The Information in DNA Is Decoded by Transcription - Nature

Then, a sequence of adenine nucleotides called a poly-A tail is added to the 3' end of the mRNA molecule (Figure 7). This sequence signals to the cell that ... Thispagehasbeenarchivedandisnolongerupdated   Aa Aa Aa TheInformationinDNAIsDecodedbyTranscription   DNAisessentiallyastoragemolecule.Itcontainsalloftheinstructionsa cellneedstosustainitself.Theseinstructionsarefoundwithingenes,whicharesectionsofDNAmade upofspecificsequencesofnucleotides.Inordertobeimplemented,the instructionscontainedwithingenesmustbeexpressed,orcopiedintoaform thatcanbeusedbycellstoproducetheproteinsneededtosupportlife. TheinstructionsstoredwithinDNAarereadandprocessedbyacellintwo steps:transcriptionandtranslation.Eachofthesestepsisaseparate biochemicalprocessinvolvingmultiplemolecules.Duringtranscription,aportionofthecell'sDNAservesasatemplatefor creationofanRNAmolecule.(RNA, orribonucleicacid,ischemically similartoDNA,exceptforthreemaindifferencesdescribedlateroninthis conceptpage.)Insomecases,thenewlycreatedRNAmoleculeisitselfa finishedproduct,anditservesanimportantfunctionwithinthecell.Inother cases,theRNAmoleculecarriesmessagesfromtheDNAtootherpartsofthe cellforprocessing.Mostoften,thisinformationisusedtomanufacture proteins.ThespecifictypeofRNAthatcarriestheinformationstoredinDNA tootherareasofthecelliscalledmessenger RNA,ormRNA. Howdoestranscriptionproceed? TranscriptionbeginswhenanenzymecalledRNApolymeraseattachestotheDNAtemplatestrandandbegins assemblinganewchainofnucleotidestoproduceacomplementaryRNAstrand.TherearemultipletypesoftypesofRNA.Ineukaryotes,therearemultipletypesofRNApolymerasewhichmakethevarioustypesofRNA.Inprokaryotes,asingleRNApolymerasemakesalltypesofRNA.Generallyspeaking, polymerasesarelargeenzymesthatworktogetherwithanumberofother specializedcellproteins.Thesecellproteins,calledtranscriptionfactors,helpdeterminewhichDNAsequencesshouldbe transcribedandpreciselywhenthetranscriptionprocessshouldoccur. Initiation Figure1: TranscriptionbeginswhenRNApolymerasebindstotheDNAtemplatestrand. Thefirststepintranscriptionisinitiation.During thisstep,RNApolymeraseanditsassociatedtranscriptionfactorsbindtothe DNAstrandataspecificareathatfacilitatestranscription(Figure1).This area,knownasapromoterregion, oftenincludesaspecializednucleotidesequence,TATAAA,whichisalsocalled theTATAbox(notshowninFigure 1) Strandelongation Figure2: RNApolymerase(green)synthesizesastrandofRNAthatiscomplementarytotheDNAtemplatestrandbelowit. OnceRNApolymeraseanditsrelated transcriptionfactorsareinplace,thesingle-strandedDNAisexposedand readyfortranscription.Atthispoint,RNApolymerasebeginsmovingdownthe DNAtemplatestrandinthe3'to5'direction,andasitdoesso,itstrings togethercomplementarynucleotides.Byvirtueofcomplementarybase-pairing, thisactioncreatesanewstrandofmRNAthatisorganizedinthe5'to3' direction.AstheRNApolymerasecontinuesdownthestrandofDNA,morenucleotides areaddedtothemRNA,therebyformingaprogressivelylongerchainof nucleotides(Figure2).Thisprocessiscalledelongation. Figure3:DNA(top)includesthymine(red);inRNA(bottom),thymineisreplacedwithuracil(yellow). ThreeofthefournitrogenousbasesthatmakeupRNA—adenine(A),cytosine(C),andguanine(G)—arealsofoundinDNA.InRNA,however,abasecalleduracil(U)replacesthymine(T)asthecomplementarynucleotidetoadenine(Figure3).Thismeansthatduringelongation,thepresenceofadenineintheDNAtemplatestrandtellsRNApolymerasetoattachauracilinthecorrespondingareaofthegrowingRNAstrand(Figure4). Figure4:AsamplesectionofRNAbases(upperrow)pairedwithDNAbases(lowerrow).Whenthisbase-pairinghappens,RNAusesuracil(yellow)insteadofthyminetopairwithadenine(green)intheDNAtemplatebelow. Interestingly,thisbasesubstitutionisnottheonlydifferencebetweenDNAandRNA.AsecondmajordifferencebetweenthetwosubstancesisthatRNAismadeinasingle-stranded,nonhelicalform.(Remember,DNAisalmostalwaysinadouble-strandedhelicalform.)Furthermore,RNAcontainsribosesugarmolecules,whichareslightlydifferentthanthedeoxyribosemoleculesfoundinDNA.Asitsnamesuggests,ribosehasmoreoxygenatomsthandeoxyribose. Thus,theelongationperiodoftranscriptioncreatesanewmRNAmoleculefromasingletemplatestrandofDNA.AsthemRNAelongates,itpeelsawayfromthetemplateasitgrows(Figure5).ThismRNAmoleculecarriesDNA'smessagefromthenucleustoribosomesinthecytoplasm,whereproteinsareassembled.However,beforeitcandothis,themRNAstrandmustseparateitselffromtheDNAtemplateand,insomecases,itmustalsoundergoaneditingprocessofsort. Figure5: Duringelongation,thenewRNAstrandbecomeslongerandlongerastheDNAtemplateistranscribed.Inthisview,the5'endoftheRNAstrandisintheforeground.Notetheinclusionofuracil(yellow)inRNA. Terminationandediting Figure6: Ineukaryotes,noncodingregionscalledintronsareoftenremovedfromnewlysynthesizedmRNA. FigureDetail Aspreviouslymentioned,mRNAcannotperformitsassigned functionwithinacelluntilelongationendsandthenewmRNAseparatesfromthe DNAtemplate.Thisprocessisreferredtoastermination.Ineukaryotes,theprocessofterminationcanoccurin severaldifferentways,dependingontheexacttypeofpolymeraseusedduring transcription.Insomecases,terminationoccursassoonasthe polymerasereachesaspecificseriesofnucleotidesalongtheDNAtemplate, knownastheterminationsequence. Inothercases,thepresenceofaspecialproteinknownasaterminationfactorisalsorequiredfor terminationtooccur. Figure7: Ineukaryotes,apoly-Atailisoftenaddedtothecompleted,editedmRNAmoleculetosignalthatthismoleculeisreadytoleavethenucleusthroughanuclearpore. Onceterminationiscomplete,themRNAmoleculefallsoff theDNAtemplate.Atthispoint,atleastineukaryotes,thenewlysynthesized mRNAundergoesaprocessinwhichnoncodingnucleotidesequences,calledintrons,areclippedoutofthemRNA strand.Thisprocess"tidiesup"themoleculeandremovesnucleotidesthatarenotinvolvedinproteinproduction(Figure6).Then,asequenceof adeninenucleotidescalledapoly-Atail isaddedtothe3'endofthemRNAmolecule(Figure7).Thissequencesignals tothecellthatthemRNAmoleculeisreadytoleavethenucleusandenterthe cytoplasm. What'snextfortheRNAmolecule? Moreontranscription Howarepolymerasesdifferentinprokaryotesandeukaryotes?Howisbacterialtranscriptionunique?Howistranscriptionregulated? OnceanmRNAmoleculeiscomplete,thatmoleculecangoontoplayakey roleintheprocessknownastranslation.Duringtranslation,theinformationthatiscontainedwithinthemRNAis usedtodirectthecreationofaproteinmolecule.Inorderforthistooccur, however,themRNAitselfmustbereadbyaspecial,protein-synthesizing structurewithinthecellknownasaribosome. Watchthisvideoforasummaryofeukaryotictranscription FurtherExploration KeyQuestionsWhatdoesRNAdointhecell?Whatareintronsandexons? KeyConceptsRNA|replication|transcription eBooks ThispageappearsinthefollowingeBook EssentialsofGenetics,Unit1.5 ABriefHistoryofGenetics:DefiningExperimentsinGenetics,Unit5.5 TopicroomswithinGenetics Close Notopicroomsarethere. Loading... BrowseVisually OtherTopicRooms Genetics GeneInheritanceandTransmission GeneExpressionandRegulation NucleicAcidStructureandFunction ChromosomesandCytogenetics EvolutionaryGenetics PopulationandQuantitativeGenetics Genomics GenesandDisease GeneticsandSociety CellBiology CellOriginsandMetabolism ProteinsandGeneExpression SubcellularCompartments CellCommunication CellCycleandCellDivision ScientificCommunication CareerPlanning Loading... 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