Gene-X-ppt(Gene10-基因十)--Chapter15

Chapter15HomologousandSite-SpecificRecombination15.1Introduction•Homologousrecombinationisessentialinmeiosisforgeneratingdiversityandforchromosomesegregation,andinmitosistorepairDNAdamageandstalledreplicationforks.FIGURE01ab:Unlinkedandlinkedgenes15.1Introduction•Site-specificrecombinationinvolvesspecificDNAsequences.•somaticrecombination–Recombinationthatoccursinnongermcells(i.e.,itdoesnotoccurduringmeiosis);mostcommonlyusedtorefertorecombinationintheimmunesystem.•Recombinationsystemshavebeenadaptedforexperimentaluse.FIGURE02:Site-specificrecombinationAdaptedfromB.Alberts,etal.MolecularBiologyoftheCell,Fourthedition.GarlandScience,2002.15.2HomologousRecombinationOccursbetweenSynapsedChromosomesinMeiosis•Chromosomesmustsynapse(pair)inorderforchiasmatatoformwherecrossing-overoccurs.•ThestagesofmeiosiscanbecorrelatedwiththemoleculareventsattheDNAlevel.FIGURE03:Recombinationoccursatspecificstagesofmeiosis15.2HomologousRecombinationOccursbetweenSynapsedChromosomesinMeiosis•sisterchromatid–Eachoftwoidenticalcopiesofareplicatedchromosome;thistermisusedaslongasthetwocopiesremainlinkedatthecentromere.–SisterchromatidsseparateduringanaphaseinmitosisoranaphaseIIinmeiosis.•bivalent–Thestructurecontainingallfourchromatids(tworepresentingeachhomolog)atthestartofmeiosis.15.2HomologousRecombinationOccursbetweenSynapsedChromosomesinMeiosis•synaptonemalcomplex–Themorphologicalstructureofsynapsedchromosomes.•jointmolecule–ApairofDNAduplexesthatareconnectedtogetherthroughareciprocalexchangeofgeneticmaterial.15.3Double-StrandBreaksInitiateRecombination•Thedouble-strandbreakrepair(DSBR)modelofrecombinationisinitiatedbymakingadouble-strandbreakinone(recipient)DNAduplexandisrelevantformeioticandmitotichomologousrecombination.•In5endresection,exonucleaseactiongenerates3′–single-strandedendsthatinvadetheother(donor)duplex.15.3Double-StrandBreaksInitiateRecombination•Whenasinglestrandfromoneduplexdisplacesitscounterpartintheotherduplex(single-strandinvasion),itcreatesabranchedstructurecalledaDloop.•StrandexchangegeneratesastretchofheteroduplexDNAconsistingofonestrandfromeachparent.FIGURE04:Doublestrandbreakrepairmodel15.3Double-StrandBreaksInitiateRecombination•NewDNAsynthesisreplacesthematerialthathasbeendegraded.•branchmigration–TheabilityofaDNAstrandpartiallypairedwithitscomplementinaduplextoextenditspairingbydisplacingtheresidentstrandwithwhichitishomologous.FIGURE06:Branchsitescanmigrate15.3Double-StrandBreaksInitiateRecombination•CaptureofthesecondDSBendbyannealinggeneratesarecombinantjointmoleculeinwhichthetwoDNAduplexesareconnectedbyheteroduplexDNAandtwoHollidayjunctions.•Thejointmoleculeisresolvedintotwoseparateduplexmoleculesbynickingtwooftheconnectingstrands.•Whetherrecombinantsareformeddependsonwhetherthestrandsinvolvedintheoriginalexchangeortheotherpairofstrandsarenickedduringresolution.15.4GeneConversionAccountsforInterallelicRecombination•HeteroduplexDNAthatiscreatedbyrecombinationcanhavemismatchedsequenceswheretherecombiningallelesarenotidentical.•Repairsystemsmayremovemismatchesbychangingoneofthestrandssoitssequenceiscomplementarytotheother.15.4GeneConversionAccountsforInterallelicRecombination•Mismatch(gap)repairofheteroduplexDNAgeneratesnonreciprocalrecombinantproductscalledgeneconversions.FIGURE07:Ascomycetestetrads,geneconversion15.5TheSynthesis-DependentStrand-AnnealingModel•Thesynthesis-dependentstrand-annealingmodel(SDSA)isrelevantformitoticrecombination,asitproducesgeneconversionsfromdouble-strandbreakswithoutassociatedcrossovers.FIGURE08:Synthesis-dependentstrandannealingmodel15.6NonhomologousEnd-JoiningCanRepairDouble-StrandBreaks•Repairofdouble-strandbreakswhenhomologoussequenceisnotavailableoccursthroughanonhomologousend-joining(NHEJ)reaction.•ImmunereceptorV(D)JrecombinationoccursthroughaspecializedNHEJpathway.FIGURE09:NHEJAdaptedfromJ.M.Jones,M.Gellert,andW.Yang,Structure9(2001):881-884.15.7TheSingle-StrandAnnealingMechanismFunctionsatSomeDouble-StrandBreaks•Single-strandannealing(SSA)occursatdouble-strandbreaksbetweendirectrepeats.FIGURE10:Single-strandannealingbetweendirectrepeats15.7TheSingle-StrandAnnealingMechanismFunctionsatSomeDouble-StrandBreaks•Resectionofdouble-strandbreakendsresultsin3′single-strandedtails.•Complementaritybetweentherepeatsallowsforannealingofthesinglestrands.•ThesequencebetweenthedirectrepeatsisdeletedafterSSAiscompleted.15.8Break-InducedReplicationCanRepairDouble-StrandBreaks•Break-inducedreplication(BIR)isinitiatedbyaone-endeddouble-strandbreak.•BIRatrepeatedsequencescanresultintranslocations.FIGURE11:BIRinitiatingtranslocations15.9RecombiningMeioticChromosomesAreConnectedbytheSynaptonemalComplex•Duringtheearlypartofmeiosis,homologouschromosomesarepairedinthesynaptonemalcomplex.•Themassofchromatinofeachhomologisseparatedfromtheotherbyaproteinaceouscomplex.FIGURE14:Synaptonemalcomplexschematic15.9RecombiningMeioticChromosomesAreConnectedbytheSynaptonemalComplex•axialelement–Aproteinaceousstructurearoundwhichthechromosomescondenseatthestartofsynapsis.•lateralelement–Astruct