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1、轉(zhuǎn)錄（Transcription）RNApolymeraserequirestemplatesandmagnesiumionswith4NTPassubstrate,andthesynthesisdirectionis5'-3',butnoprimerisneededTemplate:thecompletedoublestrandedDNAisusedastemplate,andonlyonestrandistranscribed.Thelocalunwindingofthetranscriptleadstotheformationofadoublehelixstructu
2、reafterthetranscriptionofDNA,soDNAisfullyreserved.TranscriptionprocessItisdividedintothreestages:initiation,extensionandtermination.InitiationinvolvesidentificationofspecificregionsofthedoublestrandedDNA,local(17bp)unwinding,andformationofthetwoesterbondbetweenthefirsttwonucleotides.Thepos
3、itionofthefirstnucleotideincorporationiscalledthetranscriptionalstartpoint.AllRNAinprokaryotesaresynthesizedbyaRNApolymerase.ThemolecularweightofEscherichiacoliRNA-polis460kDa,whichconsistsof5subunits,namely,alpha2,beta,beta,andsigma."Alpha2betabeta",calledthecoreenzyme,canonlyprolongtheRN
4、Achainthathasbeensynthesized,anddoesnothavetheabilitytoinitiateRNAsynthesis.OnlytheholoenzymeandthetemplateDNApromoterbindtoplayaroleintheinitiationoftranscription.TherearemanykindsofeukaryoticRNA-pol,andthemolecularweightisabout500kDaIntheDNAmolecule,anactivatedgenesegmentiscalledthetempl
5、atestrand,whichisnottranscribedbyitscomplementarychain,andiscalledthecodingstrand.Formanygenes,thetemplatechainisnotalwaysonthesamesinglestrandAfterinitiation,thestartingfactormovesaway,thecoreenzymechangesinconformation,movesalongthetemplate,transcriptionproducesthehybriddoublestrand(12bp
6、),andthentheDNAcomplementarystrandreplacestheRNAchainandrestorestheDNAdoublehelixstructure.Attheendofthepolymerase,theendoftheenzymestoppedwiththehelpofthecofactor,andtheenzymeandRNAchainsfelloffandthetranscriptionendedPromoter:aDNAsequenceofenzymerecognition,bindingandinitiationoftranscri
7、ption.Strongpromoterinitiatestranscriptionat2seconds,weakpromoteronceevery10minutes.Prokaryotes:EscherichiacolihasconservedsequenceTATAATatabout10basepairsupstreamofthestartingpoint,calledpribnowbox,whichhelpstopartiallyresolvethechain.IntheupperreachesandTTGA
