Chap11 studyppt

Chapter 11

Study Powerpoints

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INTRODUCTION

DNA replication is the process by which the genetic material is copied The original DNA strands are used as templates

for the synthesis of new strands

It occurs very quickly, very accurately, and at the appropriate time in the life of the cell This chapter examines how!

11-2Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

11.1 STRUCTURAL OVERVIEW OF DNA REPLICATION DNA replication relies on the complementarity of

DNA strands The AT/GC rule or Chargaff’s rule

The process can be summarized as follows: The two complementary DNA strands come apart Each serves as a template strand for the synthesis of

new complementary DNA strands The two newly-made DNA strands = daughter strands The two original DNA strands = parental strands

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T A

G

C

A

G

A T

T A

T

G

GA

A

C

C

CT

T

G

C G

T A

T A

C

G

A T

T A

C G

T

A T

C G

C

C G

A T

C G

CA

CGG

C

Incomingnucleotides

Original(template)strand

Original(template)strand

Newlysynthesizeddaughter strand

Replicationfork

(a) The mechanism of DNA replication (b) The products of replication

Leadingstrand

Laggingstrand

5′ 3′

3′ 5′

A T

A T

T A

T A

T A

C G

C G

G CG C

G CG C

C G

A T

5′ 3′

5′ 3′

3′ 5′

A T

A T

T A

T A

T A

C G

C G

G CG C

G CG C

C G

A T

3′ 3′

3′ 5′

A T

A T

T A

T A

T A

C G

C G

G CG C

G CG C

C G

A T


5′3′

A

A T

C

Figure 11.111-4

Identical base sequences

A pairs with T and G pairs with C during synthesis of a new strand

Experiment 11A: Which Model of DNA Replication is Correct?

In the late 1950s, three different mechanisms were proposed for the replication of DNA Conservative model

Both parental strands stay together after DNA replication

Semiconservative model The double-stranded DNA contains one parental and one

daughter strand following replication

Dispersive model Parental and daughter DNA are interspersed in both strands

following replication


Figure 11.211-6

(a) Conservative model

First round ofreplication

Second roundof replication

Originaldoublehelix

(b) Semiconservative model (c) Dispersive model


In 1958, Matthew Meselson and Franklin Stahl devised a method to investigate these models They found a way to experimentally distinguish

between daughter and parental strands

Their experiment can be summarized as follows: Grow E. coli in the presence of 15N (a heavy isotope of

Nitrogen) for many generations The population of cells had heavy-labeled DNA

Switch E. coli to medium containing only 14N (a light isotope of Nitrogen)

Collect sample of cells after various times Analyze the density of the DNA by centrifugation using

a CsCl gradient


The Hypothesis

Based on Watson’s and Crick’s ideas, the hypothesis was that DNA replication is semiconservative.

Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

Testing the Hypothesis

Refer to Figure 11.3

11-8

11-9Figure 11.3

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.Experimental level Conceptual level

2. Incubate the cells for various lengths oftime. Note: The 15N-labeled DNA isshown in purple and the 14N-labeledDNA is shown in blue.

3. Lyse the cells by the addition oflysozyme and detergent, whichdisrupt the bacterial cell wall andcell membrane, respectively.

4. Load a sample of the lysate onto a CsClgradient. (Note: The average density ofDNA is around 1.7 g/cm3, which is wellisolated from other cellularmacromolecules.)

5. Centrifuge the gradients until the DNAmolecules reach their equilibriumdensities.

6. DNA within the gradient can beobserved under a UV light.

DNA

Cell wall

Cell membrane

LightDNA

Half-heavyDNA

HeavyDNA

UVlight

(Result shown here isafter 2 generations.)

CsClgradient

Lysate

Lysecells

37°C

14Nsolution

Suspension ofbacterialcells labeledwith 15N

Up to 4 generations

Density centrifugation

Generation

0

1

Add 14N

2

1. Add an excess of 14N-containingcompounds to the bacterial cells soall of the newly made DNA will contain14N.

Light

Half-heavy

Heavy

Generations After 14N Addition

4.1 3.0 2.5 1.9 1.5 1.1 1.0 0.7 0.3


*Data from: Meselson, M. and Stahl, F.W. (1958) The Replication of DNA in Escherichia coli. Proc. Natl. Acad. Sci. USA 44: 671−682

Interpreting the Data


After one generation, DNA is “half-heavy”

This is consistent with both semi-conservative and dispersive models

After ~two generations, DNA is of two types: “light” and “half-heavy”This is consistent with only the semi-conservative model

11.2 BACTERIAL DNA REPLICATION Figure 11.4 presents an overview of the process

of bacterial chromosomal replication

DNA synthesis begins at a site termed the origin of replication Each bacterial chromosome has only one origin of replication

Synthesis of DNA proceeds bidirectionally around the bacterial chromosome

The two replication forks eventually meet at the opposite side of the bacterial chromosome This ends replication


0.25 μm

(b) Autoradiograph of an E. coli chromosome in the act of replication

(a) Bacterial chromosome replication

Replicationforks

Origin ofreplication

Replicationfork

Site wherereplicationends


From Cold Spring Harbor Symposia of Quantitative Biology, 28, p. 43 (1963). Copyright holder is Cold Spring Habour Laboratory Press.

Replicationfork

11-13

Figure 11.4


The origin of replication in E. coli is termed oriC origin of Chromosomal replication

Three types of DNA sequences in oriC are functionally important AT-rich region DnaA boxes GATC methylation sites


Initiation of Replication

11-14

11-15Figure 11.5

E. colichromosome

oriC

G GG G GGGA GAGAAAAAA GAA AAT

T

T T ATT TTTA ATTTTTC T TC ATTCT TCCC

1

CC C CCCT CTCTTTTTT CTT TTA A A T AA AAAT TAAAAAG A AG T AAGA AGG

T AG T CCTT AACAAGGAT AGC CAG T T CCT T

T

CGDnaA box

DnaA box

DnaA box

DnaA box

DnaA box

T TGGATCA T CG CTGGA GGA TC A GGAA TTGTTCCT A TCG GTC A A GGA AGCA ACCTAGT A GC GACCT CCA

T CT ACAT GAATCCTGG GAA GCA A A ATT GGAA TCTGAAA A CT ATGTG TA

A

G

C CC C GGTT TACAGCTGG CT

T

T

ATG A A TGA TCGG AGTTACG G AA AAAAC GAAG GG G CCAA ATGTCGACC GT A TAC T T ACT AGCC TCAATGC C TT TTTTG CTT

A GC A TACT GA CGTTCT GTG AGG G T CTA CTCC TGGTTCA T AA CTCTC AAAT CG T ATGA CT AGCAAGA ACCTCC C A GAT GAGG ACCAAGT A TT GAGAG TTT

GA T GTAC CAGTA CA GCA T CAGG CACT A CATG GTCAT GT A CGT A GTCC GT

A GA A TGTA CTT AGGACC CTT CGT T T T AA CCTT AGACTTT T GA T ACAC ATC

AT-rich region

5′–

–

50

51 100

101 150

201

251 275

250

151 200


3′

AT-rich region DnaA boxes

DNA helicase (DnaB protein) binds to theorigin. DnaC protein (not shown) assiststhis process.

DnaA protein


5′ 3′

AT- rich

region

3′ 5′

5′3′ 3′

5′

11-16

Figure 11.6

DNA replication is initiated by the binding of DnaA proteins to the DnaA box sequences This binding stimulates the cooperative

binding of additional ATP-bound DnaA proteins to form a large complex

Other proteins such as HU and IHF also bind. This causes the DNA to wrap

around the DnaA proteins causing the separation of the AT-rich region

11-17

Figure 11.6Composed of six subunits

Travels along the DNA in the 5’ to 3’ direction

Uses energy from ATP

Bidirectional replication is initiated

Helicase

DNA helicase separates the DNA in bothdirections, creating 2 replication forks.

ForkFork

5′3′

5′3′

3′5′

3′

5′


DNA helicase separates the two DNA strands by breaking the hydrogen bonds between them

This generates positive supercoiling ahead of each replication fork DNA gyrase travels ahead of the helicase and alleviates

these supercoils

Single-strand binding proteins bind to the separated DNA strands to keep them apart

Then short (10 to 12 nucleotides) RNA primers are synthesized by primase These short RNA strands start, or prime, DNA synthesis

The leading strand has a single primer, the lagging strand needs multiple primers

They are eventually removed and replaced with DNA11-18

Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display 11-19Figure 11.7

5′3′

5′

5′

3′

3′DNA polymerase III

Origin

Leadingstrand

Lagging strand

Linked OkazakifragmentsDirection of fork movement

Functions of key proteins involved with DNA replication

DNA polymerase III

RNAprimer

Okazaki fragment

DNAligase

RNA primer

Single-strandbinding protein

DNA helicase

Topoisomerase

Parental DNA

Primase

Replication fork


• DNA helicase breaks the hydrogen bonds between the DNA strands.

• Topoisomerase alleviates positive supercoiling.

• Single-strand binding proteins keepthe parental strands apart.

• Primase synthesizes an RNAprimer.

• DNA polymerase III synthesizes adaughter strand of DNA.

• DNA polymerase I excises theRNA primers and fills in withDNA (not shown).

• DNA ligase covalently links theOkazaki fragments together.



DNA polymerases are the enzymes that catalyze the attachment of nucleotides to synthesize a new DNA strand

In E. coli there are five proteins with polymerase activity DNA pol I, II, III, IV and V

DNA pol I and III Normal replication

DNA pol II, IV and V DNA repair and replication of damaged DNA

DNA Polymerases

11-21


DNA pol I Composed of a single polypeptide Removes the RNA primers and replaces them with DNA

DNA pol III Responsible for most of the DNA replication Composed of 10 different subunits (Table 11.2)

The subunit catalyzes bond formation between adjacent nucleotides (DNA synthesis)

The other 9 fulfill other functions

The complex of all 10 subunits is referred to as DNA polymerase holoenzyme

11-22

DNA Polymerases

11-23

(a) Schematic side view of DNA polymerase III

3′

3′ exonucleasesite3′

5′ 5′

Fingers

Thumb

DNA polymerasecatalytic site

Templatestrand

Palm

Incomingdeoxyribonucleoside triphosphates(dNTPs)


Bacterial DNA polymerases may vary in their subunit composition However, they all have the same type of catalytic subunit

11-24

Figure 11.8

Structure resembles a human right hand

Template DNA is threaded through the palm

Thumb and fingers wrapped around the DNA


11-25Unusual features of DNA polymerase functionFigure 11.9

Problem is overcome by making RNA primers using primase

DNA polymerases cannot initiate DNA synthesis on a bare template strand

DNA polymerases can attach nucleotides only in the 5’ to 3’ direction

Problem is overcome by synthesizing the new strands both toward, and away from, the replication fork

(b)

(a)

3′

5′

5′

3′

3′

5′

5′

3′Cannot link nucleotides in this direction

Able tocovalently linktogether

Can link nucleotidesin this direction

Unable tocovalently linkthe 2 individualnucleotides together


Primer


The two new daughter strands are synthesized in different ways Leading strand

One RNA primer is made at the origin DNA pol III attaches nucleotides in a 5’ to 3’ direction

as it slides toward the opening of the replication fork

Lagging strand Synthesis is also in the 5’ to 3’ direction

However it occurs away from the replication fork

Many RNA primers are required DNA pol III uses the RNA primers to synthesize small

DNA fragments (1000 to 2000 nucleotides each) These are termed Okazaki fragments after their discoverers

11-26


DNA pol I removes the RNA primers and fills the resulting gap with DNA Uses its 5’ to 3’ exonuclease activity to digest the RNA Uses its 5’ to 3’ polymerase activity to replace it with

DNA

After the gap is filled a covalent bond is still missing

DNA ligase catalyzes the formation of a covalent phosphoester bond Thereby connecting the DNA fragments

11-27

11-28Figure 11.10

Origin of replication

Replicationforks

Direction ofreplication fork

FirstOkazakifragment

First and second Okazakifragments have beenconnected to each other.

First Okazaki fragmentof the lagging strand

SecondOkazakifragment

ThirdOkazakifragment

Primer

Primer

The leading strand elongates,and a second Okazaki fragmentis made.

The leading strand continues toelongate. A third Okazakifragment is made, and the firstand second are connectedtogether.

Primers are needed to initiate DNA synthesis.The synthesis of the leading strand occurs inthe same direction as the movement of thereplication fork. The first Okazakifragment of the lagging strand ismade in the opposite direction.

5′

5′

5′

5′

3′

5′

3′

3′

5′

3′

3′

3′

5′

5′

5′

5′

3′

3′

3′

3′

5′

5′3′

3′


Leadingstrand

DNA strands separate atorigin, creating 2 replicationforks.

5′

3′ 5′

3′

Origin of replication

Replication fork Replication fork

Leadingstrand

Laggingstrand

Leadingstrand

Laggingstrand

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 11-29Figure 11.11

The synthesis of leading and lagging strands from a single origin of replication


DNA polymerases catalyzes the formation of a covalent (phosphoester) bond between the Innermost phosphate group of the incoming

deoxyribonucleoside triphosphate and

3’-OH of the sugar of the previous deoxynucleotide

In the process, the last two phosphates of the incoming nucleotide are released In the form of pyrophosphate (PPi)

Refer to figure 11.12

11-30

The Reaction of DNA Polymerase

11-31

Figure 11.12New DNA strand Original DNA strand


O

OOO

O

O

P CH2

O –

Incoming nucleotide(a deoxyribonucleosidetriphosphate)

Pyrophosphate (PPi)

Newesterbond

5′

5′ end

OOO P CH2

O –

OOO

CH2

O –

O

OO

O – P

O –

5′

OOO

O

O

P CH2

5′

5′ end

3′ end

OO P CH2

O –

O

OOO P CH2

O –O

O P

O –

O

P

O –

O –O– +

OH

3′

OH

3′

OH

3′

PP

O

Cytosine Guanine

Guanine Cytosine

Thymine Adenine

Cytosine Guanine

Guanine Cytosine

Thymine Adenine

O

OO

O

O

PH2C

H2C

O –

3′

O

OO P

O

O

H2C

O –

OO P

O –

5′

5′ end

O

OO

O

O

PH2C

H2C

O –

3′

O

OO P

O

O

H2C

O –

OO P

O –

5′

3′ end

5′ end

O

O –

O –

3′ end

Innermost phosphate


DNA polymerase III remains attached to the template as it is synthesizing the daughter strand

This processive feature is due to several different subunits in the DNA pol III holoenzyme subunit forms a dimer in the shape of a ring around

template DNA It is termed the clamp protein Once bound, the subunits can freely slide along dsDNA Promotes association of holoenzyme with DNA

complex catalyzes dimer clamping to the DNA It is termed the clamp-loader complex Includes ’, and subunits

11-32

DNA Polymerase III is a Processive Enzyme


The effect of processivity is quite remarkable

In the absence of the subunit DNA pol III falls off the DNA template after about 10

nucleotides have been polymerized Its rate is ~ 20 nucleotides per second

In the presence of the subunit DNA pol III stays on the DNA template long enough to

polymerize up to 500,000 nucleotides Its rate is ~ 750 nucleotides per second

11-33

DNA Polymerase III is a Processive Enzyme


On the opposite side of the chromosome to oriC is a pair of termination sequences called ter sequences These are designated T1 and T2

T1 stops counterclockwise forks, T2 stops clockwise forks

The protein tus (termination utilization substance) binds to the ter sequences tus bound to the ter sequences stops the movement of

the replication forks


Termination of Replication

11-34

Fork

Fork

Fork

Fork

ter(T2)

oriC

oriC

(T1)

Tus

Tus


ter

11-35

Figure 11.13


Prevents advancement of fork moving right-to-left (counterclockwise fork)

Prevents advancement of fork moving left-to-right (clockwise fork)


DNA replication ends when oppositely advancing forks meet (usually at T1 or T2)

Finally DNA ligase covalently links the two daughter strands

DNA replication often results in two intertwined molecules Intertwined circular molecules are termed catenanes These are separated by the action of topoisomerase

Termination of Replication

11-36

11-37

Figure 11.14

Catenanes

Catalyzed by DNA topoisomerase

Replication

Decatenation viatopoisomerase



DNA helicase and primase are physically bound to each other to form a complex called the primosome This complex leads the way at the replication fork

The primosome is physically associated with two DNA polymerase holoenzymes to form the replisome

Figure 11.15 provides a three-dimensional view of DNA replication

DNA Replication Complexes

11-38

11-39

Figure 11.15


Two DNA pol III proteins act in concert to replicate both the leading and lagging strands The two proteins form a dimeric DNA polymerase that

moves as a unit toward the replication fork

DNA polymerases can only synthesize DNA in the 5’ to 3’ direction Synthesis of the leading strand is continuous Synthesis of the lagging strand is discontinuous


11-40


Lagging strand synthesis is summarized as follows:

The lagging strand is looped This allows the attached DNA polymerase to synthesize the

Okazaki fragments in the 5’ to 3’ direction yet move toward the fork

Upon completion of an Okazaki fragment, DNA polymerase releases the lagging template strand

The clamp loader complex then reloads the polymerase at the next RNA primer

Another loop is then formed

This process is repeated over and over again


11-41


DNA replication exhibits a high degree of fidelity Mistakes during the process are extremely rare

DNA pol III makes only one mistake per 108 bases made

There are three reasons why fidelity is high 1. Instability of mismatched pairs 2. Configuration of the DNA polymerase active site 3. Proofreading function of DNA polymerase

Proofreading Mechanisms

11-42


1. Instability of mismatched pairs Complementary base pairs have much higher stability

than mismatched pairs Stability of base pairs only accounts for part of the fidelity

Error rate for mismatched base pairs is 1 per 1,000 nucleotides

2. Configuration of the DNA polymerase active site DNA polymerase is unlikely to catalyze bond formation

between mismatched pairs This induced-fit phenomenon decreases the error rate to

a range of 1 in 100,000 to 1 million


11-43


3. Proofreading function of DNA polymerase

DNA polymerases can identify a mismatched nucleotide and remove it from the daughter strand

The enzyme uses a 3’ to 5’ exonuclease activity to digest the newly made strand until the mismatched nucleotide is removed

DNA synthesis then resumes in the 5’ to 3’ direction Refer to figure 11.16


11-44

11-45A schematic drawing of proofreadingFigure 11.16

Site where nucleotides are removed from the 3’ end

C

T

3′

3′

5′ 5′

Mismatch causes DNA polymerase to pause,leaving mismatched nucleotide near the 3′ end.

Templatestrand

The 3′ end enters theexonuclease site.

3′

5′ 5′

At the 3′ exonuclease site,the strand is digested inthe 3′ to 5′ direction until theincorrect nucleotide isremoved.

3′

5′ 5′


Base pair mismatch near the 3′ end

3′

3′

Incorrectnucleotideremoved

exonucleasesite

11.3 EUKARYOTIC DNA REPLICATION Eukaryotic DNA replication is not as well

understood as bacterial replication

The two processes do have extensive similarities The types of bacterial enzymes described in Table 11.1 have

also been found in eukaryotes

Nevertheless, DNA replication in eukaryotes is more complex Large linear chromosomes Chromatin is tightly packed within nucleosomes More complicated cell cycle regulation



Eukaryotes have long linear chromosomes They therefore require multiple origins of replication

To ensure that the DNA can be replicated in a reasonable amount of time

In 1968, Huberman and Riggs provided evidence for multiple origins of replication Refer to Figure 11.21

DNA replication proceeds bidirectionally from many origins of replication Refer to Figure 11.22

Multiple Origins of Replication

11-63

11-65Figure 11.22

Chromosome Sister chromatids

Before S phase During S phase End of S phase

Origin

Origin

Origin

Origin

Origin

Centromere



The origins of replication found in eukaryotes have some similarities to those of bacteria

Origins of replication in Saccharomyces cerevisiae are termed ARS elements (Autonomously Replicating Sequence) They are about 50 bp in length They have a high percentage of A and T ARS consensus sequence (ACS)

ATTTAT(A or G)TTTA


11-66


Replication begins with assembly of the prereplication complex (preRC) Consists of at least 14 different proteins

An important part of the preRC is the Origin recognition complex (ORC) A six-subunit complex that acts as the initiator of

eukaryotic DNA replication Other preRC proteins include MCM Helicase

Binding of MCM completes DNA replication licensing The origin becomes capable of initiating DNA synthesis

Binding of at least 22 additional proteins is required to initiate synthesis during S phase


11-67


Mammalian cells contain well over a dozen different DNA polymerases Refer to Table 11.4

Four: alpha (), delta (), epsilon () and gamma () have the primary function of replicating DNA , and Nuclear DNA Mitochondrial DNA

Eukaryotes Contain Several Different DNA Polymerases

11-68

11-69

*The designations are those of mammalian enzymes.†Many DNA polymerases have dual functions. For example, DNA polymerases α, δ, and ε are involved in the replication of normal DNA and also play a role in DNA repair. In cells of the immune system, certain genes that encode antibodies (i.e., immunoglobulin genes) undergo aphenomenon known as hypermutation. This increases the variation in the kinds of antibodies the cells can make. Certain polymerases in this list, such as η, may play a role in hypermutation of immunoglobulin genes. DNA polymerase σ may play a role in sister chromatid cohesion, atopic discussed in Chapter 10 .


DNA pol is the only polymerase to associate with primase The DNA pol /primase complex synthesizes a short

RNA-DNA hybrid primer 10 RNA nucleotides followed by 20 to 30 DNA nucleotides

This hybrid primer is used by DNA pol or for the processive elongation of the leading and lagging strands, respectively

The exchange of DNA pol for or is required for elongation of the leading and lagging strands. This is called a polymerase switch

It occurs only after the RNA-DNA hybrid is made

11-70


DNA polymerases also play a role in DNA repair DNA pol is not involved in DNA replication It plays a role in base-excision repair

Removal of incorrect bases from damaged DNA

Recently, more DNA polymerases have been identified Lesion-replicating polymerases

Involved in the replication of damaged DNA They can synthesize a complementary strand over the

abnormal region

11-71


Flap Endonuclease Removes RNA Primers Polymerase runs into primer of adjacent

Okazaki fragment Pushes portion of primer into a short flap Flap endonuclease removes the primer Long flaps are shortened by Dna2 nuclease/helicase Refer to Figure 11.23

11-72


Figure 11.23

3′

5′

DNA polymerase δ elongates theleft Okazaki fragment and causesa short flap to occur on the rightOkazaki fragment.

Flap

5′

3′

3′

5′

Process continues until theentire RNA primer is removed.DNA ligase seals the twofragments together.

5′

3′

Flap endonucleaseremoves the flap.

5′

3′

3′

5′

DNA polymerase δcontinues to elongate andcauses a second flap.

5′

3′

3′

5′

Flap endonucleaseremoves the flap.

5′

3′

3′

5′

5′

3′

3′

5′



Linear eukaryotic chromosomes have telomeres at both ends

The term telomere refers to the complex of telomeric DNA sequences and bound proteins

Telomeres and DNA Replication

11-74


Telomeric sequences consist of Moderately repetitive tandem arrays 3’ overhang that is 12-16 nucleotides long

11-75

Figure 11.24

Telomeric sequences typically consist of Several guanine nucleotides Many thymine nucleotides

Refer to Table 11.5

Telomeric repeat sequences

OverhangCG

CG

CG

TA

AT

AT

CG

CG

CG

TA

AT

AT

CG

CG

CG

TA

AT

AT

CG

CG

CG

TA

AT

AT

CG

CG

CG

TA

AT

AT

CG

CG

CG

TA

AT

AT

CG

CG

CG

TA

AT

TAT G G GA

AT

AT G G GAT AT T T GGG

5′

3′


11-76

DNA polymerases possess two unusual features 1. They synthesize DNA only in the 5’ to 3’ direction 2. They cannot initiate DNA synthesis

These two features pose a problem at the 3’ ends of linear chromosomes-the end of the strand cannot be replicated!

11-77

Figure 11.25Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

DNA polymerase cannot linkthese two nucleotides togetherwithout a primer.

No placefor aprimer

3′

5′


Therefore if this problem is not solved The linear chromosome becomes progressively shorter

with each round of DNA replication

The cell solves this problem by adding DNA sequences to the ends of telomeres

This requires a specialized mechanism catalyzed by the enzyme telomerase

Telomerase contains protein and RNA The RNA is complementary to the DNA sequence found

in the telomeric repeat This allows the telomerase to bind to the 3’ overhang

The lengthening mechanism is outlined in Figure 11.26

11-78

11-79Figure 11.26

Step 1 = Binding

Step 3 = Translocation

The binding-polymerization-translocation cycle can occurs many times

This greatly lengthens one of the strands

The end is now copied

Step 2 = Polymerization

RNA primer is made and other strand is synthesized.

Telomerase reverse transcriptase (TERT) activity

Telomere

Telomerase

Eukaryoticchromosome

Repeat unit

3′

3

5′

T T A G G G T T A

A A T C C C A A TC CC A A U C C C

G G G A G G GT T A T TG G G

T T A G G G T T A

CCC A A U C C C

G G G T T A T T G

GG

T T AG G G A G G G

C CC A A U C C C

TT

C C C A A T A A A AT C C C U A AC UC CC C C

T T A G G G T T A G G G T T A T T GT T AG G G A G G G G G

T T A G G G T T A G G G T T A T T GT T AG G G A G G G G G GT T A G G

A A T C C C A A T

A A T C C C A A T

A A T C C C A A T

RNA

RNA primer

Telomerase synthesizesa 6-nucleotide repeat.

Telomerase moves 6nucleotides to the right andbegins to make another repeat.

The complementarystrand is made by primase,DNA polymerase, and ligase.

3′ 5′5′ 3′

3′


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complementary

display dna

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labeled dna

dna polymerases

dna pol

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