last two phosphate groups are hydrolyzed to form
pyrophosphate (P-Pi)
The exergonic hydrolysis of pyrophosphate to two inorganic
phosphate molecules (2 Pi) drives the polymerization of the nucleotide
to the new strand
A new DNA strand can only be
synthesized in the 5′3′ direction
• The sugar-phosphate backbones of DNA are
antiparallel: they run in opposite directions, like oneway streets.
• Each DNA strand has a 3′ end with a free hydroxyl
group (OH) attached to deoxyribose and a 5′ end
with a free phosphate group attached to
deoxyribose.
• The 5′3′ direction of one strand runs opposite the
3′5′ direction of the other strand.
• DNA polymerases can add nucleotides only to the
free 3′ end of a growing DNA strand.
• A new DNA strand can elongate only in the 5′3′
direction.
© 2014 Pearson Education, Inc.
The Leading Strand and the Replication Fork
Figure 16.15
•
The DNA strand made in the 5′3′ direction, called the leading strand,
requires a single RNA primer.
– Along the template’s leading strand, DNA polymerase III can
synthesize a complementary strand continuously by elongating the new
DNA in the mandatory 5′3′ direction.
– DNA polymerase III sits in the replication fork on that template’s leading
strand and continuously adds nucleotides to the new complementary
strand as the fork progresses.
– Along one template strand of DNA, the DNA polymerase III synthesizes
a leading strand continuously, moving toward the replication fork.
© 2014 Pearson Education, Inc.
Synthesis of the Leading Strand in the 5′3′
direction
Steps of leading strand in the
synthesis of new DNA in the
5′3′ direction
1.The DNA strands have been
separated by Helicase at the
replication fork.
2. A short RNA primer is made by
primase.
3. DNA polymerase III nestles in
the replication fork (via the
“sliding clamp”) and it can
continuously add nucleotides
to the new DNA strand in the
5′3′ direction.
Continuous
elongation in the
5′ to 3′ direction
© 2014 Pearson Education, Inc.
The Lagging Strand
Figure 16.15
• The other parental strand (5′3′ into the fork), the
lagging strand, is copied away from the fork and
requires several RNA primers.
• Unlike the leading strand, which elongates continuously,
the lagging stand is synthesized as a series of short
segments called Okazaki fragments (discover by
Japanese scientist Okazaki).
• Okazaki fragments are about 1,000 to 2,000 nucleotides
long in E. coli and 100 to 200 nucleotides long in
eukaryotes.
© 2014 Pearson Education, Inc.
Synthesis of the Lagging Strand in the 3′5′
direction
The DNA strands have been separated by Helicase at the
replication fork.
1. A short RNA primer is made by primase.
2. DNA polymerase III adds nucleotides to the new DNA
strand in the 3′5′ direction to form Okazaki fragment #1.
3. After reaching the next RNA primer to the right, DNA
ploymerase III detatches.
4. After Okazaki fragment #2 is primed (by primase) DNA
ploymerase III will attach and add nucleotides to this
fragment in the 3’→5’ direction until it reaches the Okazaki
fragment #1primer, then DNA polymerase III detaches.
5. This process repeats, creating several Okazaki fragments
along the lagging strand.
© 2014 Pearson Education, Inc.
Figure 16.16
Leading
strand
Overview
Lagging Origin of replication
strand
Lagging
strand
2
1
Leading
strand
Overall directions
of replication
1 Primase makes
RNA primer.
3′
5′
2
Origin of
replication
5′ 3′
5′
3′
Template
strand
3′
RNA primer
for fragment 2
5′
Okazaki
3′
fragment 2
RNA primer
for fragment 1
3′
2
3′
1
5′
3′
5′
3′
3 DNA pol III
detaches.
2
Okazaki
fragment 1
1
3′
5′
6 DNA ligase
forms bonds
between DNA
fragments.
5′
5′
5 DNA pol I
replaces RNA
with DNA.
1
5′
3′
3′
5′
1
5′
2 DNA pol III
makes Okazaki
fragment 1.
4 DNA pol III
makes Okazaki
fragment 2.
1
3′
5′
3′
5′
Overall direction of replication
DNA polymerase I replaces the primer
RNA with DNA
After DNA
polymerase III
detaches from
Okazaki fragment
#2, DNA polymerase
I replaces the RNA
on the 5’ side of
Okazaki fragment
#1with DNA by
adding it to the 3’
end of fragment #2.
Figure 16.16
© 2014 Pearson Education, Inc.
DNA Ligase
• DNA polymerase I cannot
join the final nucleotide of
the replacement DNA
segment to the DNA of
the Okazaki fragment 1.
• DNA ligase joins the
sugar-phosphate
backbones of all the
Okazaki fragments into a
continuous DNA strand.
Figure 16.16
© 2014 Pearson Education, Inc.
• To elongate the other new strand, called
the lagging strand, DNA polymerase
must work in the direction away from the
replication fork
• The lagging strand is synthesized as a
series of segments called Okazaki
fragments, which are joined together by
DNA ligase
© 2014 Pearson Education, Inc.
Table 16.1
Overview of DNA Replication
4. DNA polymerase III synthesizes a continual complementary
DNA strand in the 5’→3’ direction.
2. Single strand binding proteins stabilize the unwound template strands.
3. Primase synthesizes a short RNA primer
1. Helicase unwinds the strand
DNA pol III is completing synthesis of the 4th
Okazaki fragment. When it reaches the RNA primer
of the 3rd fragment it will dissociate, move to the fork,
and add nucleotides to the 3’ end of the 5th fragment.
8. DNA ligase connects the
3’ end of #2 to the 5’ end of #1.
5. Primase makes RNA primers for the Okazaki fragments.
7. DNA pol I removes the primer from the 5’ end of the 2nd fragment replacing it
with DNA by adding nucleotides to the 3’ end of fragment 3.
The DNA Replication Complex
• The proteins that participate in DNA
replication form a large complex, a “DNA
replication machine”
• The DNA replication machine is probably
stationary during the replication process
• Recent studies support a model in which
DNA polymerase molecules “reel in”
parental DNA and “extrude” newly made
daughter DNA molecules
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Figure 16.18
Leading strand template
DNA pol III
Parental DNA
5′
5′
3′
3′
3′
5′
5′
Connecting protein
3′
Helicase
DNA pol III
Lagging
strand
template
Leading strand
3′
5′
3′
5′
Lagging strand
Proofreading and Repairing DNA
• Mistakes during the initial pairing of template nucleotides and
complementary nucleotides occur at a rate of one error per
100,000 base pairs.
• DNA polymerase proofreads each new nucleotide against the
template nucleotide as soon as it is added.
• If there is an incorrect pairing, the enzyme removes the wrong
nucleotide and then resumes synthesis.
– The final error rate is only one mistake per 10 billion
nucleotides.
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DNA mistakes
• Mismatched nucleotides that are missed by DNA
polymerase or mutations that occur after DNA synthesis
is completed can often be repaired.
– In mismatch repair, special enzymes fix
incorrectly paired nucleotides.
– A hereditary defect in one of these enzymes is
associated with a form of colon cancer.
• Incorrectly paired or altered nucleotides can also arise
after replication.
• DNA molecules are constantly subjected to potentially
harmful chemical and physical agents.
• Reactive chemicals, radioactive emissions, X-rays,
ultraviolet light, and molecules in cigarette smoke can
change nucleotides in ways that can affect encoded
genetic information.
• DNA bases may undergo spontaneous chemical
changes under normal cellular conditions.
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Nucleotide Excision Repair of DNA
• Many cellular systems for repairing incorrectly paired
nucleotides use a mechanism that relies on the basepaired structure of DNA.
• A nuclease cuts out a segment of a damaged strand,
and the resulting gap is filled in with nucleotides, using
the undamaged strand as a template.
• One such DNA repair system is called nucleotide
excision repair.
• DNA repair enzymes in skin cells repair genetic damage
caused by ultraviolet light.
• Ultraviolet light can produce thymine dimers between
adjacent thymine nucleotides.
• This buckles the DNA double helix and interferes with
DNA replication.
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Nucleotide Excision Repair of DNA
1. A thymine dimer causes a
buckle in the DNA strand.
2. A nuclease cuts the damaged
DNA at 2 points and the
damaged section is removed.
3. A DNA polymerase fill in the
missing nucleotides
complimentary to the
undamaged strand.
4. DNA ligase combines the free
end of the new DNA with the
old DNA.
Figure 16.19
Evolutionary Significance of Altered DNA
Nucleotides
• Error rate after proofreading repair is low but not
zero
• Sequence changes may become permanent and
can be passed on to the next generation
• These changes (mutations) are the source of the
genetic variation upon which natural selection
operates
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The ends of DNA molecules are
replicated by a special mechanism
• Limitations of DNA polymerase create problems for the
linear DNA of eukaryotic chromosomes.
• The usual replication machinery provides no way to
complete the 5′ ends, so repeated rounds of replication
produce shorter DNA molecules with uneven ends
– Prokaryotes do not have this problem because
they have circular DNA molecules without ends.
• DNA polymerase provides no way to complete the 5′
ends of daughter DNA strands.
• Repeated rounds of replication produce shorter and
shorter DNA molecules, each with staggered ends.
• The ends of eukaryotic chromosomal DNA molecules,
the telomeres, have special nucleotide sequences.
© 2014 Pearson Education, Inc.
Figure 16.20
5′
Leading strand
Lagging strand
Ends of parental
DNA strands
3′
Last fragment
Lagging strand
Next-to-last
fragment
RNA primer
5′
3′
Parental strand
Removal of primers and
replacement with DNA
where a 3′ end is available
5′
3′
Second round
of replication
5′
New leading strand 3′
New lagging strand 5′
3′
Further rounds
of replication
Shorter and shorter daughter molecules
Telomeres
Figure 16-21
• Telomeres do not contain genes. Instead, the DNA
consists of multiple repetitions of one short nucleotide
sequence.
• In human telomeres, this nucleotide sequence is typically
TTAGGG, repeated between 100 and 1,000 times.
• Telomeric DNA binds to protein complexes to protect
genes from being eroded through multiple rounds of
DNA replication.
• Telomeric DNA and specific proteins associated with it
also prevent the staggered ends of the daughter
molecule from activating the cell’s system for monitoring
DNA damage.
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Telomere DNA
• Telomere DNA and specific proteins associated with it
also prevent the staggered ends of the daughter
molecule from activating the cell’s system for monitoring
DNA damage.
• Telomeres become shorter during every round of
replication.
• Telomere DNA tends to be shorter in the dividing
somatic cells of older individuals.
• It is possible that the shortening of telomeres is
somehow connected with the aging process of certain
tissues and perhaps to aging in general.
• Eukaryotic cells have evolved a mechanism to restore
shortened telomeres in germ cells, which give rise to
gametes.
• If the chromosomes of germ cells became shorter with
every cell cycle, essential genes would eventually be
lost.
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• An enzyme called telomerase catalyzes the
lengthening of telomeres in eukaryotic germ
cells, restoring their original length.
• Telomerase is not present in most cells of
multicellular organisms.
• Therefore, the DNA of dividing somatic cells and
cultured cells tends to become shorter.
– Telomere length may be a limiting factor in
the life span of certain tissues and of the
organism.
• The shortening of telomeres might protect cells
from cancerous growth by limiting the number of
cell divisions
• There is evidence of telomerase activity in
cancer cells, which may allow cancer cells to
persist
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A chromosome consists of a DNA
molecule packed together with proteins
• Most bacteria have a single, circular, doublestranded DNA molecule, associated with a
small amount of protein.
• The chromosomal DNA of E. coli consists of
about 4.6 million nucleotide pairs, representing
about 4,400 genes.
• A bacterium has a dense region of DNA called
the nucleoid.
• In a bacterium, the DNA is “supercoiled” and
found in a region of the cell called the nucleoid
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Eukaryotic DNA Packing
A eukaryote chromosome is a linear DNA molecule associated with
a large amount of protein
In eukaryotic cells DNA is packaged with protein to form chromatin
• Chromatin, a complex of DNA and protein, is found in the nucleus
of eukaryotic cells
• Chromosomes fit into the nucleus through an elaborate, multilevel
system of packing
Chromatin is organized into fibers
– The packing of chromatin varies during the course of the
cell cycle.
– It is highly extended during interphase.
– As a cell enters mitosis, the chromatin coils and condenses,
forming short, thick metaphase chromosomes.
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Eukaryotic DNA Packing-Histones and
Nucleosomes
Figure 16.22
• Histones:
– Small + charged proteins that tightly hold – charged DNA
– 4 types
• Nucleosomes:
– 10nm “beads on a string”
– A nucleosome=DNA wrapped twice around 8 total histone
molecules (2 of each histone type)
– 8 Histone tails extend out from each nucleosome
Chromatin
• Chromatin is a complex of DNA and protein, and is found
in the nucleus of eukaryotic cellsChromatin undergoes
changes in packing during the cell cycle
• At interphase, some chromatin is organized into a 10-nm
fiber, but much is compacted into a 30-nm fiber, through
folding and looping
• Though interphase chromosomes are not highly
condensed, they still occupy specific restricted regions in
the nucleus
• Histones are proteins that are responsible for the first level
of DNA packing in chromatin.
• Histones can undergo chemical modifications that result in
changes in chromatin organization
– For example, phosphorylation of a specific amino acid
on a histone tail affects chromosomal behavior during
meiosis
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Eukaryotic DNA Packing30, 300nm fibers and chromosomes
• 30nm chromatin fibers:
– Histone tail interactions cause the nucleosome fibers to coil
• 300nm fibers:
– The 30nm fibers form loops attached to protein chromosome scaffolds
• Metaphase Chromosome (made up of 2 Chromatids):
– During mitosis, looped domains coil to form a metaphase chromosome
that is about 700nm wide
– Specific genes always end up in precise locations along the
chromosomes
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5 µm
Figure 16.23 “Painting” chromosomes
• Most chromatin is loosely packed in the
nucleus during interphase and condenses
prior to mitosis
• Loosely packed chromatin is called
euchromatin
• During interphase a few regions of
chromatin (centromeres and telomeres)
are highly condensed into
heterochromatin
• Dense packing of the heterochromatin
makes it difficult for the cell to express
genetic information coded in these
regions
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Heterochromatin is compacted
• The chromatin of each chromosome occupies a specific
restricted area within the nucleus, preventing tangling of
the chromatin fibers of different chromosomes.
• Even during interphase, the centromeres and telomeres
of chromosomes exist in a highly condensed state
similar to that seen in a metaphase chromosome.
• This type of interphase chromatin is called
heterochromatin. Because of its compaction,
heterochromatin DNA is largely inaccessible to the
machinery in the cell responsible for expressing the
genetic information coded in the DNA.
• More loosely packed euchromatin makes its DNA
accessible to this machinery, so the genes present in
euchromatin can be expressed.
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