Eukaryotic DNA replication by kk sahu

Introduction
History
Definition
Classification of DNA Polymerase
Mechanism of DNA Replication
Process of DNA Replication
Initiation
Regulation
Termination
Conclusion
Reference
 
DNA replication is semi-conservative, one strand serves as the template for the second strand. Furthermore, DNA replication only occurs at a specific step in the cell cycle.
DNA replication in eukaryotes is much more complicated than in prokaryotes, although there are many similar aspects.
DNA replication is a biological process that occurs in all living organisms and copies their DNA; it is the basis for biological inheritance.
Eukaryotic cells can only initiate DNA replication at a specific point in the cell cycle, the beginning of S phase.
Due to the size of chromosomes in eukaryotes, eukaryotic chromosomes contain multiple origins of replication

1. eukaryotic DNA Replication
By
KAUSHAL KUMAR SAHU
Assistant Professor (Ad Hoc)
Department of Biotechnology
Govt. Digvijay Autonomous P. G. College
Raj-Nandgaon ( C. G. )

2. DNAreplicationissemi-conservative,onestrandservesasthe
templateforthesecondstrand.Furthermore,DNAreplication
onlyoccursataspecificstepinthecellcycle.
DNAreplicationineukaryotesismuchmorecomplicatedthan
inprokaryotes,althoughtherearemanysimilaraspects.
DNAreplicationisabiologicalprocessthatoccursinallliving
organismsandcopiestheirDNA;itisthebasisforbiological
inheritance.
EukaryoticcellscanonlyinitiateDNAreplicationataspecific
pointinthecellcycle,thebeginningofSphase.
Duetothesizeofchromosomesineukaryotes,eukaryotic
chromosomescontainmultipleoriginsofreplication
Introduction

3. History
OswaldAvery:ProvedthatDNAwastheagentoftransfer.
ErwinChargaff:determinedthatthebasesA-TandC-Gare
foundindefiniteratios.
LinusPauling,MauriceWilkins,andRoselandFranklin:
workedonthestructureofDNA.
WatsonandCrick:determinedtheshapeandstructureof
DNA.

4. Definition
‘TheprocesswherebyacopyofaDNAmoleculeismadeand
thusthegeneticinformationitcontainsisduplicated.
TheparentaldoublestrandedDNAmoleculeisreplicatedsemi
conservatively,i.e.Eachcopycontainsoneof
theoriginalstrandspairedwithanewlysynthesizedstrandthat
iscomplementaryintermsofATandGCbasepairing.’

5. DNA Polymerase
A DNA polymeraseisa cellularor
viralpolymeraseenzymethatsynthesizesDNAmoleculesfrom
theirnucleotidebuildingblocks.
DNApolymerasesareessentialforDNAreplication,and
usuallyfunctioninpairswhilecopyingonedouble-stranded
DNAmoleculeintotwodouble-strandedDNAsinaprocess
termedsemiconservativeDNAreplication.
DNApolymerasesalsoplaykeyrolesinotherprocesseswithin
cells,includingDNArepair,geneticrecombination,reverse
transcription,andthegenerationofantibodydiversityviathe
specializedDNApolymerase,terminaldeoxynucleotidyl
transferase.

6. Classification
DNApolymerasesaresubdividedintosevendifferent
families:A,B,C,D,X,Y,andRT.
1 FamilyApolymerasesfamilyincludetheextensively-
studiedT7DNApolymerase.
2 FamilyBPolζ(zeta)(polymeraseζisaBFamily
polymerase
3FamilyCPolymerasesaretheprimarybacterialchromosomal
replicativeenzymes.
4 FamilyDPolymerasesarestillnotverywell
characterized.
5 FamilyXContainsthewell-knowneukaryotic
polymerasepolβ,aswellasothereukaryoticpolymerases
suchaspolσ,polλ,polμ&TdT

7. 6.FamilyYYPolymerasesdifferfromothersinhavinga
lowfidelityonundamagedtemplatesandintheirability
toreplicatethroughdamagedDNA.
7. FamilyRT(reversetranscriptase)Thereverse
transcriptasefamilycontainsexamplesfromboth
retrovirusesandeukaryoticpolymerases.

8. Mechanism
ThebasicmechanismsofDNAreplicationarequitesimilarin
eukaryotesandprokaryotes.
DNAreplicationissemiconservativeandiscontinuousonone
strandanddiscontinuousontheother.
DNAreplicationineukaryotesoccursonlyintheSphaseof
thecellcycle.
PreparationinG1phase
ThefirststepinDNAreplicationistheformationofthepre-
initiationreplicationcomplex.Theformationofthiscomplex
occursintwostages.

9. ThefirststagerequiresthatthereisnoCDKactivity.Thiscan
onlyoccurinearlyG1.Theformationofthepre-RCisknownas
licensing,butalicensedpre-RCcannotinitiatereplicationinthe
G1phase.
SynthesisinSphase
ActivationofthecomplexoccursinS-phaseandrequiresCdk2-
CyclinE.GemininbindstoCdt1andsequestersit.
ItisaperiodicproteinthatfirstappearsinS-phaseandis
degradedinlateM-phase,possiblythroughtheactionof
theanaphasepromotingcomplex(APC).

10. Process of DNA Replication
DNAReplicationproceedsinthreeenzymaticallycatalyzed
andcoordinatedsteps:initiation,elongationandtermination.
Origins
ThisprocessisinitiatedatparticularpointsintheDNA,
knownas"origins",whicharetargetedbyproteinsthatseparate
thetwostrandsandinitiateDNAsynthesis
OriginscontainDNA sequencesrecognizedby
replicationinitiatorproteins(e.g.theOriginRecognition
Complexinyeast).Theseinitiatorsrecruitotherproteinsto
separatethestrandsandinitiatereplicationforks.

11. Fourdistinctmechanismsforsynthesishavebeendescribed.
I.ManyDNAviruses,phagesandplasmidsuseaprimaseto
synthesizeashortRNAprimerwithafree3′OHgroup
whichissubsequentlyelongatedbyaDNApolymerase.
II.Theretroelements(includingretroviruses)employa
transferRNAthatprimesDNAreplicationbyprovidinga
free3′OHthatisusedforelongationbythereverse
transcriptase.
III.Intheadenovirusesthe3'OHgroupisprovidedbytheside
chainofanaminoacidofthegenomeattachedproteinto
whichnucleotidesareaddedbytheDNApolymeraseto
formanewstrand.
IV.InthesinglestrandedDNAvirusesthemanyphages
andplasmidsthatusetherollingcirclereplication(RCR)
mechanism,

12. Proteins in DNA replication
DNAHelicase
Alsoknownashelixdestabilizingenzyme.UnwindstheDNA
doublehelixattheReplicationFork.
DNAPolymerase
BuildsanewduplexDNAstrandbyaddingnucleotidesinthe
5'to3'direction.Alsoperformserrorcorrection.
Single-StrandBinding(SSB)Proteins
BindtossDNAandpreventtheDNAdoublehelixfromre-
annealingafterDNAhelicaseunwindsitthusmaintainingthe
strandseparation.

13. DNALigase
Re-annealsthesemi-conservativestrandsand
joinsOkazakiFragmentsofthelaggingstrand.
Primase
ProvidesastartingpointofRNA(orDNA)forDNA
polymerasetobeginsynthesisofthenewDNA
strand.
Telomerase.
LengthenstelomericDNAbyaddingrepetitive
nucleotidesequencestotheendsofeukaryotic
chromosomes.

14. Replicationfork
Thereplicationforkisastructurethatformswithinthenucleus
duringDNAreplication.Itiscreatedbyhelicases,whichbreakthe
hydrogenbondsholdingthetwoDNAstrandstogether.
Leadingstrand
TheleadingstrandisthetemplatestrandoftheDNAdoublehelix
sothatthereplicationforkmovesalongitinthe3'to5'direction.
Ontheleadingstrand,apolymerase"reads"theDNAand
addsnucleotidestoitcontinuously.ThispolymeraseisDNA
polymeraseIII(DNAPolIII)inprokaryotesandpresumablyPolεin
yeasts.
Laggingstrand
ThelaggingstrandisthestrandofthetemplateDNAdoublehelix
thatisorientedsothatthereplicationforkmovesalongitina5'to3'
manner.

15. Onthelaggingstrand,primase"reads"theDNAandaddsRNA
toitinshort,separatedsegments.Ineukaryotes,primaseis
intrinsictoPolα.
DNApolymeraseIIIorPolδlengthenstheprimedsegments,
formingOkazakifragments.

16. Regulation
Withineukaryotes,DNAreplicationiscontrolledwithinthe
contextofthecellcycle
DNAreplicationoccursduringtheSphase(synthesisphase).
TheG1/Scheckpointregulateswhethereukaryoticcellsenter
theprocessofDNAreplicationandsubsequentdivision.
Cellsthatdonotproceedthroughthischeckpointremaininthe
G0stageanddonotreplicatetheirDNA.

17. Termination
EukaryotesinitiateDNAreplicationatmultiplepointsinthe
chromosome,soreplicationforksmeetandterminateatmany
pointsinthechromosome;thesearenotknowntoberegulated
inanyparticularway.
Becauseeukaryoteshavelinearchromosomes,DNAreplication
isunabletoreachtheveryendofthechromosomes,butendsat
thetelomereregionofrepetitiveDNAclosetotheend.
Thisisanormalprocessinsomaticcells.Asaresult,cellscan
onlydivideacertainnumberoftimesbeforetheDNAloss
preventsfurtherdivision.
Withinthegermcellline,whichpassesDNAtothenext
generation,telomeraseextendstherepetitivesequencesofthe
telomereregiontopreventdegradation.Telomerasecan
becomemistakenlyactiveinsomaticcells,sometimesleading
tocancerformation.

18. Conclusion
DNAreplicationissemiconservative,witheachexistingstrand
servingasatemplateforthesynthesisofanewstrand.
Replicationbeginsatspecificlocationscalledoriginsof
replication.Replicationrequiresaprimer.

19. References
Cell&MolecularBiology-JeraldCarp
Biotechnology-USatyanarayana
Internet-http://en.wikipedia.org/wiki/DNA_replication
http://www.ndsu.edu/pubweb/~mcclean/plsc431/eukarych
rom/eukaryo3.htm