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Biology 12 Ms. Kim Lesson 2: DNA Replication and Repair Main Point Notes Semiconservative replication • A mechanism of DNA replication in which each of the two strands of parent DNA is incorporated into a new strand of DNA DNA Replication Overview Three steps: 1. __________________ of the parent DNA strands 2. Building of __________________ (daughter) strands 3. __________________ and __________________ errors in the new strands Step 1 – Strand Separation • The __________________ is a specific sequence of DNA = the starting point for replication • The Y-shaped structure that forms as the two strands separate is known as the __________________ Two challenges: 1. __________________ is created ahead of the fork Q. What enzyme is used to relieve tension in DNA strands? 2. Tendency to re-join, or __________________ (complementary) Q. What proteins help to prevent annealing? • __________________ enzyme binds to these origins are begins to unwind the two strands of DNA from the origin (can separate in both directions) by breaking H-bonds b/w complementary base pairs • As the forks proceed in opposite directions, the space between them is called a __________________, soon filled with the newly replicated DNA

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Page 1: Lesson 4 - DNA replication and repair (ANS) · • Repairing: DNA repair mechanisms of proteins and enzymes (such as DNA Polymerase I and II) may locate distortions in the strands

Biology12 Ms.KimLesson2:DNAReplicationandRepair

MainPoint NotesSemiconservativereplication

• AmechanismofDNAreplicationinwhicheachofthetwostrandsofparentDNAis

incorporatedintoanewstrandofDNA

DNAReplicationOverview

Threesteps:

1. __________________oftheparentDNAstrands2. Buildingof__________________(daughter)strands3. __________________and__________________errorsinthenewstrands

Step1–StrandSeparation

• The__________________isaspecificsequenceofDNA=thestartingpointfor

replication• TheY-shapedstructurethatformsasthetwostrandsseparateisknownasthe

__________________Twochallenges:

1. __________________iscreatedaheadoftheforkQ.WhatenzymeisusedtorelievetensioninDNAstrands?

2. Tendencytore-join,or__________________(complementary)Q.Whatproteinshelptopreventannealing?

• __________________enzymebindstotheseoriginsarebeginstounwindthetwostrandsofDNAfromtheorigin(canseparateinbothdirections)bybreakingH-bondsb/wcomplementarybasepairs

• Astheforksproceedinoppositedirections,thespacebetweenthemiscalleda__________________,soonfilledwiththenewlyreplicatedDNA

Page 2: Lesson 4 - DNA replication and repair (ANS) · • Repairing: DNA repair mechanisms of proteins and enzymes (such as DNA Polymerase I and II) may locate distortions in the strands

Biology12 Ms.Kim

Step2–BuildingComplementaryStrands

• NewnucleotidesarejoinedbyagroupofenzymescalledDNApolymerases• MostofourunderstandingoftheprocessofDNAreplicationcomesfromthestudyof

thebacteriumE.coli.àSimilartoreplicationofeukaryoticDNAbutthesheervolumeinaeukaryoticcellmeansthattheprocessismorecomplex

• ___________________________”read”thetemplatestrandinits___à___direction• Theycanonlyaddnucleotidestothe3’endofanexistingDNAstrand• Thus,thenewstrandisalwaysassembledinthe___à___direction

Priming

• _________________enzymesbeginthereplicationprocessbybuildingasmallcomplementaryRNAsegmentcalled____________(10-60ribonucleotideslong)

Replicating

• ________________________beginstoaddDNAnucleotidestotheprimer• SinceDNApolymeraseIIIonlybuilds5’à3’,thetwonewstrandsbegintobeassembled

inoppositedirectionsLeadingStrand

• DNApolymeraseIIIisabletocontinuecontinuously• NoneedfortheRNAprimasetoaddadditionalprimers• Thisiscalledthe_______________________

Page 3: Lesson 4 - DNA replication and repair (ANS) · • Repairing: DNA repair mechanisms of proteins and enzymes (such as DNA Polymerase I and II) may locate distortions in the strands

Biology12 Ms.KimLaggingStrand

• Ontheoppositestrand,DNApolymeraseIIIismovingawayfromthereplicationforkà_______________________

• RNAprimaseattachesanotherprimerallowingDNApolymeraseIIItobeginfromanewpoint

OkazakiFragments

• ThepatterncreatedonthesecondstrandisaseriesofRNAprimersandshortDNAfragmentscalled______________________

• Aseachfragmentextendsinthe5’à3’direction,iteventuallyrunsintotheRNAprimerattachedtotheOkazakifragmentahead

ReplaceRNAandSeal

• ______________________• removestheRNAnucleotidesandreplacesthemwithDNAnucleotides• _______________catalyzestheformationofphosphodiesterbondstosealthestrand

Step3–DealingwithErrors(repairing)

• DNApolymerasesthatcarryoutreplicationalsoplayanotherimportantrole• AstheyassemblenewDNAstrands,theyproof-readandcorrecterrors(base-pair

mismatches)Proof-reading

• Proof-reading:Whilecreatingthecomplementarystrand,ifamismatchoccurs,DNApolymeraseIIImaybackup,repair,andcontinue

• ____________________continuesaddingnucleotidesintheforwarddirection• Iftheenzymeaddsamismatchednucleotide,theenzymeactsasan_______________

toremovethemismatchednucleotide• TheenzymeresumesactivityasDNApolymerase

Repairing

• Repairing:DNArepairmechanismsofproteinsandenzymes(suchasDNAPolymeraseIandII)maylocatedistortionsinthestrandsbetweenreplicationeventsandremoveapieceofthestrand,DNApolymeraseswillfillthegap,andDNAligasewillsealthestrand

• _____________________repairsdamagetoDNAthatoccursbetweenreplicationevents

• Repaircomplexesremoveseveraltomanybases,leavingagapintheDNA• GapisfilledinbyaDNApolymerase,usingthetemplateasaguide• Nickissealedby_______________tocompleterepair

Page 4: Lesson 4 - DNA replication and repair (ANS) · • Repairing: DNA repair mechanisms of proteins and enzymes (such as DNA Polymerase I and II) may locate distortions in the strands

Biology12 Ms.Kim

Questions:

1. (a)Withoutlookingatthetext,trytolistandsummarizethethreestepsthatareinvolvedinreplication.(b) Checkwhatyouwrote.Whatdidyoumiss?Didyoumissanythingthatiscrucialtothereplicationprocess?

2. PlacethefollowingenzymesintheorderinwhichtheyareusedinDNAreplication:topoisomerase,DNAligase,DNApolymeraseIII,RNAprimase,helicase,DNApolymeraseI

3. WhatwouldhappenifeachofthefollowingwerenotavailableforDNAreplication?Howwouldthisaffecttheprocess?

(a) Topoisomerase(b) DNAligase(c) Single-strandedbindingprotein

4. ExplainwhytheprocessofDNAreplicationissloweronthelaggingstrandthanontheleadingstrand5. HelicaseseparatesandunwindstheDNAstrandsfromeachother.Whyisanenzymenotrequiredtobringthe

strandsbacktogether(re-anneal)them?


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