3.A.1 DNA and RNA: Structure and Replication...3.A.1 DNA and RNA: Structure and Replication • Each...
Transcript of 3.A.1 DNA and RNA: Structure and Replication...3.A.1 DNA and RNA: Structure and Replication • Each...
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3.A.1DNAandRNA:StructureandReplication
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• EachDNApolymerismadeofNucleotides(monomer)whicharemadeof:
a) Phosphate group:Negativelychargedandpolar
b) Sugar:deoxyribose- a5carbonsugar
c) Nitrogenbase– A,C,T,G
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NitrogenBases
• Fourbasesdividedintotwogroups:• Purines Pyrimidines• Doublering Singlering• Adenine(A) Thymine(T)• Guanine(G) Cytosine(C)
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ADNAstrandisdirectional;ithasa3’endanda5’end.Itisantiparalleltotheoppositestrand
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antiparallel =runparalleltoeachotherbutwithoppositealignments. Ex.thetwocomplementary strandsof DNA,whichruninopposite directionsalongside eachother.
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DNAreplication ensurescontinuityofhereditaryinformation.
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ReplicationoccursduringSphaseofthecellcycle.
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Replicationisasemi
conservativeprocess;thatis,onestrandservesasthetemplate
foranew,complementary
strand.
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Replicationrequires manyessentialcellularenzymes.
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EssentialReplicationEnzymes• DNAisunwoundbyHELICASE• PRIMASE- producestheRNAprimertoinitiatereplication
• DNAPOLYMERASEIIIaddsnewcomplementarynucleotidestothegrowingstrandinthe5’à 3’direction
• DNAPOLYMERASEIproofreadsandcorrectsmistakes
• LIGASE- actslike‘moleculartape’joiningnewDNAbasestogether.
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ThecopyingofDNAisremarkableinitsspeedandaccuracy.
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InDNAreplication,theparentmoleculeunwindsandtwonewdaughterstrandsare
builtfollowingbase-pairingrules.
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ThereplicationofaDNAmoleculebeginsatspecialsitescalledoriginsofreplication(ORI),wherethetwostrandsareseparated.Replicationisbidirectional.
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Aeukaryoticchromosomemayhavehundredsoreventhousandsofreplicationorigins.
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Areplicationforkisformedbytheactionoftwoenzymes,topoisomerase andDNAhelicase.Topoisomerasehelpsrelievethestraincreatedbyunwindingtheparentstrand,whileDNAhelicasebreaksthehydrogenbondsbetweenbasepairs.
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DNApolymeraseIIIaddsnucleotidestothegrowingstrandinthe 5’à 3’ direction.
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DNAisantiparallel,sothetwonewstrandshavetobemadedifferently.
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First,anRNAprimerisformedbytheenzymeDNAprimase.AprimerisnecessarybecauseDNA
polymerasecannotinitiatethesynthesisofnewstrand;itcanonlyaddnucleotidestothe3’DNA
template strand
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Alongtheleadingstrand, DNApolymerasecansynthesizeacomplementarystrand
continuously,movingtowardthereplicationforkinthe5’à3’ direction.
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ToelongatetheothernewstrandofDNA,thelaggingstrand,DNApolymerasemustworkinthedirectionawayfromthereplicationfork.Thelaggingstrandisdiscontinuous,resultinginfragments:eachwithitsownRNAprimer.
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ThelaggingstrandissynthesizedasaseriesofsegmentscalledOkazakifragments,whicharethenjoinedtogetherbyDNA
ligase.TheprimersarethenremovedandreplacedwithDNA.
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DNApolymeraseIproofreadsnewlymadeDNA,replacinganyincorrectnucleotides.Inmismatch
repairofDNA,repairenzymescorrecterrorsinbasepairing.
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TheendsofeukaryoticchromosomalDNAget
shorterwitheachroundofreplication.Theprimersare
removedandreplacedwithDNA. Telomeres(endsofDNA)delaytheshortening.
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TheendsofeukaryoticDNAmoleculeshavenucleotidesequencescalledtelomeresthatpostponetheerosionofgenesneartheendsofDNAmolecules.Anenzyme
calledtelomerase catalyzesthelengtheningoftelomeresingermcellssothattheendsdonotget
shortereachtimeagermcellisproduced.
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DNAReplicationProcess:Summary
• Theweakhydrogenbonds betweenthebasepairs arebrokenandthetwostrands attheoriginofreplication.
• Thepartialunwindingofthedoublehelixoccursatthereplicationfork bywhichenzyme?
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• Primase doeswhat?• Whatisthedifferencebetweenleadingandlaggingstrands?
• Whichenzymeaddsfreenucleotidestoagrowingstrand?Whichoneproofreads?
• WhatareOkazakifragments?• WhichenzymejoinstheDNAbasestogether?• Whatisthedifferencebetweenprokaryoticandeukaryoticreplication?
• Whatisatelomereandwhatdoestelomerasecatalyze?
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LearningObjectives:
LO3.3Thestudentisabletodescriberepresentationsandmodelsthatillustratehowgeneticinformationiscopiedfortransmissionbetweengenerations.[SeeSP1.2]