Fundamental
Molecular Biology
Second Edition
Lisabeth A. Allison
Chapter 6
DNA Replication and Telomere Maintenance
Copyright © 2012 John Wiley & Sons, Inc. All rights reserved.
Cover photo: Julie Newdoll/www.brushwithscience.com “Dawn of the
Double Helix”, oil and mixed media on canvas, © 2003
Chapter 6:
DNA Replication
and Telomere
Maintenance
It has not escaped our notice that the
specific pairing we have postulated
immediately suggests a possible copying
mechanism for the genetic material.
James D. Watson and Francis Crick, Nature (1953),
171:737
6.1 Introduction
DNA replication involves:
• The melting apart of the two strands of the
double helix followed by the polymerization of
new complementary strands.
• Decisions of when, where, and how to initiate
replication to ensure that only one complete
and accurate copy of the genome is made
before a cell divides.
6.2 Early insights into the mode
of bacterial DNA replication
Three possible modes of replication
hypothesized based on Watson and
Crick’s model:
•Semiconservative
•Conservative
•Dispersive
The Meselson-Stahl experiment
• 1958 experiment designed to distinguish
between semiconservative, conservative,
and dispersive replication.
• Results were consistent only with
semiconservative replication.
Visualization of replicating
bacterial DNA
• Semiconservative mechanism of DNA
replication visually verified by J. Cairns in 1963
using autoradiography.
• Bidirectional replication of the E. coli
chromosome.
• One origin of replication.
• Replication intermediates are termed theta ()
structures.
()
Thymine
labeled with
Tritium 3H
Autoradiography
6.3 DNA polymerases are the
enzymes that catalyze DNA
synthesis from 5′ to 3′
DNA polymerases
• Can only add nucleotides in the 5′→3′
direction.
• Cannot initiate DNA synthesis de novo.
• Require a primer with a free 3′-OH group
at the end.
• Deoxynucleoside 5′ triphosphates
(dNTPs) are added one at a time to the 3′
hydroxyl end of the DNA chain.
• The dNTP added is determined by
complementary base pairing.
• As phosphodiester bonds form, the two
terminal phosphates are lost, making the
reaction essentially irreversible.
Problem
• DNA polymerases can only add nucleotides
from 5′→3′ but, the two strands of the double
helix are antiparallel.
Solution
• Semidiscontinuous replication.
Semidiscontinuous DNA replication
• Major form of replication in
eukaryotic nuclear DNA, some
viruses, and bacteria.
Leading strand synthesis is continuous
• Once primed, continuous replication is possible
on the 3′→ 5′ template strand (leading strand).
• Leading strand synthesis occurs in the same
direction as movement of the replication fork.
Leading strand synthesis is continuous
• Discontinuous replication occurs on the 5′→3′
template strand (lagging strand).
• DNA is copied in short segments called
“Okazaki fragments” moving in the opposite
direction to the replication fork.
• Repetition of primer synthesis and formation of
Okazaki fragments. 100-200 or 1000 – 2000 Nt
Synthesis of both strands occurs
concurrently
• Nucleotides are added to the leading and
lagging strands at the same time and rate.
• Two DNA polymerases, one for each strand.
• Fundamental features of DNA replication are
conserved from E. coli to humans.
• 1984: A cell-free system allowed scientists to
make progress in studying replication in
eukaryotic cells.
• Model system: Simian virus 40 (SV40)
replication.
6.4 Multi-protein machines
mediate bacterial DNA
replication
Bacterial DNA polymerases have
multiple functions
DNA polymerase I
• What can do:
– Primer removal
– Gap filling between Okazaki fragments
– Nucleotide excision repair pathway
• Two subunits: Klenow fragment has 5′→3′
polymerase activity; other subunit has both
3′→5′ and 5′→3′ exonuclease activity.
E. coli
Bacterial DNA Poly I
Thermus aquaticus
DNA Pol-I
• Unique ability to start replication at a nick
in the DNA sugar-phosphate backbone.
• Used extensively in molecular biology
research.
DNA polymerase III
• Main replicative polymerase.
DNA polymerase II
• Involved in DNA repair mechanisms.
DNA polymerases IV and V
• Mediate translesion synthesis (see Chapter 7).
Initiation of replication
• An origin of replication is a site on chromosomal DNA
where a bidirectional replication fork initiates or “fires.”
• Most bacteria have a single, well-defined origin (e.g.
oriC in E. coli)
• Some Archaea have as many as three origins (e.g.
Sulfolobus).
• Usually A-T rich.
• In E. coli the initiator protein DnaA can only bind to
negatively supercoiled origin DNA.
Replication is mediated by the
Replisome
Major parts of this multi-protein machine
are:
• A helicase which unwinds the parental double
helix.
• Two molecules of DNA polymerase III.
• A primase that initiates lagging strand Okazaki
fragments.
Trombone Model
ATP
ATP
Major parts of this multi-protein
machine, cont:
• Two sliding clamps that tether DNA
polymerase to the DNA.
• A clamp loader that uses ATP to open and
close the sliding clamps around the DNA.
• Single-strand DNA binding proteins (SSB)
that protect the DNA from nuclease attack.
Lagging strand synthesis by the replisome:
• As the replication fork advances, the lagging
strand polymerase remains associated with the
replisome forming a loop.
• The loop grows until the Okazaki fragment is
complete.
• DNA polymerase III is released.
• New clamps are assembled; DNA polymerase III
hops aboard to make the next Okazaki fragment.
• This process occurs around the circular genome
until the replication forks meet.
• In E. coli, the replication forks meet at a terminus
region containing sequence-specific replication
arrest sites.
• DNA polymerase I removes the RNA primers
and replaces them with complementary dNTPs.
• DNA ligase catalyzes the formation of a
phosphodiester bond between adjacent Okazaki
fragments.
Figure 16.16a
Leading
strand
Overview
Lagging
Origin of replication
strand
Lagging
strand
2
1
Overall directions
of replication
Leading
strand
Figure 16.16b-1
1 Primase makes
3′
RNA primer.
5′
Template
strand
3′
Origin of
replication
5′ 3′
5′
Figure 16.16b-2
1 Primase makes
3′
RNA primer.
5′
3′
Template
strand
3′
2 DNA pol III
RNA primer
for fragment 1
5′
Origin of
replication
5′ 3′
5′
makes Okazaki
fragment 1.
1
3′
5′
3′
5′
Figure 16.16b-3
1 Primase makes
3′
RNA primer.
5′
Origin of
replication
5′ 3′
5′
3′
Template
strand
3′
2 DNA pol III
RNA primer
for fragment 1
5′
makes Okazaki
fragment 1.
3′
1
5′
3′
5′
3 DNA pol III
3′
detaches.
5′
Okazaki
fragment 1
1
3′
5′
Figure 16.16c-1
RNA primer
for fragment 2
5′
Okazaki
3′
fragment 2
2
4 DNA pol III
makes Okazaki
fragment 2.
1
3′
5′
Figure 16.16c-2
RNA primer
for fragment 2
5′
Okazaki
3′
fragment 2
2
4 DNA pol III
makes Okazaki
fragment 2.
3′
5′
1
5′
3′
2
5 DNA pol I
replaces RNA
with DNA.
1
3′
5′
Figure 16.16c-3
RNA primer
for fragment 2
5′
Okazaki
3′
fragment 2
2
4 DNA pol III
makes Okazaki
fragment 2.
3′
5′
1
5′
3′
2
5 DNA pol I
replaces RNA
with DNA.
1
3′
5′
6 DNA ligase
forms bonds
between DNA
fragments.
5′
3′
2
1
3′
5′
Overall direction of replication
Figure 16.17
Overview
Origin of
replication
Leading
strand
Leading strand
template
Single-strand
binding proteins
Leading strand
Helicase
5′
3′
Parental
DNA
DNA pol III
3′ Primer
5′
3′ Primase
5
DNA pol III
5′
4
3′
5′
Lagging
strand
3′
Leading
Lagging
strand
strand
Overall directions
of replication
Lagging
strand
3
Lagging strand
template
DNA pol I
2
5′
DNA ligase
3′
1
5′
Figure 16.17a
Overview
Origin of
replication
Leading
strand
Lagging
strand
Lagging
strand
Leading
strand
Overall directions
of replication
Figure 16.17b
Leading strand
template
Single-strand
binding proteins
Leading
strand
Helicase
5′
3′
Parental
DNA
DNA pol III
3′ Primer
5′
Primase
3′
5
Figure 16.17c
DNA pol III
5′
4
3′
5′
Lagging
strand
3
Lagging strand
template
DNA pol I
2
DNA ligase
3′
1
5′
Movement of the replication fork machinery
results in:
• Positive supercoiling ahead of the fork.
• Negative supercoiling in the wake of the fork.
• Torsional strain that could inhibit fork
movement is relieved by DNA topoisomerase.
Topoisomerases relax supercoiled DNA
Topoisomers are forms of DNA that have the same
sequence but differ in:
• linkage number
• mobility in an electrophoresis gel
Topoisomerases are enzymes that convert
(isomerize) one topoisomer of DNA to another by
changing the linking number (L).
Type I topoisomerases cause transient
single-stranded breaks in DNA
• Type 1A only relax negative supercoils.
• Type 1B can relax both negative and
positive supercoils.
• Do not require ATP.
Type II topoisomerases cause transient
double-stranded breaks in DNA
• Relax both negative and positive supercoils.
• Unknot or decatenate entangled DNA molecules.
• Usually ATP-dependent.
• Bacterial “gyrase” can introduce negative
supercoils.
Is leading strand synthesis really
continuous?
• DNA polymerase III can be blocked by a
damaged site on the template DNA.
• Sometimes DNA polymerase collides with RNA
polymerase and is stalled.
• In both cases, replication can be jumpstarted on
the leading strand by formation of a new primer
at the replication fork.
6.5 Multi-protein machines trade
places during eukaryotic DNA
replication
Eukaryotic origins of replication
• Internal sites on linear chromosomes.
• Mice have 25,000 origins, spanning ~150
kb each.
• Humans have 10,000 to 100,000 origins.
• In the budding yeast Saccharomyces
cerevisiae there is a consensus sequence
called an autonomous replicating sequence
(ARS).
• Mammalian origin sequences are usually
AT rich but lack a consensus sequence.
Mapping eukaryotic DNA replication
origins
• Analysis by two-dimensional agarose gel
electrophoresis.
• Other techniques allow detection of the start
site for DNA synthesis at the nucleotide level.
• Data suggest that there is a single defined start
point.
Selective activation of origins of
replication
• The overall rate of replication is largely
determined by the number of origins used and
the rate at which they initiate.
• During early embryogenesis, origins are
uniformly activated.
• At the mid-blastula transition, replication
becomes restricted to specific origin sites.
Replication factories
• Replication forks are clustered in
“replication factories.”
• Forty to many hundreds of forks are
active in each factory.
• Shown by a pulse-chase technique using
BrdU labeling of cells in S-phase and
detection with anti-BrdU antibodies.
Histone removal at origins of replication
• Histone modification and chromatin remodeling
factors.
• Disassembly of the nucleosomes.
• Template DNA is accessible to the replication
machinery.
Prereplication complex formation
and replication licensing
• DNA replication is restricted to S phase of the
cell cycle.
• Origin selection is a separate step from
initiation.
• Formation of a prereplication complex.
• Prevents overreplication of the genome.
Assembly of the origin recognition
complex (ORC)
• The ATP-dependent origin recognition complex
(ORC) binds origin sequences.
• Recruits Cdc6 and Mcm proteins.
• The SV40 T antigen functions as a viral ORC.
The naming of genes involved in
DNA replication
• Many genes first characterized in the yeast
Saccharomyces cerevisiae.
• Mutations that affect the cell cycle were isolated as
conditional, temperature-sensitive mutants.
– At the permissive temperature, the gene product can function.
– At the restrictive temperature, mutant yeast accumulate at a
particular point in the cell cycle.
Assembly of the replication
licensing complex
• In association with Cdc6 and Cdt1, ORC loads
the licensing protein complex, Mcm2-7.
• Mcm2-7 is a hexameric complex with helicase
activity.
• Only licensed origins containing Mcm2-7 can
initiate a pair of replication forks.
• ATP hydrolysis by ORC stimulates
prereplication complex assembly.
• Prereplication complex assembly is
inhibited when ORC is bound by a
nonhydrolyzable analog of ATP (ATP-S)
Regulation of the replication
licensing system by CDKs
• Replication licensing is regulated by the activity
levels of cyclin-dependent kinases (CDKs).
• For catalysis, CDKs must associate with a
cyclin.
• Cyclins accumulate gradually during interphase
and are abruptly destroyed during mitosis.
• ORC, Cdc6, Cdt1, and Mcm2-7 are
downregulated by high CDK activity.
• The mode of downregulation differs for each
protein.
 No further Mcm2-7 can be loaded onto origins in S
phase, G2, and early mitosis when CDK activity is
high.
Figure 12.16a
M
G1
S
G2
M
MPF
activity
G1
S
G2
M
Cyclin
concentration
Time
(a) Fluctuation of MPF activity and cyclin
concentration during the cell cycle
G1
Figure 12.16b
Cdk
Degraded
cyclin
Cdk
Cyclin is
degraded
MPF
Cyclin
G2
checkpoint
(b) Molecular mechanisms that help regulate
the cell cycle
Duplex unwinding at replication forks
• DNA helicases are enzymes that use the
energy of ATP to melt the DNA duplex.
• They catalyze the transition from doublestranded to single-stranded DNA in the
direction of the moving replication fork.
• Mcm2-7 helicase is bound to the leading
strand template and moves 3′→5′.
RNA priming of leading and lagging
strand DNA synthesis
• In eukaryotes, the RNA primer is synthesized
by DNA polymerase (pol)  and its associated
primase activity.
• The pol /primase enzyme synthesizes a
short strand of 10 bases of RNA, followed by
20-30 bases of initiator DNA (iDNA).
Polymerase switching
• A key feature of the replication process is the
ordered hand-off, or “trading places”, from one
protein complex to another.
• Polymerase switching: The hand-off of the
DNA template from one polymerase to another.
Elongation of leading strands and
lagging strands
At least 14 different eukaryotic DNA polymerases
• Chromosomal DNA replication
DNA pol , pol , pol 
• Mitochondrial DNA replication
DNA pol 
• Repair processes
All the rest (Chapter 7)
• Leading strand: switch from DNA
polymerase  to pol 
• Lagging strand: switch from pol  to
pol 
• Polymerase switching is regulated by
PCNA.
• Once DNA pol  is recruited to the
leading strand, synthesis is continuous.
• Lagging strand synthesis requires
repeated cycles of polymerase switching
from DNA pol  to pol .
PCNA: a sliding clamp with many
protein partners
• PCNA: Proliferating Cell Nuclear Antigen.
• Plays an important role in many cellular
processes.
• In DNA replication, acts as a sliding clamp to
increase DNA polymerase processivity.
PCNA structure
• PCNA is a ring-shaped trimer.
• In the presence of ATP, the clamp loader RFC
opens the trimer and passes DNA into the ring
and then reseals it.
• RFC locks onto DNA in a screw-cap-like
arrangement.
• The RFC spiral matches the minor grooves of the
DNA double helix.
Proofreading
• Replicative polymerases are high fidelity
but not perfect: 10 -4 to 10 -5 errors per
base pair (bp).
• Proofreading exonuclease activity
reduces the error rate to 10 -7 to 10 -8
errors per bp.
• DNA polymerase has a hand-shaped
structure.
• 5′→3′ polymerase activity is within the
fingers and thumb.
• 3′→5′ exonuclease activity is at the base
of the palm.
Nucleotide selectivity largely depends on
the geometry of Watson-Crick base pairs
• The abnormal genometry of mismatched base
pairs results in steric hindrance at the active
site.
• Base-base hydrogen bonding also contributes
to fidelity.
Maturation of DNA nascent strands
• RNA primer removal.
• Gap fill-in.
• Joining of Okazaki fragments on the
lagging strand.
Two different pathways proposed for
RNA primer removal:
1. Ribonuclease H1 nicks the RNA primer and
the primer is degraded by FEN-1 (flap
endonuclease 1)
2. DNA pol  causes strand displacement and
FEN-1 removes the entire RNA containing 5′
“flap.”
• FEN-1 is a structure-specific 5′ nuclease
with both exonuclease and endonuclease
activity.
• PCNA-coordinated rotary handoff
mechanism of DNA from DNA pol  to
FEN-1.
Gap fill-in and joining of the Okazaki
fragments
• The remaining gaps left by primer removal are
filled in by DNA polymerase  or .
• End product is a nicked double-stranded DNA.
• Nicks are sealed by DNA ligase I.
• In association with PCNA, DNA ligase I joins
the Okazaki fragments by catalyzing the
formation of new phosphodiester bonds.
• DNA binding domain encircles DNA and
interacts with the minor groove.
• Stabilizes distorted structure with A-form helix
upstream of the gap.
Histone deposition
• Nucleosomes re-form within approximately 250
bp behind the replication fork.
• Chromatin assembly factor 1 (CAF-1) brings
histones to the DNA replication fork in
association with PCNA.
• Histones H3 and H4 form a complex and are
deposited first, followed by two histone H2AH2B dimers.
Two models for nucleosome assembly
after DNA replication:
• The tetrameric model: histones H3 and H4
are deposited on DNA as parental or newly
synthesized tetramers.
• The dimeric model: histones H3 and H4 are
deposited on DNA as parental or newly
synthesized dimers.
Topoisomerase untangles the newly
synthesized DNA
• In eukaryotes, replication continues until one
fork meets a fork from the adjacent replicon.
• The progeny DNA molecules remain
intertwined.
• Toposiomerase II is required to resolve the two
separate progeny genomes.
Topoisomerase-targeted
anti-cancer drugs
• Target rapidly growing cells.
• Act either as inhibitors of at
least one step in the catalytic
cycle or as poisons.
• Topoisomerase I is a target
for a number of anti-cancer
drugs.
e.g. Camptothecin
6.6 Alternative modes of
circular DNA replication
Rolling circle replication
• Multiplication of many bacterial and eukaryotic
viral DNAs, bacterial F factors during mating,
and in certain cases of gene amplification.
• A phosphodiester bond is broken in one of the
strands of a circular DNA.
• Synthesis of a new circular strand occurs by
addition of dNTPs to the 3′ end using the intact
strand as a template.
Phage X174 replication
• When one round of replication is complete,
a full-length, single-stranded circle of DNA
is released.
• The process repeats over and over to yield
many copies of the phage genome.
Xenopus oocyte ribosomal DNA
(rDNA) amplification
• In oocytes of the South African clawed frog,
rDNA is amplified to form extrachromosomal
circles.
• The double stranded DNA replicates to form
many rDNA repeat units in length, then one
repeat’s worth is cleaved off by a nuclease.
• DNA ligase joins the end to form a circle.
Models for organelle DNA
replication
• There is no consensus on the mode of
replication of organelle DNA.
Models for chloroplast DNA (cpDNA)
replication
• A subject of debate particularly since there is
controversy over whether cpDNA is linear or
circular.
• Some evidence for a strand displacement
model.
• Other models include a theta replication
intermediate, rolling circle replication, and
recombination-dependent replication.
Models for mitochondrial DNA (mtDNA)
replication
• DNA polymerase  is used exclusively for
mtDNA replication.
• Two models for replication have been
proposed:
1. The strand displacement model
2. The strand coupled model
Strand displacement model:
• The most widely accepted model.
• Replication is unidirectional round the
circle and there is one replication fork for
each strand.
Later removed by RNase MRP
RNA
Strand coupled model:
• Semidiscontinous, bidirectional replication.
• Synthesis of Okazaki fragments on the
lagging strand.
Disease Box
RNase MRP and cartilage-hair
hypoplasia
• RNase MRP is an RNP that plays a
role in:
– Cleavage of RNA primers in
mtDNA replication.
– Nucleolar processing of pre-rRNA.
• Mutations in the RNA component
cause a rare form of dwarfism called
cartilage-hair hypoplasia.
6.7 Telomere maintenance:
the role of telomerase in DNA
replication, aging, and cancer
The end replication problem
• When the final primer is removed from the
lagging strand, an 8-12 nucleotide region is left
unreplicated.
• Predicts that chromosomes would get shorter
with each round of replication.
Telomeres
• Eukaryotic chromosomes end with tandem
repeats of a simple G-rich sequence.
Humans: TTAGGG
Tetrahymena: TTGGGG
• Seal the ends of chromosomes.
• Confer stability by keeping the chromosomes
from ligating together.
Solution to the end replication
problem
• Solution reported by Carol Greider and
Elizabeth Blackburn in 1985.
• Studied Tetrahymena thermophila, a singlecelled eukaryote with over 40,000 telomeres.
• Discovered the enzyme Telomerase.
• Shared the 2009 Nobel prize in physiology or
medicine with Jack Szostak.
• Telomerase is a ribonucleoprotein (RNP)
complex with reverse transcriptase activity.
• Contains an essential RNA component that
provides the template for telomere repeat
synthesis.
– RNA: Telomerase RNA component (TERC)
– Protein: Telomerase reverse transcriptase
(TERT)
Maintenance of telomeres by
telomerase
• Telomerase elongates the 3′ end of the
template for the lagging strand (G-rich
overhang).
• A pseudoknot in telomerase RNA is important
for processivity of repeat additions.
• Repeated translocation and elongation steps
results in chromosome ends with an array of
tandem repeats.
• Elongation of the shorter lagging strand (C-rich
strand) occurs by the normal replication
machinery.
• Alternatively, the 3′ overhang folds into a t-loop
structure, which prevents telomerase access.
Other modes of telomere
maintenance
• Telomerase-mediated telomere maintenance is
widespread among eukaryotes from ciliates to
yeast to humans.
• A striking exception is the fruitfly Drosophila
melanogaster, which maintains telomeres by
the addition of large retrotransposons.
• In human and fungi, telomeres can also be
maintained by a recombination-based
mechanism.
Regulation of telomerase activity
• Telomere length regulation involves the
accessibility of telomeres to telomerase.
• Length control involves a number of factors
including:
– Proteins POT1, TRF1, and TRF2
– t-loop formation
• A telomere-specific protein complex forms
called shelterin.
Model for length control
• POT1 binds to the TRF1 complex on the
double-stranded portion of telomeres.
• TRF1 (and TRF2) “count” the number of G-rich
repeats.
• Transfer of POT1 to the 3′ overhang.
When the telomere is long enough:
• POT1 levels are high at the 3′ overhang.
• The action of telomerase is blocked.
When the telomere is too short:
• Little or no POT1 is present at the 3′ end.
• Telomerase is no longer inhibited.
A model for t-loop formation
• The 3′ single-stranded DNA tail invades the
double-stranded telomeric DNA.
• A loop forms in which the 3′ overhang is base
paired to the C strand sequence.
• The t-loop may aid in preventing telomerase
access.
Telomerase, aging, and cancer
• In most unicellular organisms, telomerase has
a “housekeeping function.”
• In most human somatic cells, not enough
telomerase is expressed to maintain a constant
telomere length: Progressive shortening of
telomeres.
• High levels of telomerase activity in ovaries,
testes, rapidly dividing somatic cells, and
cancer cells.
Telomerase and aging: the Hayflick limit
• The Hayflick limit is the point at which cultured
cells stop dividing and enter an irreversible
state of cellular aging (senescence).
• Proposed to be a consequence of telomere
shortening.
Telomere shortening:
a molecular clock for aging?
• Telomerase: A target for anti-aging therapy or
anti-cancer therapy?
• Cellular senescence may be a mechanism to
protect multicellular organisms from cancer.
• Cancer cells become immortalized and thus
can grow uncontrolled.
• In most cancer cells, telomerase has been
reactivated.
Direct evidence for a relationship
between telomere shortening and aging
• Evidence from experiments in human cells in
culture and in transgenic mice.
• However, there are reports of instances where
short telomere length does not correlate with
entry into cellular senescence.
1. Effect of experimental activation of
telomerase on normal human somatic
cells
• Experiment carried out in telomerase-negative
normal human cell types.
• Demonstrated a link between telomerase
activity and cellular immortality.
2. Insights from telomerase-deficient
mice
Cells from mice engineered to lack a
telomerase RNA component:
• Progressive telomere shortening after 300 cell
divisions.
• After 450 divisions, cell growth stopped.
Sixth-generation mice lacking
telomerase RNA component
• Defects in spermatogenesis.
• Impaired proliferation of hematopoietic cells.
• Premature graying and hair loss.
Dyskeratosis congenita:
loss of telomerase activity
• Premature aging syndrome.
• Problems in tissues where cells multiply rapidly
and where telomerase is normally expressed.
• Two forms of dyskeratosis congenita:
– X-linked recessive
– Autosomal dominant
X-linked recessive dyskeratosis congenita
• Mutations in dyskerin gene.
• Dyskerin is a pseudouridine synthase that
binds to small nucleolar RNAs and to
telomerase RNA.
• Patients with dyskerin mutations have 5-fold
less telomerase activity than unaffected
siblings.
Autosomal dominant dyskeratosis congenita
• Mutations in telomerase RNA gene in the
pseudoknot domain.
• Partial loss of function of telomerase RNA.
3. Gene therapy for liver cirrhosis
• Inhibition of liver cirrhosis in mice by
telomerase gene delivery.
• Why hasn’t this gene therapy strategy
progressed to human trials?