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