Gene-X-ppt(Gene10-基因十)--Chapter17

Chapter17TransposableElementsandRetroviruses17.1Introduction•transposon(transposableelement)–ADNAsequenceabletoinsertitself(oracopyofitself)atanewlocationinthegenomewithouthavinganysequencerelationshipwiththetargetlocus.•retrovirus–AnRNAviruswiththeabilitytoconvertitssequenceintoDNAbyreversetranscription.FIGURE01:DNArearrangementshavemanycauses17.1Introduction•retrotransposon(retroposon)–AtransposonthatmobilizesviaanRNAform;theDNAelementistranscribedintoRNA,andthenreverse-transcribedintoDNA,whichisinsertedatanewsiteinthegenome.Itdoesnothaveaninfective(viral)form.–Commonly,retrotransposonscontaininglongterminalrepeats(LTRs)arereferredtoasretrotransposons,whilenon-LTR-containingretrotransposonsarereferredtoasretroposons.17.1IntroductionFIGURE02:Onlyretrovirusesexistextracellularly17.2InsertionSequencesAreSimpleTranspositionModules•Aninsertionsequence(IS)isatransposonthatcodesfortheenzyme(s)neededfortranspositionflankedbyshortinvertedterminalrepeats.•Thetargetsiteatwhichaninsertionsequenceisinsertedisduplicatedduringtheinsertionprocesstoformtworepeatsindirectorientationattheendsofthetransposon(directrepeats).FIGURE03:ISelementshaveinvertedrepeatsandgeneratetargetrepeats17.2InsertionSequencesAreSimpleTranspositionModules•Thelengthofthedirectrepeatis5to9bpandischaracteristicforanyparticularinsertionsequence.•transposase–Theenzymeactivityinvolvedininsertionoftransposonatanewsite.17.3TranspositionOccursbyBothReplicativeandNonreplicativeMechanisms•MosttransposonsuseacommonmechanisminwhichstaggerednicksaremadeintargetDNA,thetransposonisjoinedtotheprotrudingends,andthegapsarefilled.FIGURE04:Directrepeatsaregeneratedbyinsertion17.3TranspositionOccursbyBothReplicativeandNonreplicativeMechanisms•TheorderofeventsandexactnatureoftheconnectionsbetweentransposonandtargetDNAdeterminewhethertranspositionisreplicativeornonreplicative.FIGURE05:TransposoniscopiedtonewsiteFIGURE06:Transposonmovestonewsite17.3TranspositionOccursbyBothReplicativeandNonreplicativeMechanisms•resolvase–Theenzymeactivityinvolvedinsite-specificrecombinationbetweentwocopiesofatransposonthathasbeenduplicated.•compositetransposons–TransposableelementsconsistingoftwoISelements(whichcanbethesameordifferent)andtheDNAsequencesbetweentheISelements;thenon-ISsequencesoftenincludegene(s)conferringantibioticresistance.17.4TransposonsCauseRearrangementofDNA•HomologousrecombinationbetweenmultiplecopiesofatransposoncausesrearrangementofhostDNA.•Homologousrecombinationbetweentherepeatsofatransposonmayleadtopreciseorimpreciseexcision.FIGURE08:InvertedrepeatrecombinationinvertsmaterialFIGURE07:Directrepeatsrecombinetoexcisematerial17.5ReplicativeTranspositionProceedsthroughaCointegrate•Replicationofastrandtransfercomplexgeneratesacointegrate,whichisafusionofthedonorandtargetreplicons.•Thecointegratehastwocopiesofthetransposon,whichliebetweentheoriginalreplicons.FIGURE09:Donorandrecipientfuseintocointegrate17.5ReplicativeTranspositionProceedsthroughaCointegrate•Recombinationbetweenthetransposoncopiesregeneratestheoriginalreplicons,buttherecipienthasgainedacopyofthetransposon.•Therecombinationreaction(resolution)iscatalyzedbyaresolvasecodedbythetransposon.FIGURE10:Mutranspositionusesacrossoverintermediate17.6NonreplicativeTranspositionProceedsbyBreakageandReunion•Nonreplicativetranspositionresultsifacrossoverstructureisnickedontheunbrokenpairofdonorstrandsandthetargetstrandsoneithersideofthetransposonareligated.FIGURE11:Acrossovercanbereleasedbynicking17.6NonreplicativeTranspositionProceedsbyBreakageandReunion•Twopathwaysfornonreplicativetranspositiondifferaccordingtowhetherthefirstpairoftransposonstandsarejoinedtothetargetbeforethesecondpairarecut(Tn5),orwhetherallfourstrandsarecutbeforejoiningtothetarget(Tn10).FIGURE12:TranspositioncanusecleavageandligationFIGURE13:Tn5iscleavedfromflankingDNA17.6NonreplicativeTranspositionProceedsbyBreakageandReunion17.7MaizeTransposonsCanCauseBreakageandRearrangements•Transpositioninmaizewasdiscoveredbecauseoftheeffectsofthechromosomebreaksgeneratedbytranspositionof“controllingelements.”•sector–Apatchofcellsmadeupofasinglealteredcellanditsprogeny.•Thebreakgeneratesonechromosomethathasacentromere,abrokenend,andoneacentricfragment.FIGURE15:Transpositionsareclonallyinherited17.7MaizeTransposonsCanCauseBreakageandRearrangements•Theacentricfragmentislostduringmitosis;thiscanbedetectedbythedisappearanceofdominantallelesinaheterozygote.•Fusionbetweenthebrokenendsofthechromosomegeneratesdicentricchromosomes,whichundergofurthercyclesofbreakageandfusion.FIGURE16:Acentricfragmentsarelost17.7MaizeTransposonsCanCauseBreakageandRearrangements•Thefusion-breakage-bridgecycleisresponsiblefortheoccurrenceofsomaticvariegation.FIGURE17:Dscausesabreakage-fusion-bridgecycle17.8TransposonsFormFamiliesinMaize•Eachfamilyoftransposonsinmaizehasbothautonomousandnonautonomousmembers.•Autonomoustransposonscodeforproteinsthatenablethemtotranspose.•Nonautonomoustransposonscannotcatalyzetransposition,buttheycantransposewhenanautonomouselementprovidesthenecessaryprotei