dna replication 09-10

8/8/2019 DNA Replication 09-10

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Heavenly Father, in

whom we live and moveand have our being: Wehumbly pray thee so toguide and govern us bythy Holy Spirit, that in all

the cares and occupationsof our life we may notforget thee, but mayremember that we are

ever walking in thy sight;through Jesus Christ ourLord. Amen


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DNA Replication and Transcription

a). DNA replication

i). Cell cycle/ semi-conservative replicationii). Initiation of DNA replicationiii). Discontinuous DNA synthesisiv). Components of the replication apparatus

b). Transcriptioni). Process of RNA transcriptionii). Types of RNA molecules

iii). RNA processing


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Central Dogma

RNA

DNA

Protein

Transcription

Translation

Replication

Reverse

Transcription


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DNA is double stranded

DNA strands are antiparallel

G-C pairs have 3 hydrogen bonds

A-T pairs have 2 hydrogen bonds

Cellular DNA is almost exclusively

B DNA

B-DNA has ~10.5 bp/turn of the helix

DNA: structure


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The mammalian cell cycle

G1

S

G2

M

G0

DNA synthesis andhistone synthesis

Growth andpreparationforcell division

Rapid growth andpreparation forDNA synthesis

Quiescent cells

phase

phase

phase

phase


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SYNTHESIS OF HISTONES OCCURS DURINGTHE S PHASE

HISTONES ASSEMBLE INTO OCTAMERSTRUCTURES


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DNA replicationistheprocess by whichthe

genetic materialiscopied

Theoriginal DNA strandsare usedastemplates for

thesynthesisofnew strands

Itoccursvery quickly, very accurately andatthe

appropriatetimeinthelifeofthecell


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DNA REPLICATION is SIMILARBETWEEN

PROKARYOTES & EUKARYOTES :

-however, the E coli genome is a single circular DNA molecule:

- the human genome is on 46 linear molecules (chromosomes)


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DNA replicationreliesonthecomplementarity of

DNA strands

TheAT/GCruleorChargaffsrule

Theprocesscan besummarizedassuch

Thetwo DNA strandscomeapart

Eachservesasa templatestrandforthesynthesisof

new strands

Thetwonewly-madestrands = daughterstrands

Thetwooriginalones = parentalstrands


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Figure 11.1 11-4

Identical

base sequences


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Inthelate 1950s, threedifferent mechanisms

wereproposed forthereplicationof DNA

Conservative model Bothparentalstrandsstay togetherafterDNA replication

Semiconservative model

Thedouble-stranded DNA containsoneparentalandone

daughterstrand following replication

Dispersive model

ParentalanddaughterDNA areinterspersedin bothstrands

following replication


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Three

theoreticalmodelsproposed

Each had

differentexpectations forproducts of oneor two roundsof replications


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1958

E. coli

Naturally occurring (N14) and heavy (N15)isotopes of nitrogen

CsCl

Ultracentrifuge


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Interpreting the Data

Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display11-12

After one generation,

DNA is half-heavy

This is consistent with both semi-

conservative anddispersive models

After ~ two generations, DNA is of

two types: light and half-heavy

This is consistent with only

the semi-conservative model


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Requirements for DNA Replication


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Both DNA StrandsCouldBe Expressed

3 5

DNA

5 3

DNA

DNA issynthesizedin 5 3 direction

1. TEMPLATE


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2. Substrate

3. Enzymes

4. Primers


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DNA polymerasesaretheenzymesthatcatalyze

theattachmentofnucleotidesto makenew DNA

InE. colithereare fiveproteins withpolymeraseactivity

DNA pol I, II, III, IV and V

DNA pol I and III Normalreplication

DNA pol II, IV and V

DNA repairandreplicationofdamaged DNA

11-21


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DNA pol I

Composedofasinglepolypeptide

RemovestheRNA primersandreplacesthem with DNA

DNA pol III

Composedof 10 differentsubunits (Table 11.2)

TheE subunitsynthesizes DNA

Theother9 fulfillotherfunctions

Thecomplex ofall 10 isreferredtoastheDNA pol III

holoenzyme


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11-23


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DNA polymerase III remainsattachedtothe

templateasitissynthesizing thedaughterstrand

This processivefeatureisduetoseveraldifferentsubunitsinthe DNA pol III holoenzyme

F subunitisintheshapeofaring

Itistermedtheclampprotein

K subunitisneeded forFtoinitially clampontothe DNA Itistermedtheclamp-loaderprotein

HH and] subunitsareneeded fortheoptimal functionof

theEandFsubunits


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Moran et al 2006


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DNA polymerasescatalyzesaphosphodiesterbond

betweenthe

Innermostphosphate groupoftheincoming

deoxynucleosidetriphosphate

AND

3-OH ofthesugarofthepreviousdeoxynucleotide

Intheprocess, thelasttwophosphatesoftheincoming nucleotidearereleased

Inthe form ofpyrophosphate (PPi)


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Figure 11.10

Innermostphosphate


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E. colichromosomeiscircular, double-stranded DNA

(4.6x103 kilobasepairs)

Replication beginsata uniquesite (origin) Proceeds bidirectionally untilthetworeplicationcomplexes

meet (termination site)

Replisome - protein machinery forreplication (one

replisomeateachof 2 replication forks)

Thereplication forkseventually meetattheoppositesideof

the bacterialchromosome

Thisendsreplication


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Figure 11.4


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TheoriginofreplicationinE. coliistermedoriC

originofChromosomalreplication

Threetypesof DNA sequencesinoriCarefunctionally significant

AT-richregion

DnaA boxes

GATC methylationsites


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11-17

DNA replicationisinitiated by the binding of

DnaA proteins totheDnaA box sequences

This binding stimulatesthecooperative

binding ofanadditional 20 to 40 DnaA

proteinsto form alargecomplex

Otherproteinssuchas HU and

IHF also bind.

Thiscausestheregionto

wraparoundthe DnaA

proteinsandseparatesthe

AT-richregion


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Composedofsix subunits

Travelsalong the DNA in

the 5 to 3 direction

Usesenergy from ATP

Bidirectionalreplication


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Figure 11.7

Breaks the hydrogen

bonds between the

two strands

Alleviates

supercoiling

Keep the parental

strands apart

Synthesizes an

RNA primer


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Unusual features of DNA polymerase function

Problem isovercome by

theRNA primers

synthesized by primase

DNA polymerasescannot

initiate DNA synthesis

DNA polymerasescanattachnucleotidesonly in


Problem isovercome by

synthesizing the 3 to 5

strandsinsmall fragments


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DNA helicaseandprimasearephysically boundto

eachotherto form acomplex calledtheprimosome

Thiscomplex leadsthe way atthereplication fork

Theprimosomeisphysically associated withthe

DNA polymeraseholoenzyme forming the replisome


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DNA pol I removestheRNA primersand fillsthe

resulting gap with DNA It usesits 5 to 3 exonucleaseactivity todigesttheRNA

andits 5 to 3 polymeraseactivity toreplaceit with DNA

Afterthe gapis filledacovalent bondisstill

missing

DNA ligasecatalyzesaphosphodiesterbond Thereby connecting the DNA fragments


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Lagging strandsynthesisissummarizedassuch:

Thelagging strandislooped

Thisallowstheattached DNA polymerasetosynthesizetheOkazaki fragmentsinthenormal 5 to 3 direction

Uponcompletionofan Okazaki fragment, theenzyme

releasesthelagging templatestrand

Anotherloopisthen formed

Thisprocessedisrepeatedoverandoveragain


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1. Instability of mismatchedpairs

Complementary basepairshave muchhigherstability

than mismatchedpairs

This featureonly accounts forpartofthe fidelity Ithasanerrorrateof 1 per1,000 nucleotides

2. Configurationofthe DNA polymeraseactivesite

DNA polymeraseis unlikely tocatalyze bond formationbetween mismatchedpairs

Thisinduced-fitphenomenondecreasestheerrorrateto

arangeof 1 in 100,000 to 1 million


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3. Proofreading functionof DNA polymerase

DNA polymerasescanidentify a mismatchednucleotide

andremoveit from thedaughterstrand

Theenzyme usesits 3 to 5 exonuclease activity to

removetheincorrectnucleotide

Itthenchangesdirectionandresumes DNA synthesisin



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OppositetooriCisapairofterminationsequences

calledtersequences

ThesearedesignatedT1 andT2

Theproteintus (terminationutilizationsubstance)

bindstothesesequences

Itcanthenstopthe movementofthereplication forks


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Tus binds to specificsequences at the

termination site of DNAreplication

Tus blocks movement ofthe replisome


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DNA replicationends whenoppositely advancing

forks meet (usually atT1 orT2)

Finally DNA ligasecovalently linksall fourDNAstrands

DNA replicationoftenresultsintwointertwined

molecules

Intertwinedcircularmoleculesaretermedcatenanes

Theseareseparated by theactionoftopoisomerases


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

QUICKREVIEW


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Eukaryotic DNA replicationisnotas well

understoodas bacterialreplication

Thetwoprocessesdohaveextensivesimilarities, The bacterialenzymesdescribedinTable 1.1 havealso been

foundineukaryotes

Nevertheless, DNA replicationineukaryotesis more

complex Largelinearchromosomes

Tightpackaging withinnucleosomes

Morecomplicatedcellcycleregulation

The Eukaryotic DNA Replication


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Eukaryoteshavelong linearchromosomes

They thereforerequire multipleoriginsofreplication

Toensurethatthe DNA can bereplicatedinareasonabletime

In 1968, HubermanandRiggsprovidedevidence

forthe multipleoriginsofreplication

DNA replicationproceeds bidirectionally from many

originsofreplication


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Figure 11.20

Part (b) shows a micrograph of a replicating DNA chromosome

Bidrectional

DNA synthesis

Replication

forks will

merge


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Theoriginsofreplication foundineukaryoteshave

somesimilaritiestothoseof bacteria

OriginsofreplicationinSaccharomyces cerevisiae aretermed ARSelements (Autonomously Replicating

Sequence)

They are 100-150 bpinlength

They haveahighpercentageof A andT

They havethreeorfourcopiesofaspecificsequence

Similartothe bacterial DnaA boxes


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Originrecognitioncomplex (ORC)

A six-subunitcomplex thatactsastheinitiatorof

eukaryotic DNA replication

Itappearsto be foundinalleukaryotes

Requires ATPto bind ARSelements

Single-stranded DNA stimulates ORCtohydrolyze AT

P


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Mammaliancellscontain welloveradozendifferent

DNA polymerases

Four:alpha (E), delta (H), epsilon (I) and gamma (K)

havetheprimary functionofreplicating DNA

E, H andI NuclearDNA

K Mitochondrial DNA


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P ti f DNA l


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Properties of DNA polymerases

DNA polymerases of humans

EFKHILocation nucl nucl mito nucl nuclReplication yes no yes yes yesRepair no yes no yes yes3

Functions5 to 3 polymerase yes yes yes yes yes3 to 5 exonuclease no no yes yes yes

5 to 3 exonuclease1 no no no no noPrimase yes no no no no

Associates with PCNA2 no no no yes yesProcessivity low highStrand synthesis lagging* repair both leading* lagging*

1 activity present in associated proteins2 Proliferating Cell Nuclear Antigen sliding clamp3 involved in transcription-linked DNA repair


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DNA polE istheonly polymerasetoassociate with

primase

The DNA polE/primasecomplex synthesizesashortRNA-DNA hybrid

10 RNA nucleotides followed by 20 to 30 DNA nucleotides

Thisis used by DNA polHorIfortheprocessive

elongationoftheleading andlagging strands Currentevidencesuggestsa greaterrole forDNA polH

Theexchangeof DNA polE forHorIiscalleda

polymeraseswitch

Itoccursonly aftertheRNA-DNA hybridis made

Referto Figure 11.21


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Figure 11.21

Proteins at the replication fork in humans


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5

3

35

Proteins at the replication fork in humans

helicase

SSB

pol E(or pol H

DNA ligase

topoisomerases I and II

PCNA

primase activityassociated with pol E

pol H

leading strand

lagging strand

5 to 3exoassociated

with the

complex

pol I


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DNA polymerasesalsoplay arolein DNA

repair

DNA polF isnotinvolvedin DNA replication

Itplaysaroleinbase-excisionrepair

Removalofincorrect bases from damaged DNA

Recently, more DNA polymeraseshave been

identified

Lesion-replicating polymerases

Involvedinthereplicationofdamaged DNA

They cansynthesizeacomplementary strandoverthe

abnormalregion


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Replicationdoublestheamountof DNA

Thereforethecell mustsynthesize morehistonesto

accommodatethisincrease

Synthesisofhistonesoccursduring theSphase Histonesassembleintooctamerstructures

They associate withthenewly made DNA very nearthe

replication fork

Thus following DNA replication, eachdaughter

strandhasa mixtureof oldand newhistones

Referto Figure 11.22


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Lineareukaryoticchromosomeshavetelomeresatbothends

Theterm telomere referstothecomplex oftelomeric DNA sequencesand boundproteins


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Telomericsequencesconsistof

Moderately repetitivetandem arrays

3 overhang thatis 12-16 nucleotideslong

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Figure 11.23

Telomericsequencestypically consistof

Several guaninenucleotides

Often many thyminenucleotides


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DNA polymerasespossesstwo unusual features

1. They synthesize DNA only inthe 5 to 3 direction

2. They cannotinitiate DNA synthesis Thesetwo featuresposeaproblem atthe 3 endof

linearchromosomes

Figure 11.24


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Thereforeifthisproblem isnotsolved

Thelinearchromosome becomesprogressively shorter

witheachroundof DNA replication Indeed, thecellsolvesthisproblem by adding DNA

sequencestotheendsoftelomeres

Thisrequiresaspecialized mechanism catalyzed

by theenzymetelomerase

TelomerasecontainsproteinandRNA

TheRNA iscomplementary tothe DNA sequence found

inthetelomericrepeat

Thisallowsthetelomeraseto bindtothe 3 overhang

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F

igure 11.25

Step 1 = Binding

Step 3 = Translocation

The binding-

polymerization-

translocation cycle can

occurs many times

This greatly lengthens

one of the strands

The complementarystrand is made by primase,

DNA polymerase and ligase

RNA primer

Step 2 = Polymerization


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GENE TRANSCRIPTION AND

RNA MODIFICATION


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Transcription literally means the act orprocess of making a copy

In genetics, the term refers to the copyingof a DNA sequence into an RNA sequence


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Sequence of nucleotides inhuman beta-globin gene

The DNA sequences

highlighted in color show

the three regions of thegene that specify the

amino acid sequence.

Not all of the

DNA in genes isused to encode the

proteins that they

specify; much of

the rest is

concerned with

determining when,

and in what

amounts, the

protein encoded ismade regulatory

regions ofgenes

What is transcribed?


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Bacterial mRNA may be polycistronic, whichmeans it encodes two or more polypeptides

Start codon: specifies the first amino acid inaprotein sequence, usually a formylmethionine(in bacteria) or a methionine (in eukaryotes)

Signals the end ofprotein synthesis

Eukaryotic:monocistronic

CISTRON


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The promoter functions as a recognitionsite for transcription factors

The transcription factors enable RNApolymerase to bind to the promoterforming a closed promoter complex

Following binding, the DNA is denaturedinto a bubble known as the openpromoter complex, or simply an opencomplex

Initiation

Elongation

RNA polymerase slides along the DNAin an open complex to synthesize theRNA transcript

Termination A termination signal is reached that

causes RNA polymerase to dissociatedfrom the DNA


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Once they are made, RNA transcripts play differentfunctional roles

A structural gene is a one that encodes apolypeptide

When such genes are transcribed, the product is an RNAtranscript called messenger RNA (mRNA)

Well over 90% of all genes are structural genes


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Types of RNA %total cellular RNAmass

Ribosomal RNA (rRNA) 85-the RNA structural component of the ribosome-in eukaryotes there are 4 major forms: 28S, 18S and 5.8S and 5S-in prokaryotes there are only 3: 23S, 16S, and 5S

S refers to a Svedberg Unit, which is a measure of size based upon themolecular sedimentation rate during ultracentrifugation

Messenger RNA (mRNA) 2-the RNA that transfers genetic information stored in DNA into a formuseable for protein synthesis

Transfer RNA (tRNA) 12-assists in decoding the information contained within mRNA during

translation by recruiting the correct amino acid to the growing peptidechain

Other forms (snRNA, snoRNA) 1-small nuclear RNAs that participate in RNA processing


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The RNA transcripts from nonstructural genes arenot translated

They do have various important cellular functions

In some cases, the RNA transcript becomes part of acomplex that contains protein subunits

For example

Ribosomes

Spliceosomes

Signal recognition particles

12-10

TRANSCRIPTION IN BACTERIA


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Promoters are DNA sequences that promote geneexpression

More precisely, they direct the exact location for the

initiation of transcription

Promoters are typically located just upstream of thesite where transcription of a gene actually begins

The bases in a promoter sequence are numbered inrelation to the transcription start site

12-13

TRANSCRIPTION IN BACTERIA


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Figure 12.3 The conventional numbering system of promoters

Bases precedingthis are numbered

in a negativedirection

There is no basenumbered 0

Bases to the right arenumbered in a

positive direction

Sometimes termedthe Pribnow box,

after its discoverer

Sequence elements that playa key role in transcription


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Figure 12.4 Examples of 35 and 10 sequences within avariety of bacterial promoters

The most commonlyoccurring bases


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Binding of the RNA Polymerase to the bindingsites (prokaryotic)


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36.5kD

70kD

151kD

155kD

11kD

36.5kD

Corecomplex

HOLOENZYME

Prokaryotic RNA polymerase structure


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Prokaryotic RNA polymerase structure

RNA polymerase of bacteria is a multisubunit protein

Subunit Number Role

E 2 uncertainF(Rifampicin target) 1 forms phosphodiester bondsF 1 binds DNA templateW 1 recognizes promoter and

facilitates initiation

EFFW EFF + W

holoenzyme core polymerase sigma factor


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The binding of the RNA polymerase to thepromoter forms the closed complex

Then, the open complex is formed when theTATAAT box is unwound

A short RNA strand is made within the opencomplex

The sigma factor is released at this point

This marks the end of initiation

The core enzyme now slides down the DNA tosynthesize an RNA strand


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12-20


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Similar to the

synthesis of DNAvia DNApolymerase

Figure 12.7

Mechanism of RNA synthesis


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Mechanism of RNA synthesis

RNA synthesis usually initiated with ATP or GTP (the first nucleotide) RNA chains are synthesized in a 5 to 3 direction

A = T

U = A

A = T

U = A

RNA RNA


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Termination is the end of RNA synthesis

It occurs when the short RNA-DNA hybrid of the opencomplex is forced to separate

This releases the newly made RNA as well as the RNA polymerase

12-24


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5

Nelson & Cox, 2005, p. 1001


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RNAPol.

5

RNA

Terminator

RNAPol.

5RNA

V

RNAPol.

5

RNA

Help, rho

hit me!

V


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DNA

Cytoplasm

NucleusExport

G AAAAAA

RNA

Transcription

G AAAAAA

RNAProcessing

mRNA


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Many of the basic features of genetranscription are very similar in bacteria andeukaryotes

However, gene transcription in eukaryotes ismore complex

Larger organisms Cellular complexity

Multicellularity



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Nuclear DNA is transcribed by three different RNApolymerases

RNA pol I

Transcribes all rRNA genes (except for the 5S rRNA) RNA pol II

Transcribes all structural genes

Thus, synthesizes all mRNAs

Transcribes some snRNA genes

RNA pol III

Transcribes all tRNA genes

And the 5S rRNA gene


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Eukaryotic promoter sequences are more variableand often more complex than those of bacteria

For structural genes, at least three features arefound in most promoters

Transcriptional start site

TATA box

Regulatory elements

Figure 12 11


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Usually anadenine

The core promoter is relatively short

It consists of the TATA box Important in determining the precise start point for

transcription

The core promoter by itself produces a low level oftranscription This is termed basal transcription

Figure 12.11

Figure 12 11


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Figure 12.11

Regulatory elements affect the binding of RNA

polymerase to the promoter They are of two types Enhancers

Stimulate transcription

Silencers Inhibit transcription

They vary in their locations but are often found in the50 to 100 region

Usually anadenine


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Factors that control gene expression can be dividedinto two types, based on their location

cis-acting elements DNA sequences that exert their effect only on nearby

genes

Example: TATA box, enhancers and silencers

trans-acting elements

Regulatory proteins that bind to such DNA sequences


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Three categories of proteins are required for basaltranscription to occur at the promoter

RNA polymerase II

Five different proteins called general transcription factors(GTFs)

A protein complex called mediator

Figure 12.12 shows the assembly of transcriptionfactors and RNA polymerase II at the TATA box

12-35

Figure 12 12


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12-36

Figure 12.12


Figure 12 12A closed complex


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Figure 12.12


TFIIH plays a major role in theformation of the open complex It has several subunits that

perform different functions

One subunit hydrolyzes ATP and phosphorylates adomain in RNA pol II known as the carboxyl terminaldomain (CTD) This releases the contact between TFIIB and

RNA pol II Other subunits act as helicases

Promote the formation of the open complex

Released after

the opencomplex is

formed

RNA pol II can nowproceed to the

elongation stage


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Positions of promoter elements in

tRNA and 5S rRNA genes.

Initiation of transcription of a tRNA gene:

1. The TFIIIC transcription factor binds viarecognition of the A and B sites

2. This permits subsequent binding of thetrimeric TFIIIB factor immediatelyupstream of the transcription start site.

3. In response to TFIIIB binding, RNApolymerase III is recruited and initiatestranscription.

In the case of 5S rRNA genes, the process issimilar, except that an additional factor,TFIIIA, is required. TFIIIA binds the C box,which permits subsequent binding of TFIIIBand TFIIIC, then recruitment of RNA pol III.


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One model:1. Two identical subunits of the

upstream binding factor bind tothe upstream core element and thecore promoter element.

2. Protein:protein interactionsbetween UBF molecules forcethese two DNA sequences to comeinto close proximity.

3. This enables subsequent binding ofselectivity factor I, which consists

of four subunits.

4. Ultimately, this stabilized structurepermits binding of other factors(not shown), and finally RNA pol I.


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Analysis of bacterial genes in the 1960s and 1970revealed the following: The sequence of DNA in the coding strand corresponds

to the sequence of nucleotides in the mRNA

This in turn corresponds to the sequence of amino acid inthe polypeptide

This is termed the colinearity of gene expression

Analysis of eukaryotic structural genes in the late1970s revealed that they are not always colinearwith their functional mRNAs


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Instead, coding sequences, called exons, areinterrupted by intervening sequences or introns

Transcription produces the entire gene product Introns are later removed or excised

Exons are connected together or spliced

This phenomenon is termed RNA splicing It is a common genetic phenomenon in eukaryotes

Occurs occasionally in bacteria as well


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35 Exon 2 Exon 3Int. 2Exon 1 Int. 1Protein Coding Region

3 Untranslated Region5 Untranslated Region

3AAAAA

3 Poly A Tail

5G

5 Cap

Exon 2 Exon 3Exon 1

RNA processing achieves three things: Removal of introns

Addition of a 5cap

Addition of a 3 tail

l This signals the mRNA is ready to move outof the nucleus and may control its life span

in the cytoplasm


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is then methylated by 2'-O-methyltransferases.


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Most mature mRNAs have a string of adeninenucleotides at their 3ends

This is termed the polyA tail

The polyA tail is not encoded in the gene sequence

It is added enzymatically after the gene is completelytranscribed

The attachment of the polyA tail is shown inFigure 12.20

Figure 12.20


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Consensus sequence inhigher eukaryotes

Appears to be important in thestability of mRNA and the

translation of the polypeptide

Length varies betweenspecies

From a few dozen adeninesto several hundred


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The spliceosome is a large complex that splicespre-mRNA

It is composed of several subunits known assnRNPs (pronounced snurps)

Each snRNP contains small nuclear RNA and a set ofproteins

In eukaryotes, the


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12-62

Figure 12.16

transcription of structuralgenes, produces a long

transcript known as pre-mRNA

Also as heterogeneous nuclearRNA (hnRNA)

This RNA is altered by splicingand other modifications,before it leaves the nucleus

Splicing in this case requiresthe aid of a multicomponentstructure known as thespliceosome


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The subunits of a spliceosome carry out severalfunctions

1. Bind to an intron sequence and precisely recognizethe intron-exon boundaries

2. Hold the pre-mRNA in the correct configuration

3. Catalyze the chemical reactions that remove intronsand covalently link exons

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Intron RNA is defined by particular sequences within the


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Figure 12.21

intron and at the intron-exon boundaries

The consensus sequences

Sequences shown in boldare highly conserved

Corresponds to the boxedadenine in Figure 12.22

Serve as recognition sites forthe binding of the spliceosome


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Intron loops out andexons brought closer

together


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Figure 12.22

Intron will be degraded andthe snRNPs used again


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Transcription factors exhibit a number of

different motifs found in the area known tobind DNA:

Zinc finger -First found in TFIIIA

Helix-turn-helix - First described from phagereceptors

Amphipathic Helix-loop-helix - Identified insome development regulators

Leucine zipper - Held together byinteractions between leucine amino acids


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Zn++

H

H

C

C

Zn++

H

H

C

C

Zn++

H

H

C

C

23Aminoacids

7 - 8 Amino acid linker

2- 4Aminoacids

6Aminoacids


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actinomycin D, acridine:

-intercalate between successive G=C base

pairs in duplex DNA-inhibit transcriptional elongation in pro- andeukaryotes

rifampicin:

-binds theF subunit of bacterial RNApolymerase

-blocks promoter clearance (elongation)

E-amanitin:-produced by fungusAmanita phalloides(death cap mushroom)

-potent inhibitor of RNA pol II and weakinhibitor of RNA pol III

Nelson & Cox, 2005, p. 1006


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Novobiocin

Nalidixic acid and ciprofloxacin

Camptothecin

2,3 deoxyinosine cytosine arabinoside (cytarabin, araC)


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Give us, O Lord, a steadfast heart, which no unworthy affectionmay drag downwards;

Give us an unconquered heart, which no tribulation can wear out;

Give us an upright heart, which no unworthy purpose may temptaside.

Bestow upon us also, O Lord our God, understanding to knowyou, diligence to seek you, wisdom to find you, and a faithfulnessthat may finally embrace you; through Jesus Christ our Lord.

St. ThomasAquinas


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dna replication 09-10

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