《线粒体基因组》PPT课件

MitochondrialmoleculargeneticsMitochondrialmoleculargeneticsMitochondriaarethemainsiteofATPsynthesisineukaryotecellsandassucharevitalforthehealthandsurvivalofthecellTheyarealsooneofthesitesatwhichapoptosisismediatedTheselectureswillexplorethemoleculargeneticsofmitochondria,howtheyaremade,thestructureoftheirgenome,howtheyevolved,andhowmitochondrialgeneexpressioniscontrolled.Mitochondrialmoleculargenetics•focusonmitochondria:briefoverviewoftheirfunctionandstructure•mtDNAstructureandreplication:-animals-yeast-plants•inheritanceofmitochondria-petitemutantsofyeast•biogenesisofmitochondriabyfissionMITOCHONDRIA•essentialforcelllife-ATPsynthesis-manymetabolicintermediates•essentialforcelldeath-unprogrammeddeath:necrosis(eg,duetolossofenergystatus)-programmedcelldeath(apoptosis-controlledcelldestruction)•Twomembranes•Innermembraneinvaginated•Numbersofmitochondriapercellvarybutusually100s/cellMatrixcontainstheTCAcycle(andother)solubleenzymesInnermembranecontainsmetabolitetransportersandtheelectrontransportchainMitochondrialstructureTheribosomescanactuallybevisualizedinsomemitochondria.Inthesefigures,theyareseeninthematrixassmalldarkbodies.DNAcanalsobevisualizedinmitochondria.TheDNAiscircularandresemblesthatofabacteriuminitsbasicstructure.MitochondriaalsohavetheirownribosomesandtRNA:•22tRNAs•rRNAs(16Sand12S)MitochondriacontainDNAmoleculeswithanassortmentofgenes.MitochondrialgeneticsystemconsistofDNAandthemolecularmachineryneededtoreplicateandexpressthegenescontainedinthisDNA.Thismachineryincludesthemacromoleculesneededfortranscriptionandtranslation.Mitochondriaevenpossesstheirownribosomes.Manyofthesemacromoleculesareencodedbymitochondrialgenes,butsomeareencodedbynucleargenesandarethereforeimportedfromthecytosol.MitochondriahavetheirownDNAandRibosomesMitochondriahavesomeoftheirownDNA,ribosomes,andcanmakemanyoftheirownproteins.TheDNAiscircularandliesinthematrixinstructurescalled"nucleoids". Eachnucleoidmaycontain4-5copiesofthemitochondrialDNA(mtDNA).mitochondrialDNAMitochondrialDNA(mtDNA)mtDNAwasdiscoveredinthe1960s;revealedasDNA-likefiberswithinmitochondria.ThecompletenucleotidesequencesofmtDNAmoleculesfrommanydifferentspecieshavenowbeendetermined.mtDNAvaryenormouslyinsize,fromabout6kbinPlasmodiumto2500kbinsomeofthefloweringplants..EachmitochondrionappearstocontainseveralcopiesofDNA.MitochondrialDNA(mtDNA)Inavertebrateoocyte,forexample,ithasbeenestimatedthatasmanyas100millioncopiesofthemtDNAarepresent.Somaticcells,however,havefewercopies,perhapslessthan1000.MostmtDNAmoleculesarecircular,butinsomespecies,suchasalgaChlamydomonasreinhardtii莱茵衣藻,theyarelinear.Inthevertebrates37distinctgenesarepackedintoa16to17-kbcircleleavinglittleornospacebetweengenes.MITOCHONDRIALDNA(YELLOW)INTHEUNICELLULARORGANISMEUGLENAGRACILIS.THENUCLEARDNA(RED)ISALSOVISIBLE.PlantmtDNAInsomeofthefloweringplantsanunknownnumberofgenesaredispersedoveraverylargecircularDNAmoleculehundredsorthousandsofkilobaseinsize.Intheseplantsthemitochondrialgenesmaybecomeseparatedontodifferentcircularmoleculesbyaprocessofintramolecularrecombination.PlantmtDNAThisrecombinationismediatedbyrepetitivesequenceslocatedinthemtDNA.Anexchangebetweentwooftherepetitivesequencescanpartitionthe“master”mtDNAcircleintotwosmallercircle,aprocessthatsuperficiallyresemblestheexcisionofalambdaprophagefromE.colichromosome.Insomespecies,severalDNAcirclesofdifferentsizesareformedbyrecombinationbetweenpairsofrepetitivesequenceslocatedatdifferentpositionsaroundthemasterDNAcircle.Thesemoleculesisdifficulttostudy,andmoreresearchisneededtoelucidatethemechanismthatproducesthem.IntramolecularrecombinationinthemtDNAoftheChinesecabbage,Brassicacampestris油菜.RecombinationbetweentherepeatedelementsinthelargecircularDNAmoleculepartitionsthismoleculeintotwosmallerones.Alternatively,therepeatedelementsinthetwosmallmoleculesmayrecombinewitheachothertoproduceasinglelargemolecule.ThestructureofmtDNAThestructureofmtDNAmoleculeshasbeenstudiedbyDNAsequencing.AnimalmtDNAissmallandcompact.Inhumanbeings,forexample,themtDNAis16,659basepairslongandcontains37genes,includingtwothatencoderibosomalRNAs,22encodetransferRNAs,and13thatencodepolypeptidesinvolvedinoxidativephosphorylation,theprocessthatmitochondriausetorecruitenergy.Inmice,cattle,andfrogs,themtDNAissimilartothatofhumanbeings–anindicationofabasicconservationofstructurewithinthevertebratesubphylum.MapofhumanmtDNAshowingthepatternoftranscription.GenesontheinnercirclearetranscribedfromtheLstrandoftheDNA,whereasgenesontheoutercirclearetranscribedfromtheHstrandoftheDNA.Arrowsshowthedirectionoftranscription.ND1-6aregenesencodingsubunitsoftheenzymeNADHreductase;thetRNAgenesinthemtDNAareindicatedbyabbreviationsfortheaminoacids.ThestructureofmtDNAInvertebratemtDNAisaboutthesamesizeasvertebratemtDNA,butithasasomewhatdifferentgeneticorganization.ThesedifferencesseemtohavebeencausedbystructuralrearrangementsofthegeneswithincircularmtDNAmolecule.ThestructureofmtDNAInfungi,themtDNAisconsiderablylargerthanitisinanimals.Yeast,forexample,possessescircularmtDNAmolecules78kblong.Thesemoleculescontainatleast33genes,including2thatencoderibosomalRNAs,23to25thatencodetransferRNAs,1thatencodesaribosomalprotein,and7encodedifferentpolypeptidesinvolvedinoxidativephosphorylation.TheyeastmtDNAislargerthananimalmtDNAbecauseseveralofitsgenescontainintronsandtherearelongnoncodingsequencesbetweensomeofthegenes.AnimalmtDNAdoesnotcontainintrons.ThestructureofmtDNAPlantmtDNAismuchlargerthanthemtDNAofothersorganisms.Itisalsomorevariableinstructure.TheseconclusionscomefromcrudephysicalandchemicalanalysisandfromDNAsequencing.\OneofthefirstplantmtDNAstobesequencedisfromtheliverwort地钱,Marchantiapolymorpha.ThemtDNAfromthisprimitive,nonvascular(非维管)plantisa186-kbcircularmoleculewith94substantialopenreadingframes(ORFs),somecorrespondingtoknowngenesandothershavingstillunassignedgeneticfunctions.ThelatterORFsarethereforecalledURFs,forunassignedreadingframes.32distinctintronshavebeenfoundintheMarchantiamtDNA,accountingforabout20%ofthemolecule.Invascular(维管)plants,themtDNAislargerthanitisinMarchantia;forexample,itisa570-kbcircularmoleculeinmaizeanda300-kbcircleinthewatermelon西瓜.ThestructureofmtDNAHigherplantmtDNAmoleculescontainmanynoncodingsequences,includingsomethatareduplicated.TheactualnumberofgenespermtDNAmoleculeisunknown.PhysicalmappingofsomeofthesegeneshasshownthattheyarelocatedindifferentpositioninthemtDNAcirclesofdifferentspecies,evenwhenthespeciesarefairlycloselyrelated.ThisimpliesthatmtDNAofhigherplantshasundergonemanygeneticrearrangementsduringitsevolution.ExpressionofMitochondrialgenesThesimplemtDNAofvertebratesareorganizedintotwolargetranscriptionunit,eachencodingtheinformationofseveralgenes.WhenthetwostrandofhumanmtDNAareseparatedbycentrifugation,oneprovestobedenser–theHstrand(forheavy),thantheotherreferredastheL(forlight).ThepromotersfortheHandLtranscriptionunitsaresituatedjustupstreamofthephenylalaninetRNAgene.ThetranscriptsareextendedinoppositedirectionsaroundthemtDNAmolecule.ThetranscriptfromtheHstrandencodes2ribosomalRNAs,14tRNAs,and12polypeptides.ThetranscriptfromtheLstrandencodes8tRNAsand1polypeptide.EachtranscriptiscleavedtoseparatethetRNAsfromtherRNAsandmRNAs,andmRNAsarepolyadenylated.EachmRNAisthentranslatedintopolypeptides,usingthemitochondrialribosomesandacombinationofnuclearandribosomaltRNAs.ExpressionofMitochondrialgenesTranslationinthemitochondriaproceedsmuchasitdoesontheribosomesofthecytosol,exceptthatsomeofthecodonshaveadifferentmeaning.AGAandAGGareterminationcodonsinmammalianmitochondria,whereasinthecytosoltheyspecifytheincorporationofarginine;UGA,whichisterminationcodoninthecytosol,isatryptophancodoninthemitochondria;andAUA,whichencodesisoleucineincytosol,isthemethionineinitiationcodoninthemitochondria.ExpressionofMitochondrialgenesInfungiandplantsthemtDNAisorganizedintomanyseparatetranscriptionunits,somecontainingtheinformationformorethanonegene.Littleisknownaboutthedetailsoftranscription,butinyeast,themitochondrialRNApolymeraseisasinglepolypeptideencodedbyanucleargene.RNAprocessingseparatesplantmitochondrialtranscriptsintotheirconstituentpartsandalsoremovestheintrons,whicharepresentinseveralplantmitochondrialgenes.THEMECHANICSOFTHESEEVENTSAREPOORLYUNDERSTOOD.ExpressionofMitochondrialgenesAnotherpeculiarityofplantmitochondrialgeneexpressionisthatmanyofthemtRNAtranscriptsundergoediting;thatis,someofthenucleotidesarechangedaftertranscripthasbeensynthesized.ThemostfrequentchangeisCtoU(occasionallyUtoC).Thus,RNAeditingaltersthecompositionofcodonsinplantmitochondrialtranscript.EditingalterstheinformationthatisactuallyencodedinthemtDNAandallowsfunctionalpolypeptidestobesynthesized.Editingisnotfoundinthenonvascularplants(mossesandalgae藻类和苔藓植物).Theeditingmechanismprobablyevolvedsometimeafterplantshadbecomeestablishedontheland.ExpressionofMitochondrialgenesYetathirdpeculiarityofplantmitochondrialgeneexpressionisthatsomemitochondrialmRNAsareformedbytheprocessoftrans-splicing.ItoccurswhensegmentsofagenearescatteredoverthemtDNAmolecule.Eachgenesegmentistranscribedindependently,andexonsofthedifferenttranscriptsaresplicedtogetherbyinteractionsbetweentheintronsthatflankthem.ExpressionofMitochondrialgenes分子内(intramolecular)剪接(cissplicing)以及分子间(intermolecular)剪接(transsplicing)Trans-splicinginwheatmitochondria.FourdifferentRNAscontributetothefinalmRNAencodingapolypeptideoftheenzymeNDHreductase.INTERPLAYBETWEENMITOCHONDRIALANDNUCLEARGENEPRODUCTSMost–perhapsall-mitochondrialgeneproductsfunctionsolelywithinmitochondrion.However,theydonotfunctionalone.Manynucleargeneproductsareimportedtoaugmentorfacilitatetheirfunction.Manyofthepolypeptidesneededforaerobicmetabolismarealsosynthesizedincytosol(ATPase–thatisresponsibleforbindingtheenergyofaerobicmetabolismintoATP).However,becausesomeofthesubunitsofthisproteinaresynthesizedinthemitochondria,thecompleteproteinisactuallyamixtureofnuclearandmitochondrialgeneproducts.INTERPLAYBETWEENMITOCHONDRIALANDNUCLEARGENEPRODUCTSThisdual(双重的)compositionsuggeststhatnuclearandthemitochondrialgeneticsystemsarecoordinatedinsomewaysothatequivalentamountsoftheirproductsaremade;possiblemolecularmechanismsforthiscoordinationarecurrentlyunderinvestigation.KEYPOINTSmtDNAmoleculesrangefrom6-kbto2500-kbinsize,andmostofthemappeartobecircular.mtDNAmoleculescontaingenesforsomeoftheribosomalRNAs,transferRNAs,andpolypeptidesusedwithinthemitochondrion.Thestructure,organization,andexpressionofmitochondrialgenesvaryamongspecies.Insomeorganisms,thetranscriptsofmitochondrialgenesareeditedaftertheyaresynthesized.Bothmitochondrialandnucleargeneproductsareneededfornormalmitochondrialfunction.mtDNAandhumandiseaseRecentresearchhasdemonstratedthatseveralhumandiseasesarecausedbymitochondrialdefects,andinsomecases,thesedefectsareduetomutationsinthemtDNA.OnesuchdiseaseisLeber’shereditaryopticneuropathy(LHON),aconditioncharacterizedbythesuddenonsetofblindnessinadults.Thisdiseaseisassociatedwiththedeathoftheopticnerve(ataphysiologicallevel),andwithmutationinanyofseveralmitochondrialgenes(atamolecularlevel).Eachmutationchangesanaminoacidinoneofthemitochondrialproteins–reducingtheefficiencyofoxidativephosphorylation.Thereductionisgreatenoughtodestroythefunctionoftheopticnerveandcausetotalblindness.Itisnotknownwhythiseffectislimitedtotheopticnerve.LHONisinheritedstrictlythroughthematernalline.AnotherdisordercausedbyamutationinthemtDNAisaPearsonmarrow-pancreassyndrome,characterizedbyalossofbone-marrowcellsduringchildhood,isfrequentlyfatal.ItiscausedbyafairlylargedeletionsinthemtDNA.Peoplewiththissyndromealmostneverhaveaffectedparents.Thus,thecausativedeletionprobablyoccursspontaneouslyduringdevelopmentinthechildorduringoogenesisinthemother.IndividualswithPearsonsyndromeactuallyhaveamixtureofdeletedandnormalmtDNA–anexampleofmitochondrialheteroplasmy.Homoplasmicindividualshaveneverbeenobserved.mtDNAandhumandiseaseThemoleculargeneticsofChloroplastsChloroplastscontainDNAmoleculeswithanassortmentofgenes.Chloroplastarespecializedformsofageneralclassofplantorganellescalledplastids质体.Botanistsdistinguishamongseveralkindsofplastids,includingchloroplasts(plastidscontainingpigments),amyloplasts(plastidscontainingstarch造粉体：一种形成淀粉的植物性白色体),andelaioplasts(油质体plastidscontainingoilorlipid).Allthreetypesseemtodevelopfromsmallmembrane-boundedorganellescalledproplastids前质体,and,withinaparticularplantspecies,allseemtocontainthesameDNA.ThisDNAisgenerallyreferredtoaschloroplastDNA,abbreviatedsimplyascpDNA.CHLOROPLASTDNAInhigherplants,cpDNAtypicallyrangefrom120to160kbinsize,andinalgae,from85to292kb.InafewspeciesofgreenalgaethecpDNAismuchlarger,about2000kb.ThecpDNAseemstobeorganizedasaclosedcircularmolecule,butinsomespecies(withlargecpDNAs)alineararrangementcannotberuledout.ThenumberofcpDNAmoleculesinacelldependsontwofactors:thenumberofchloroplastsandthenumberofcpDNAmoleculeswithineachchloroplast.CHLOROPLASTDNAAllcpDNAmoleculescarrybasicallythesamesetofgenes,butindifferentspeciesthesegenesarearrangedindifferentways.ThebasicgenesetincludesgenesforribosomalRNAs,transferRNAs,someribosomalproteins,variouspolypeptidecomponentsofphotosystemsthatareinvolvedincapturingsolarenergy,foursubunitsofaribulose1,5-biphosphatecarboxylase磷酸核酮糖梭化酶,andfoursubunitofachloroplast-specificRNApolymerase.MostcpDNAshaveapairoflargeinvertedrepeatsthatcontainthegenesforribosomalRNAs.Theserepeatsrangeanywherefrom10to76kbinlengthandarevariouslylocatedindifferentcpDNAmolecules..GeneticorganizationofthechloroplastDNAintheliverwortMarchantiapolymorpha.Symbols:rpo,RNApolymerase;rps,ribosomalproteinsofsmallsubunit;rplandsecX,ribosomalproteinsoflargesubunit;4.5S,5S,16S,23S,rRNAsoftheindicatedsize;rbs,ribulosebisphosphatecarboxylse;psa,photosystemI;psb,photosystemII;pet,cytochromeb/fcomplex;atp,ATPsynthesis;infA,initiationfactorA;frx,iron-sulfurproteins;ndh,putativeNADHreductase;mph,chloroplastpermease(?);tRNAgenesareindicatedbyabbreviationsfortheaminoacids.CHLOROPLASTBIOGENESISAllplastidsdevelopfromproplastids.Chloroplastsdevelopmentisstimulatedbylight,andinvolvesthetranscriptionofmanygenes,somelocatedinthenucleus.Alltheproteinsandchlorophyllpigmentsneededforphotosynthesisaremadeandtargetedtotheirappropriatelocationswithintheemergingchloroplast.Theformationsoffunctionalchloroplastsisaprocessreferredtoasbiogenesis.Onlysomeofthedetailsareknown.Lightplaysanimportantrole.Somegenesaretranscribedwhenlightisprovided.CHLOROPLASTBIOGENESISAspecialclassofpigmentedproteinscalledphytochromes(光敏色素)seemstomediatethisandothersresponsestolight.Byabsorbinglightenergy,theseproteinsacquiretheabilitytotriggerotherproteinstostimulatethetranscriptionofgenesinvolvedinchloroplastbiogenesis.Theformationofchloroplastsandthemaintenanceoftheirstructureandfunctionduringthelifeofaplantdependonthecoordinatedexpressionofnuclearandchloroplastgenes.Chloroplastbiogenesis.Amaturechloroplastcontainingstacks(堆)ofthylakoid类囊体membranes(grana基粒)withinitsprotoplasmicstroma(基质)developsfromaproplastidafterexposuretolight.KEYPOINTSChloroplastDNA(cpDNA)moleculesaretypically120to292kbinsize,andtheycontainatleast100genes.TheorganizationofcpDNAmoleculesvariesamongspeciesofplantsandalgae.Lightinduceschloroplaststodevelopfromunpigmentedplastidsthroughaprocessthatinvolvestheinterplayofchloroplastandnucleargeneproducts.MitochondrialInheritanceYeasthasbeenusedextensivelytostudymitochondrialinheritance.ThereisaYeaststrain,called"Petite小型菌聚落"thathavestructurallyabnormalmitochondriathatareincapableofoxidativephosphorylation. ThesemitochondriahavelostsomeoralloftheirDNA.Geneticcrossesbetweenpetiteandwt(wild-type)strainsshowedthatinheritanceofthistraitdidnotsegregatewithanyofthenuclearchromosomes.Mitochondrialinheritancefromyeastisbiparental双亲的,andbothparentcellscontributetothedaughtercellswhenthehaploidcellsfuse. Aftermeiosisandmitosis,thereisrandomdistributionofmitochondriatodaughtercells. Ifthefusioniswithyeastthatarepetiteandyeastthatarenot,acertainpercentageofthedaughtercellswillbe"petite".  MitochondrialInheritanceMitochondrialInheritanceinYeast复制分离又称不均等的有丝分裂分离复制分离在连续的细胞分裂过程中，异质性细胞中突变型mtDNA和野生型mtDNA的比例会发生漂变，向同质性的方向发展。分裂旺盛的细胞（如血细胞）往往有排斥突变mtDNA的趋势，经无数次分裂后，细胞逐渐成为只有野生型mtDNA的同质性细胞。突变mtDNA具有复制优势，在分裂不旺盛的细胞（如肌细胞）中逐渐积累，形成只有突变型mtDNA的同质性细胞。漂变的结果， 表 关于同志近三年现实表现材料材料类招标技术评分表图表与交易pdf视力表打印pdf用图表说话 pdf 型也随之发生改变。Thisledtothesuggestionthatsomegeneticelementexistedinthecytoplasmandwasinheritedinadifferentmannerfromnucleargenes.Thisiscalled“non-Mendelianinheritance”or“cytoplasmicinheritance”.MitochondrialInheritanceInyeastandanimals,thisindicatedinheritanceofmitochondrialgenes:inplantsitalsoincludesinheritanceofchloroplastgenesMitochondrialreplicationMitochondriareplicatemuchlikebacterialcells.Whentheygettoolarge,theyundergofission.Thisinvolvesafurrowingoftheinnerandthentheoutermembraneasifsomeonewaspinchingthemitochondrion.Thenthetwodaughtermitochondriasplit.Ofcourse,themitochondriamustfirstreplicatetheirDNA.Anelectronmicrographdepictingthefurrowingprocessisshowninthesefigures.MitochondrialreplicationSometimesnewmitochondriaaresynthesizedincentresthatarerichinproteinsandpolyribosomesneededfortheirsynthesis.Theelectronmicrographinthefollowingfigureshowssuchacentre.Itappearsthattheclusterofmitochondriaaresittinginamatrixofproteinsandothermaterialsneededfortheirproduction.Certainmitochondrialproteinsareneededbeforethemitochondriacandivide.TheyshowedtheresultoftheremovalofanoutermembraneproteinfrommitochondriacalledMDM10.Thisfigureshowstheresults.Themitochondriaareabletotakeincomponentsandproducemembranesandmatrixenzymes.However,fissionisnotallowedandtheresultisagiantmitochondrion.giantmitochondrionHumanmtDNA•small,doublestrandedcircularchromosome•16,569bpintotal•nonon-codingDNA•nointrons•polycistronicreplicationwhichisinitiatedfromtheD(displacement)-loopregion•followedbysplicingoftranscripttoformmessages.OrganisationofthemitochondrialchromosomehumanmtDNAyeastmtDNAYeastmitochondrialchromosomeHumanDNA•16,569bp;•nonon-codingDNA•nointrons•polycistronicreplicationfollowedbysplicingtoformmessages.YeastmtDNA•68-75kb,similarinstructuretobacterialgenome•containsintronsandnon-regionsbetweengenes.•Sameproteinsmadeasinanimals•genestranscribedseparatelyDespitehavingtheirowngenome,mostmitochondrialproteinsareencodedinthenucleus,madeinthecytosolandimportedintothemitochondria.Inallorganisms,onlyafewoftheproteinsofthemitochondrionareencodedbymtDNA,buttheprecisenumbervariesbetweenorganisms•Subunits1,2,and3ofcytochromeoxidase•Subunits6,8,9oftheFo ATPase•ApocytochromebsubunitofcomplexIII•SevenNADH-CoQreductasesubunits(exceptinyeast)Thenucleusencodestheremainingproteinswhicharemadeinthecytosolandimportedintothemitochondrion.SynthesisofmitochondrialproteinsPlantmtDNA•chromosomesizeismuchbiggerbutvariesdramaticallybetweenspecies(200-2000kb)•arrangedasdifferentsizecircles,sometimeswithplasmids.•TheplantmtDNAcontainschloroplastsequences,indicatingexchangeofgeneticinformationbetweenorganellesinplants.•MuchoftheplantmtDNAisnon-coding,butcodingregionsarelargerthananimalsandfungi.•Numberofproteinssynthesisednotknowndefinitelybutmorethaninanimalsandyeast(probablyabout50)Plantmitochondriahavespecialisedfunctions•inleavestheyparticipateinphotorespiration•sitesofvitaminsynthesis(vitC,folicacid,biotin)maizemitochondrialgenomeInplants,respirationandphotosynthesisoperatesimultaneouslyinthelightNIGHTDAYChloroplastsarethesiteofphotosynthesisandbelongtotheplastidfamilyoforganelles-theydevelopfromproplastidsinthelightproplastidRicemitochondrialandchloroplastgenomesPlantmitochondriacontainchloroplastgenes-suggestingthatgenetictransferoccursbetweenthetwoorganellesMitochondrialDNAofanimalsandfungiusesadifferentgeneticcodethanthe“universal”codeRNAprocessinginmitochondriaPlantmitochondria“edit”theirRNAtranscripts.ThiswasfirstnoticedwhencomparingcDNAsequenceswithgenomicDNAsequences.ThemostcommonchangeistoreplaceCwithU,althoughinsomeinstancesotherchangescanoccur.Matrixenzymesarethoughttoberesponsibleforthis,butthereasonfortheeditingisnotknown.MostoftheDNAinplantmitochondriaisnon-coding,onlysomeofwhichistranscribed.RNAeditingoccurseveninnon-codingregionssuchasintrons.MitochondriaandchloroplastsNon-codingDNAisrareinmitochondriaandchloroplasts.Mitochondriaoriginatedmorethanabillionyearsagowhenafree-livingbacterium,theclosestlivingrelatives