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BiologyLecture1-2:MOLECULARGENETICS-Transcription,TranslationandControlofgeneexpression
• Describetheessentialfeaturesoftranscriptionandtranslationincellsofbothprokaryotes(bacteria)andeukaryotes(fungi,plantsandanimals).
• Understandhowpointandframe-shiftmutationscanaffectproductionoffunctionalgeneproducts/proteins.• Understandthebasicprocessesbywhichgeneexpressioncanbecontrolledattheleveloftranscriptioninprokaryotes
andeukaryotes.
Transcription: thesynthesisofRNAusinginformationintheDNA.Thetwonucleicacidsarewrittenindifferentformsofthesamelanguageandtheinformationisjustsimply‘rewritten’fromDNAtoRNA.
MessengerRNA(mRNA): carriesageneticmessagefromtheDNAtotheprotein-synthesisingmachineryofthecell.Translation: thesynthesisofapolypeptideusingtheinformationinthemRNA.Duringthisstage,thereis
achangeinlanguage:ThecellmusttranslatethenucleotidesequenceofanmRNAmoleculeintotheaminoacidsequenceofapolypeptide.Thesitesoftranslationareribosomes,molecularcomplexesthatfacilitatetheorderlylinkingofaminoacidsintopolypeptidechains.
Transcription/Translationinbacteriavs.eukaryotes:
Becausebacteriadonothavenuclei,theirDNAisnotseparatedbynuclearmembranesfromribosomesandotherprotein-synthesisingequipment.ThislackofcompartmentalisationallowstranslationofanmRNAtobeginwhileitstranscriptionisstillinprogress.Inaeukaryoticcell,bycontrast,thenuclearenvelopeseparatestranscriptionfromtranslationinspaceandtime.Transcriptionoccursinthenucleus,andmRNAisthentransportedtothecytoplasm,wheretranslationoccurs.
Primarytranscript: theinitialRNAtranscriptfromanygene,includingthosespecifyingRNAthatisnottranslatedintoprotein.Templatestrand:thestrandofDNAthatprovidesthepatternortemplateforthesequenceofnucleotidesinanRNAtranscript.
*ThemRNAisreadinthe5’-3’direction.
TRANSCRIPTION
MolecularcomponentsofTranscription:
• RNApolymerase:priesthetwostrandsofDNAapartandjoinstogetherRNAnucleotidescomplementarytotheDNAtemplatestrand.RNApolymerasescanassembleapolynucleotideonlyinits5’!3’direction.UnlikeDNApolymerasehowever,RNApolymerasesareabletostartsachainfromscratch;theydon’tneedaprimer.
• Promoter:theDNAsequencewhereRNApolymeraseattachesandinitiatestranscription.
• TranscriptionUnit:ThestretchofDNAdownstreamfromthepromoter.
SynthesisofanRNATranscript:
Therearethreestagesoftranscription:initiation,elongationandterminationoftheRNAchain.
1. Initiation• RNApolymerasebindstoapromoterthatdetermineswhere
transcriptionstartsandwhichofthetwostrandsoftheDNAhelixisusedasthetemplate.
• Ineukaryotes,acollectionofproteinscalledtranscription
factorsmediatethebindingofRNApolymeraseandtheinitiationoftranscription.OnlyaftertranscriptionfactorsareattachedtothepromoterdoesRNApolymeraseIIbindtoit.ThecomplexoftranscriptionfactorsandRNApolymeraseIIboundtothepromoteriscalledatranscriptioninitiationcomplex.
• OncetheappropriatetranscriptionfactorsarefirmlyattachedtothepromoterDNAandthepolymeraseisboundinthecorrectorientation,theenzymeunwindsthetwoDNAstrandsandbeginstranscribingthetemplatestrandatthestartpoint.
2. Elongation• AsRNApolymerasemovesalongtheDNA,ituntwists
thedoublehelix,exposingDNAnucleotidesforpairingwithRNAnucleotides.
• Theenzymeaddsnucleotidestothe3’endofthegrowingRNAmoleculeasitcontinuesalongthedoublehelix.
• ThenewRNAmoleculepeelsawayfromitsDNAtemplateandtheDNAdoublehelixre-forms.
3. Termination• Terminationdiffersbetweenbacteriaandeukaryotes.Inbacteria,transcriptionproceedsthroughaterminator
sequenceintheDNA.Thetranscribedterminator(anRNAsequence)functionsastheterminationsignal,causingthepolymerasetostitchfromtheDNAandreleasethetranscript,whichrequiresnofurthermodificationbeforetranslation.
• Ineukaryotes,RNApolymeraseIItranscribesasequenceontheDNAcalledthepolyadenylationsignalsequence,whichspecifiesapolyadenylationsignalinthepre-mRNA.Thepre-mRNAthenundergoesprocessing,thetopicofthenextsection.Al-thoughthatcleavagemarkstheendofthemRNA,theRNApolymeraseIIcontinuestotranscribe.Sincethenew59endisn’tprotectedbyacap,however,enzymesdegradetheRNAfromthe59end.Thepolymerasecontinuestranscribing,pursuedbytheenzymes,untiltheycatchuptothepolymeraseanditdissociatesfromtheDNA.
Q.WhatenablesRNApolymerasetostarttranscribingageneattherightplaceontheDNAinabacterialcell?Inaeukaryoticcell?
ModifyingRNAaftertranscription:
• 5’endreceivesa5’capandapoly-Atailisaddedtothe3’end(termedpre-RNA).ThesefeaturesfacilitatetheexportofmaturemRNAfromthenucleus,protectthemRNAfromdegradationbyhydrolyticenzymesandhelpribosomesattachtothe5’endofthemRNAoncethemRNAreachesthecytoplasm.
• RNAsplicing:noncodingregions(introns)arecutoutoftheRNAmolecule(bysmallRNAscalledspliceosomes)whilstexonsaremaintainedandexpressed.
AlternateRNAsplicing: Alternativesplicingisaregulatedprocessduringgeneexpressionthatresultsinasinglegenecodingformultipleproteins.Inthisprocess,particularexonsofagenemaybeincludedwithinorexcludedfromthefinal,processedmessengerRNA(mRNA)producedfromthatgene.
TRANSLATION
MolecularcomponentsofTranslation:
• tRNAtranslatesmessagesintheformofseriesofcodonsalonganmRNAmolecule.ThefunctionoftRNAistotransferaminoacidsfromthecytoplasmicpoolofaminoacidstoagrowingpolypeptideinaribosome.
• EachtRNAmoleculetranslatesagivenmRNAcodonintoacertainaminoacid.ThisispossiblebecausetRNAbearsaspecificaminoacidatoneend,whichattheotherendarenucleotidetripletsthatcanbasepairwiththecomplementarycodononmRNA.
• TheanticodonisthepartofthenucleotidetripletthatbasepairstoaspecificmRNAcodon.• LikemRNAandothertypesofcellularRNA,transferRNAmoleculesaretranscribedfromDNAtemplates.Ina
eukaryoticcell,tRNA,likemRNA,ismadeinthenucleusandthentravelsfromthenucleustothecytoplasm,whereitwillparticipateintheprocessoftranslation.Inbothbacterialandeukaryoticcells,eachtRNAmoleculeisusedrepeatedly,pickingupitsdesignatedaminoacidinthecytosol,depositingthiscargoontoapolypeptidechainattheribosome,andthenleavingtheribosome,readytopickupanotherofthesameaminoacid.
InadditiontoabindingsiteformRNA,eachribosomehasthreebindingsitesfortRNA,asdescribedinFigure17.17.ThePsite(peptidyl-tRNAbindingsite)holdsthetRNAcarryingthegrowingpolypeptidechain,whiletheAsite(aminoacyl-tRNAbindingsite)holdsthetRNAcarryingthenextaminoacidtobeaddedtothechain.DischargedtRNAsleavetheribosomefromtheEsite(exitsite).TheribosomeholdsthetRNAandmRNAincloseproximityandpositionsthenewaminoacidsothatitcanbeaddedtothecarboxylendofthegrowingpolypeptide.Itthencatalysestheformationofthepeptidebond.Asthepolypeptidebecomeslonger,itpassesthroughanexittunnelintheribosome’slargesubunit.Whenthepolypeptideiscomplete,itisreleasedthroughtheexittunnel.
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