1415Translation and the Genetic Code翻译和遗传密码.docx
- 文档编号:11337244
- 上传时间:2023-02-28
- 格式:DOCX
- 页数:27
- 大小:27.37KB
1415Translation and the Genetic Code翻译和遗传密码.docx
《1415Translation and the Genetic Code翻译和遗传密码.docx》由会员分享,可在线阅读,更多相关《1415Translation and the Genetic Code翻译和遗传密码.docx(27页珍藏版)》请在冰豆网上搜索。
1415TranslationandtheGeneticCode翻译和遗传密码
TranslationandtheGeneticCode
Chapters14and15
I.Translation
A.Fourcomponentsrequiredfortranslation
B.Polyacrylamidegelelectrophoresis
C.Two-dimensionalgelelectrophoresis
D.Translation-overview
E.Initiation
F.Elongation
G.Termination
H.Translation-DependentregulationofmRNAandproteinstability
II.TheGeneticCode
A.TheGeneticCode
B.Wobblehypothesis
C.Antibiotics
TranslationismuchmorecomplicatedthanDNAreplicationandtranscription.
Theproductionofasingleproteinrequiresmorethan100proteinsandRNAs.
Intranslationthegeneticcodethatiswrittenwith4nucleotidebasepairsmustbetranslatedintoasecondcodewrittenwith20aminoacids.
A.Thereare4componentsthatarerequiredfortranslation:
1.mRNA
2.tRNA
3.aminoacyltRNAsynthetases
4.ribosomes
1.MessengerRNA
EachmessengerRNA(mRNA)containsregionsthatencodeproteinsequence.Inthissequencethereare3nucleotideunitscalledcodonsthatindicateaparticularaminoacid.
TheDNAthatwastranscribedintoRNAcontainsthesequencesrequiredtoencodeapolypeptide.Thisiscalledanopenreadingframe(ORF).
ThemRNAofanORFbeginswithatranslationstartcodon,followedbyasequenceofcodonsthatspecifyaminoacids,andendswithastopcodon.
Translationbeginsnearthe5’endofthemRNAandmovesonecodonatatimetothe3’end.
Thefirstcodoniscalledthestartcodon.Thelastcodoniscalledthestopcodon.
EukaryotesalwaysbegintranslationwiththestartcodonAUG.BacteriaalsoalmostalwaysuseAUGasthestartcodon,though,theycanrarelyuseGUGorUUG.
Thestartcodonencodesthefirstaminoacidandsetsthereadingframeforallthecodonsthatfollow.
Thereare3possiblereadingframesinanysequence.Thestartcodonsetsthereadingframefortherestofthecodons.
Thetopsequencecontainsthecodingsequenceforapolypeptide.
Thereare3stopcodons:
1.UAG
2.UGA
3.UAA
Thissignalstheendoftranslation.
EachmRNAwillencodeatleastonepolypeptide.ProkaryoticmRNAoftenencodes2ormorepolypeptides.ThesearecalledpolycistronicmRNAs
(poly=many;cistron=gene,soitismanygenes).
EukaryoticmRNAnearlyalwaysencodesasinglepolypeptide.ThisiscalledmonocistronicmRNA.(mono=one)
OfteninbacteriamanypolypeptidesareencodedonthesamemRNA.
EncodingseveralgenesonthesamemRNA
(calledapolycistronicmRNA)allowsthesegenestoberegulatedtogether.
BetweentheRNAsequencesencodingpolypeptides,theremaybenoncodingsequencesknownasspacers.ManymRNAshavealeadersequencethatisnottranslated.The3’sequencesarealsononcoding(nottranslated).
RibosomesmustbindtothemRNAfortranslationtooccur.ProkaryoticmRNAshavearibosomebindingsite(RBS)sequencethatisupstreamofthestartcodon.ThissequenceisalsocalledtheShine-Dalgarnosequenceandwasnamedafterthescientistswhodiscoveredit.
InprokaryotestheconsensussequenceisAGGAGGU.Thissequenceiscomplementarytoasequenceinthe16SrRNA.
Thus,the16SrRNAbasepairstotheribosomebindingsite.
AsequenceinthemRNAknownastheribosomebindingsiteiscomplementarytoasequenceinthe16SrRNAthatispartoftheribosome.ThisallowstheribosometobindtothemRNA.
OfteneachcistroninpolycistronicmRNAwillhaveanRBS.Sometimesacistronwilllackaribosomebindingsitebutitcanstillbeactivelytranslatedbecauseitiscloseenoughtoapreviousupstreamcistronthattheribosomesdonotdissociateandtranslatethesecondcistron.Thisiscalledtranslationalcoupling.
Translationalcoupling:
whenacistroninapolycistronicmRNAdoesnothavearibosomebindingsite,butdependsontheribosomescontinuingtotranslatefollowingtranslationofanupstreamcistron.
ForeukaryoticmRNA,the5’capatthe5’endofthemRNAactsasaribosomebindingsite.
Thereare3stepsinaddinga5’captoaeukaroyticmRNA.Whatarethey?
1.aphosphateisremovedfromthe5’endoftheRNA.
2.aGTPisadded.
3.AmethylgroupisaddedtotheGTP
Aftertheribosomebindstothe5’cap,itmovesalongthemRNAscanningforastartcodon(AUG).Thisprocessiscalledscanning.
SomemRNAsalsohaveaKozaksequence.Thisstimulatesahigherleveloftranslation.
2.transferRNA
OneofthekeycomponentsoftranslationistRNA.ItbindstobothanaminoacidandthecodonsinthemRNA.
2.tRNA
ThetransferRNAortRNAmoleculesbindtoboththemRNAcodonsandtheaminoacid.TherearemanydifferentkindsoftRNAmoleculesandeachisattachedtoaspecificaminoacidandrecognizesaparticularcodonorcodons.
ThestructureoftRNAisimportant.Ithasprimary,secondaryandtertiarystructuresimilartoproteins.
WhatistheprimarystructureoftRNA?
tRNAsare75-95basepairslong.FirstalongerRNAistranscribedandthenpartsofitareremoved.
SomeofthebasesinthetRNAaremodifiedfollowingtranscription.Thisiscalledpost-transcriptionalmodification.Enzymeschangethebases.(post=after)
Post-transcriptionalmodification:
whenchangesaremadeaftertranscription
SomeofthebasesintRNAarepost-transcriptionallymodified.
Forexample,uracilischangedtopseudouridine[writtenψU(psi)]ordihydrouridine.
tRNA
TransferRNAiscutoutfromalargerstrandofRNA.
Inpost-transcriptionalmodificationoftRNA,someofthebasepairsaremodified.
ThemodifiedbasesarenotessentialfortRNAfunction.However,growthisslowedincellsthatlacktheenzymesthatmodifythebases.ThissuggeststhatthemodifiedbasesmaketRNAmoreefficient.
TransferRNAhasaDloop,aTloop,avariableloopandananticodonloop.TheTloopisalsocalledtheψUloopbecauseitcontainsthealteredbasepseudouridine(ψU).
TheaminoacidsareattachedtotheacceptorstempartofthetRNA.
AminoacidsareaddedtotheacceptorstemattheCCAsite.
ThetRNAsecondarystructurefoldsintoatertiarystructure,whichhasbeendetermined.
HowdoyouthinkscientistsdeterminedthestructureoftRNA?
x-raycrystallography
ThreekindsofbondsstabilizethetRNAtertiarystructure:
1.Hydrogenbondsbetweenbases,butthesearenotnormalWatson-Crickbasepairing
2.interactionsbetweenthebasesandthesugar-phosphatebackbone
3.basestacking
ThreedimensionalstructureoftRNA.
AttachmentofAminoAcidstotRNA
tRNAsthatlackanaminoacidaresaidtobeuncharged.
Anaminoacidisaddedwithanacylbondbetweenthecarboxylgroupoftheaminoacidandthe2’or3’OHgroupoftheadenosinenucleotideontheacceptorstemofthetRNA.
HowdoestheaminoacidgetattachedtothetRNA?
3.AminoacyltRNAsynthetase
Thereare2reactionsinattachinganaminoacidtoatRNA.
1.Thefirststepisanadenylylation
2.tRNAcharging
1.adenylylation
AdenylylationisthetransferofAMP.
TheaminoacidandATPinteractandthroughadenylylationAMPisaddedtotheaminoacid.Apyrophosphateisreleased.
Whatprovidestheenergytodrivethisreaction?
Thereactionisdrivenbytheenergyprovidedbyhydrolysisofthepyrophosphatebyapyrophosphatase.
ThephosphorylgroupoftheAMPisattachedtothecarbonylgroupoftheaminoacidwithahighenergyesterbond
2.tRNAcharging
Theaminoacidistransferredtothe3’endofthetRNA.Thereisanadenineatthispositionandtheaminoacidcanbeaddedtothe2’OHorthe3’OH.
TheAMPisreleased.
ClassItRNAsynthetasesaddtheaminoacidtothe2’OH.
ClassIItRNAsynthetasesaddtheaminoacidtothe3’OH.
AfterthechargedtRNAisreleasedtheaminoacidcanmovefromthe2’tothe3’orviceversa.Equilibriumisreachedbetweenthe2sites.
ClassIaddtheaminoacidtothe2’OH.
ClassIIaddtheaminoacidtothe3’OH.
Eachofthe20aminoacidsisattachedtothetRNAbyasingletRNAsynthetase.
SometRNAsynthetasesrecognizemorethanonetRNA,butthesametRNAsynthetasechargesalltRNAsforaparticularaminoacid.
ThreedifferenttRNAsynthetasemoleculesareshownboundto3differentaminoacids.
WhatpartofthetRNAisrecognizedbytheaminoacyltRNAsynthetase?
Thereare2partsofthetRNAthatarerecognized.
1.theacceptorstem
2.theanticodonloop
Bindingto2siteshelpstheenzymedistinguishbetweendifferenttRNAs.
TheaminoacyltRNAsynthetaserecognizesspecificsequencesintheacceptorstem.ThisallowsittodeterminewhichtRNAtoaddanaminoacidto.
Onebasepairintheacceptorstemiscalledthediscriminatorandgreatlycontributestotherecognition.
TheaminoacyltRNAsynthetasealsobindstosequencesintheanticodonloop.
TheaminoacyltRNAsynthetasemustbeabletoacceptalltheanticodonsthatspecifyaparticularaminoacid.
Forexampleserineisencodedby6codons(UCU,UCC,UCA,UCG,AGU,andAGC).Thus,theaminoacyltRNAsynthetasethataddsaserinemustacceptthesediversesequencesintheanticodonloop.
ItisimportantthattheaminoacyltRNAsynthetaseaddsthecorrectaminoacidtothetRNA.Generallyonly1in1,000tRNAsisboundtothewrongaminoacid.
SomeaminoacidsareverydifferentanditiseasyfortheaminoacyltRNAsynthetasetodistinguishbetweenthembasedonsizeandfunctionalgroups.
Tyrosineandphenylalaninehaveasimilarsize,butthepresenceofanOHallowsthemtobedistinguishedfromeachother.
Someoftheaminoacidssuchasisoleucineandvalinearesimilarinsizeandstructure.Theonlydifferenceisamethylgroup.
TheactivesiteontheaminoacyltRNAsynthetaseistoosmallforaminoacidsthatarelargerthanthecorrectonetobind.
Smalleraminoacidscanbind,sovalinecanbindinsteadofisoleucine,buttheoppositedoesnothappen.(Isoleucinecanno
- 配套讲稿:
如PPT文件的首页显示word图标,表示该PPT已包含配套word讲稿。双击word图标可打开word文档。
- 特殊限制:
部分文档作品中含有的国旗、国徽等图片,仅作为作品整体效果示例展示,禁止商用。设计者仅对作品中独创性部分享有著作权。
- 关 键 词:
- 1415Translation and the Genetic Code翻译和遗传密码 1415 Translation Code 翻译 遗传 密码
![提示](https://static.bdocx.com/images/bang_tan.gif)