《分子生物学》第15章 遗传编码（英文教学课件）非AI生成 (2).ppt

Figure15-2WobblebasepairingTheribose-ribosedistancesforthewobblepairsareclosetothoseofA:UorG:Cbasepairs*CriticalThinkingThewobbleconceptpredictedthatatleastthreetRNAsexistforthesixserinecodons(UCU,UCC,UCA,UCG,AGU,andAGC).Why?*Whywobbleisallowedatthe5’anticodon?The3-DstructureoftRNAshowsthatthestackinginteractionsbetweentheflatsurfacesofthe3anticodonbases+2followedbasespositionthefirst(5’)anticodonbaseattheendofthestack,thuslessrestrictedinitsmovements.The3’baseappearsinthemiddleofthestack,resultingintherestrictionofitsmovements.*Figure15-3StructureofyeasttRNA(Phe)TheadjacentbaseTheadjacentbaseisalwaysabulkymodifiedpurineresidue.*1-3ThreeCodonsDirectChainTerminationThreecodons,UAA,UAG,andUGAsignifychaintermination.TheyarenotreadbytRNAsbutbyproteinscalledreleasefactors(RF1andRF2inbacteriaandeRF1ineukaryotes).*1-4HowtheCodeWasCracked？SeeChapter2,Page35:EstablishingtheGeneticCodeTheuseofartificialmRNAsandtheavailabilityofcell-freesystemsforcarryingoutproteinsynthesisbegantomakeitpossibletocrackthecode.氨基酸与三联密码子之间的对应关系是分子生物学史上最伟大的成就之一。*ExperimentalResults:UUUcodesforphenylalanine.CCCcodesforproline.AAAcodesforlysine.Theguanineresiduesinpoly-Gfirmlyhydrogenbondtoeachotherandformmultistrandedtriplehelicesthatdonotbindtoribosomes.*1-6MixedCopolymersAllowedAdditionalCodonAssignmentsPoly-ACcontain8codons:CCC,CCA,CAC,ACC,CAA,ACA,AAC,andAAA.TheycodeforAsp,Glu,His,ThrPro(CCC),Lys(AAA).Theproportionsofthe8codonsincorporatedintopolypeptideproductsdependontheA/Cratio.*Suchexperimentcandeterminethecompositionofthecodons,butnottheorderofthethreenucleotides.SeeTable15-3onPage528*1964年，人们研发出一种直接确定某些三联密码子中3个核苷酸顺序的方法.即发现了特定的氨基酰tRNA分子与核糖体-mRNA复合物结合。如poly-