3.A.1 DNA and RNA: Structure and Replication...3.A.1 DNA and RNA: Structure and Replication • Each DNA polymer is made of Nucleotides (monomer) which are made of: a)Phosphategroup:

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]