第一题：THEFEATURE：ThelargestsubunitinRNApolymeraseIIhasacarboxy-terminaldomain(CTD)（羧基末端域）,whichconsistsofmultiplerepeatsofaconsensussequenceof7aminoacids.TheCTDcanbehighlyphosphorylatedonserineorthreonineresidues.
BIOLOGICALROLES：(1)Duringpromoterescape,phosphorylationoftheCTDbythekinaseactivityofTFIIHmayhelpthepolymeraseleavebehindthepromoterandGTFsandenterelongationphase.(2)Duringtranscriptionelongationandprocessing,manyelongationfactorsandRNAprofessingproteinsareloadedtopolymerase,nascentmRNAand/orprocessingmachinerythroughtheCTDtails.Therefore,throughtheCTDtail,synthesisandprocessingofthenewmRNAsarehighlycoupledandcoordinatedtoensuretheproductionofhighqualitymRNAfortranslation.
(Note:区分真核和原核.题目中指明为RNAPⅱ,说明为真核生物.另外原核生物pol的CTD称为α-CTD.)
第二题CHEMISTRY：splicingoccursastwosequentialester-transferreactions（两个连续的转酯反应）.Firstly,the2’OHofthebranchpointAattacksthephosphorylgroupofaconserved5’Gathe5’splicesite,resultinginthefreeof5’exonfromtheintron.Then,the3’OHofthe
free3’endof5’exonattacksaphosphorylgroupatthe3’splicesite,resultingintheligationofthe5’and3’exonsandreleaseofalariatform(套索结构)ofintron.
MECHANISM：步骤一,formationoftheE(early)complexbyrecognitionofthe5’splicesite,3’splicesiteandAbranchpointbyU1snRNP,U2AFandBBPrespectively.
步骤2,U2snRNPbindtothebranchsitetoreplaceBBPformsAcomplex.Thebase-pairingbetweenU2snRNAwiththebranchsitemaketheconservedAresidueinthebranchsiteextrudedfromthepairedregion,andthusthisAisreadytocarrythenucleophileattack.Thetri-snRNPU4/U6/U5joinsandAcomplexisarrangedtoBcomplexinwhichthethreesplicesitesarebroughttogether.InthiscomplexU4/U6snRNPsareheldtogethertightlybyextensivebase-pairingbetweenU4andU6snRNAs.
步骤三,U1leavesthecomplex,andU6occupiesthe5’splicesitebybase-pairing.U4leavesthecomplex,allowingtheRNAcomponentsofU2andU6tobasepairtoproducetheactivesite.ThebranchsiteAattacksthe5’splicesite,formingthe3-wayjunctionandCcomplex.The5’splicesitethenattacksthe3’splicesite,freeingtheintronlariatandformingthemRNAproduct.
二类内含子剪接:thechemistryisthesameasthatofthe
spliceosome-mediatedmRNAsplicing,butthesplicingiscatalyzedbytheintronRNAitself,whichisalsonamedasself-splicing.
一类内含子剪接：Chemistry:the3’OHofanexogenousGnucleotideornucleosideattacksthe5’splicesitetoundergothefirstester-transferreaction.Thenthe3’OHofthe5’exonattacks5’phosphateofatthe3’splicesite,resultedinthereleaseofalinearintronandligatedexons.ThesplicingiscatalyzedbytheintronRNAitselfasthegroupIIintrondoes.
第三题：Alternativesplicing（选择性剪接）:manyprotein-encodedgenesinhighereukaryotescontainmultipleintronsandcanbesplicedinalternativewaystogeneratetwoormoredifferentmRNAs.
Biologicalfunctions（生物学作用）:Alternativesplicingallowsindividualgenestoproducemultipleproteinisoforms,therebyplayingacentralpartinexpandingproteindiversityfromalimitednumberofgenesandregulatinggeneexpressioninhighereukaryotes.Alternativesplicingalsohasalargelyhiddenfunctioninquantitativegenecontrol,bytargetingR