Класификация
Acellinthehumanbodycontainsabout10pgofRNA(about7pgofDNA).ComparedwithDNA,RNAhasawidevarietyoftypes,smallermolecularweights,andlargechangesincontent.RNAcanbedividedintomessengeрРНКandnon-codingRNAaccordingtoitsstructureandfunction.Non-codingRNAisdividedintonon-codinglargeRNAandnon-codingsmallRNA.Largenon-codingRNAincludesribosomalRNAandlongnon-codingRNA.Non-codingsmallRNAsincludetransfeрРНК,ribozymes,smallmoleculeRNAsandsoon.SmallRNA(20~300nt)includesmiRNA,SiRNA,piRNA ,scRNA,snRNA,snoRNA и т.н. Бактериите също имат малка РНК (50~500nt).
MessengeрРНК
MessengeрРНК(тРНК)wasfirstdiscoveredin1960.Itisresponsiblefortransmittinggeneticinformationanddirectlyguidingproteinsynthesisduringproteinsynthesis.Ithasthefollowingcharacteristics.
1. Ниско съдържание, което представлява 1% до 5% от общата клетъчна РНК.
2.Therearemanytypes,upto105species.DifferentgenesexpressdifferentтРНКs.
3.Thelifespanisshort,anddifferentтРНКguidesthesynthesisofdifferentproteins,whichwillbedegradedaftercompletingthemission.Theaveragehalf-lifeofbacterialтРНКisabout1.5minutes.Thehalf-lifeofvertebrateтРНКvariesgreatly,withanaverageofabout3hours.
4.LargedifferencesinlengthmammalianтРНКlengthis5×102~1×105nttheтРНКofprokaryotesandeukaryotesalthoughTherearedifferencesinstructure,butthesamefunction,theyaretemplatesforguidingproteinsynthesis.
Трансфер РНК
Трансферна РНК (тРНК) е отговорна за транспортирането на аминокиселини и интерпретирането на генетичния код на тРНК по време на протеиновия синтез. На тРНК се падат 10% до 15% от цялата клетъчна РНК, повечето от които са локализирани в цитоплазмата. Съществуването на РНК е предложено от Крикин през 1955 г. и е идентифицирано от Замечник и Хоагланд през 1957 г.
1. Първичната структура на РНК
Той има следните характеристики:
①Това е тип едноверижна малка молекула РНК, дължина73~95nt(консенсусна последователност76nt),коефициент на утаяване4S.
②Това е РНК с най-много базови бази, съдържащи 7-15 редки базови бази (отчитащи 15%-20% от фалшивите бази), разположени в несдвоената област.
③5′терминална база есофтенгуанин.
④Краят на 3' завършва последователността CCA, в която дениловата киселина се нарича A76, а нейният 3'-OH е мястото за свързване на аминокиселината.
2.тРНК вторична структура
About50%basepairing,formingafour-segmentdoublehelix,formingaclover-shapedstructurewithfiveunpairedsequences.Therearefourarmsandfourloopsinthisstructure:
①Аминокиселини.
②Дихидроурациларм (DHUarm, Darm) и дихидроурацилов пръстен (DHUring, Dring), характеризиращи се със съдържание на дихидроурацил (DHU, D).
③Theanticodonarmandanticodonlooparecharacterizedbythefactthattheanticodonloopcontainsanticodons.The5'endoftheanticodonisconnectedtouridineacid,andthe3'endisconnectedtopurinenucleotides.TheTΨCarm(Tarm)andtheTΨCring(Ψring)arecharacterizedbytheTΨCringcontainingthymineribonucleotideT54pseudouridineΨ55cytidineC56.
④Extraloop3~21nt.
3. Третична структура на РНК
ItisL-shaped,withtheaminoacidbindingsiteatoneend,theanticodonloopattheotherend,andtheDHUloopAlthoughtheandTΨCringsarelocatedonbothsidesinthesecondarystructure,theyareadjacentinthetertiarystructure.AlthoughthelengthandsequenceofvariousтРНКsarenotthesame,theirtertiarystructureissimilar,suggestingthatthetertiarystructureiscloselyrelatedtoitsfunction.
Рибозомна РНК
Рибозомна РНК(рРНК)andribosomalproteinformakindofnucleoproteinparticlescalledribosomes.Thereareabout15,000ribosomesinanE.coli.
1.Състав и структура на рибозомите
Theribosomesofprokaryotesandeukaryotesarecomposedofalargesubunitandasmallsubunit.BothsubunitsarecomposedofрРНКandribosomalprotein.Thesizeofribosomes,ribosomalsubunitsandрРНКisgenerallyexpressedbythesedimentationcoefficient.
2.Характеристики на рибозомната РНК
(1) Високо съдържание, рРНК е най-високото съдържание на RNA в клетката, което представлява 80% от общата RNA на клетката~85%.
(2) Дълъг живот, бавна актуализация на РНК и дълъг живот.
(3)Therearefewtypes.Prokaryoteshave5S,16S,and23sрРНКs,whichaccountfor66%ofthemassofribosomes(5Sand23SрРНКsaccountfor70%ofthelargeribosomalsubunits,and16SрРНКItaccountsfor60%ofthesmallribosomalsubunits);eukaryotesmainlyhave5S,5.8S,18S,28SрРНК,andasmallamountofmitochondrialрРНКandchloroplastрРНК.Escherichiacoli16SрРНКhasaconservedsequenceACCUCCUatthe3'end,whichcancomplementallybindtotheSDsequenceintheтРНК.5TwoconservedsequencesofSрРНКhavealsobeenidentified:
①CGAAC,whichcancomplementtheGTCGoftheTΨCloopofтРНК.
②GCGCCGAAUGGUAGU може да бъде комплементарна на последователност от 23SрРНК.
3.Типове фрибозоми
Prokaryoteshaveonlyonetypeofribosomes,whileeukaryoteshavethefollowingtypeslocatedindifferentpartsofthecell:ribosomes,Freeribosomes,endoplasmicreticulumribosomes(alsocalledattachmentribosomes),mitochondrialribosomesandchloroplastribosomes(plants).Freeribosomesandendoplasmicreticulumribosomesareactuallythesametypeofribosomes.TheyarelargerthanprokaryoticribosomesandcontainmoreрРНКandprotein.Mitochondrialribosomesandchloroplastribosomesaresmallerthanprokaryoticribosomes.However,thebasicstructureandfunctionoftheseribosomesarethesame.
Рибозим
WhenscientistsarestudyingRNApost-transcriptionalprocessing,theyhavefoundthatcertainRNAshavecatalyticactivityandcancatalyzethesplicingofRNA.TheseRNAsaresynthesizedbylivingcellsandplayacatalyticrole.Knownasribozymes.ThesubstrateofmanyribozymesisalsoRNA,evenitsown,anditscatalyticreactionisalsospecific.
Thenaturalribozymesthathavebeenelucidatedincludehammerheadribozyme,hairpinribozyme,typeIintron,typeIIintron,hepatitisDvirusribozyme,ribonucleaseP,Peptidyltransferaseandsoon.Howtoevaluatethetheoreticalandpracticalsignificanceofribozymes,andhowtotreatthestatusofribozymesandtraditionalenzymesinmetabolism,allneedtobefurtherstudied.
1. Откриване на фрибозими
РибозимswerefirstdiscoveredbyCechandAltman(theNobelPrizewinnerinChemistryin1989).In1967,Woese,Crick,andOrgelproposedthatitmayhavecatalyticactivitybasedonthecomplexityoftheRNAsecondarystructure;in1982,Cechdiscoveredthatitsintronhasself-splicingactivitywhenstudyingthesplicingoftheTetrahymenaрРНКprecursor;in1983,AltmanfoundthattheMRNAinribonucleasePisinvolvedinthepost-transcriptionalprocessingofтРНКprecursorswhenstudyingbacterialтРНКprecursors;in1982,Krugeretal.suggestedthatthecatalyticallyactiveRNAbenamed"ribozyme(ribozyme)".
2.Характеристики на рибозима
Thevariousribozymesdiscoveredsofarhavethefollowingcharacteristics.
(1)ThechemicalnatureofribozymesisRNAoрРНКfragments.Someribonucleoproteinsalsohaveacatalyticeffect,buttheactivecenterislocatedontheirproteincomponentsandisnotaribozyme,suchastelomerase.However,iftheRNAoftheribonucleoproteincontainsanactivecenter,theRNAcomponentistheribozyme,suchastheM1RNAintheribonucleasePmolecule.
(2)Рибозимshaverelativelyfewtypesofsubstrates,mostofwhichareself-RNAorotheрРНКmolecules.Therefore,theyaredividedintotwotypes:autocatalysisandheterocatalysis.Inaddition,thereareothersubstrates.Forexample,thesubstratesofpeptidyltransferaseareaminoacylтРНКandpeptidylтРНК.
(3)Thecatalyticefficiencyofribozymesismuchlowerthanthatofenzymes.
(4)Рибозимsarealsospecific.Forexample,M1RNAonlycutstheextranucleotidesatthe5'endoftheRNAprecursor,butdoesnotcuttheextranucleotidesandothersequencesatthe3'end.
(5)Thereactionscatalyzedbyribozymesareirreversible.
(6)Mg2+isrequiredfortheribozymetocatalyzethereaction.Mg3+notonlymaintainstheactiveconformationoftheribozyme,butalsoparticipatesinthecatalyticreaction.
(7) Съдържанието на повечето трибозими в клетките е изключително ниско.
3.Значение на фрибозимите
①ThediscoveryandresearchofribozymeshasgivenusafurtherunderstandingofthephysiologicalfunctionsofRNA,thatis,itisbothgeneticThecarrierofinformationisalsoabiocatalyst,whichhasthefunctionsoftwotypesofbiologicalmacromolecules,DNAandprotein.
②Откриването на фрибозими разтърси традиционната концепция, че всички биокатализатори са протеини.
③Thediscoveryofribozymesisofgreatsignificanceforunderstandingtheevolutionoflife,andRNAmaybethefirstbiologicalmacromoleculetoappear.
4.Приложение на рибозим
①Генетерапия; ②Специфично разграждане на РНК; ③Биосензор; ④Функционална геномика; ⑤Откриване на ген.
Разпределение в клетките
90%ofeukaryoticRNAisdistributedinthecytoplasm,andasmallamountisfoundinmitochondria,chloroplastsandnucleoli.
РНК на прокариотите, разпределени в цитоплазмата.
Композиционна структура
LikeDNA,RNAisalsoapolynucleotidechaincomposedofvariousnucleotidesconnectedby3′,5′-phosphodiesterbonds,butwithDNAThereareaseriesofdifferences.
1.Intermsofchemicalcomposition,RNAcontainsribosebutnotdeoxyribose.Containsuracilbutdoesnotcontainthymidine.TheexceptionisthateachtNAmoleculecontainsathymine,whichismethylatedbyuracilaftertheRNAstrandissynthesized.Inaddition,asmentionedearlier,asmallnumberofDNAcontainsasmallamountofribose,buttheseindividualexceptionscannotbeusedThisnegatesthedifferenceinthecompositionofthetwotypesofnucleicacids.
2.AlthoughtheconceptofRNAprimarystructureisthesameasDNA.Butitsbasicstructuralunitisribonucleotideinsteadofdeoxyribonucleotide.Inaddition,partofRNAhasaspecialnucleotidesequenceatthe5'endor3'end,andthereisnocomplicatedsequenceorganizationlikeDNAintheRNAprimarystructure.
3.MosтРНКsaresingle-strandedmolecules,whichcanfoldthemselvestoformahairpin-likestructureandhavethecharacteristicsofalocaldoublehelixstructure.ThisisthecommonfeatureofvariousRANspatialstructures.feature.TheruleofbasecomplementarypairinginthelocaldoublehelixstructureofRNAisAtoUandGtoC.SincethebasepairingcannotbefullyformedinsidetheRNAmolecule,thebasemolarratioAisnotequaltoU,GisnotequaltoC,andthereisnoChargafflawofDNAbaseratio.
Механизъм на намеса
In1998,twoAmericanscientistsAndrewFallandCraigMellowjointlypublishedthediscoveryofRNA(ribonucleicacid)inthejournalNature.Thepaperontheinterferencemechanismiscalled"oneofthemostexcitingdiscoveriesinmolecularbiologyinrecenttimes"bycolleagues.
AndrewFarrwasborninSantaClaraCounty,California,USAin1959.HemajoredinmathematicsattheUniversityofCalifornia,Berkeley,andobtainedhisdegreeinjustthreeyears.In1983,hereceivedhisPh.D.degreeinbiologyfromMassachusettsInstituteofTechnology.Hegraduallybecameinterestedingeneticsinvolvingthemysteriesoflifeandregardeditashislifelongacademicpursuit.
CraigMellowwasbornin1960.Hisfatherwasapaleontologist.Duringhischildhood,MellowoftenfollowedhisfathertosearchforfossilsinthewesternUnitedStates.
Inthehighschoolera,Merlot'sinterestgraduallyshiftedtogeneticengineering.Atthattime,scientistsclonedthehumaninsulingeneandputitsDNA(deoxyribonucleicacid)intobacteria,sothataninfiniteamountofinsulincanbeartificiallysynthesized.Thisachievementhasbroughtgoodnewstomillionsofdiabeticpatientsworldwide.Melorecalled:"Scientificresearchcanreallyhaveanimpactonhumanhealth.Thisideaarousedmyinterest."
In1998,duringtheworkofFarrandMeloattheCarnegieInstitutionintheUnitedStates,TheycollaboratedtodiscoverthemechanismofRNAinterference.
AndrewFarrsaid:"TheworkofCraigandIistostudywhysomegenesstopfunctioning.Wetriedtocontrolthem.Wefoundsomethingthatcaneffectivelystopthem.Thesegenesdon’tIcan'ttellyouwhattheycando,soifyoucanstopthem,youcanstarttounderstandwhattheycando.However,itwasaChinesescholarwhofirstdiscoveredtheRNAphenomenon.Itisapitythathedidnotfurtherunderstandwhy."
Whattheydiscoveredwasakeymechanismforcontrollingtheflowofgeneticinformation.ThehumangenomesendsinstructionsforproteinproductionfromtheDNAinthenucleustotheproteinsynthesismechanism,andtheseinstructionsaretransmittedthroughтРНК.TheyfoundawaytodegradeтРНКwithspecificgenes.InthisRNAinterferencephenomenon,double-strandedRNAinhibitsgeneexpressioninaveryclearway.Thistechnologyisusedinlaboratoriesaroundtheworldtodeterminewhichgenesplayanimportantroleinvariousdiseases.
RNAinterferenceexistsinplants,animals,andhumans,whichisofgreatsignificanceforthemanagementofgeneexpression,participationintheprotectionofviralinfections,andcontrolofactivegenes.RNAinterferenceisabiologicalprocessinwhichdouble-strandedRNAinhibitsgeneexpressioninaveryclearway.Sinceitsdiscoveryin1998,RNAinterferencehasemergedasapowerful"genesilencing"technology.RNAinterferencehasbeenwidelyusedinbasicscienceasaresearchmethodforstudyinggeneoperation,anditmayproducemorenewertreatmentmethodsinthefuture.ScientistsbelievethaтРНКinterferencetechnologyisnotonlyapowerfultoolforstudyinggenefunctions.Inthenearfuture,thistechnologymaybeusedtodirectly“silence”disease-causinggenesfromthesourcetotreatcancerandevenAIDS.Itisalsousedinagriculture.Thereisalottobedone.
функция
тРНК
тРНКcontainsfournucleotidesofA,U,G,andC,eachofwhichisconnectedtoformatriplet,namelyThecoderepresentstheinformationofanaminoacid,socalculatedaccordingtothepermutationandcombinationruleinmathematics,43=64differentcodescanbeformed.Accordingtotheexperimentalresults,thecorrespondingrelationshipbetween64codesandaminoacidsisdeducedasshowninthetablebelow.
Amongthe64codes,61codesrepresentvariousaminoacids.Thereisonlyonecodeforeachkindofaminoacid,andtherecanbe6more,but2and4arethemajority.Inaddition,thethreecodesUAA,UAG,andUGAaretheterminationsignalsforpeptidechainsynthesisanddonotrepresentanyaminoacids.Ineukaryotes,AUGisboththecodeformethionineandtheinitiationsignalforpeptidechainsynthesis.Inprokaryotes,GUG(thecodeforvalineineukaryotes)andAUGarebothItisthecodeofformylmethionineandthestartingphasenumberofpeptidechainsynthesis.Itcanbeseenthat,exceptGUG,allpasswordscanbeappliedfrombacteriatohigherorganisms,whichprovidesstrongevidenceforthetheoryofcommonoriginoforganisms.
Itmustbepointedout:⑨IntheentireтРНКmolecule,fromthestartsignaltothestopsignal,thetripletofthecodeiscontinuous,andthereisnointervalbetweenthecodeandthecode;②thestartsignalAUGisnotthestart(5'end)oftheтРНК,butcanbeseparatedfromthe5'endbyseveralnucleotides;andtheterminationsignalisnotatthe3'endoftheтРНК.
тРНК
TherearemanykindsofтРНКsas"transportationtools".The20aminoacidsinthebodyhavetheirownuniqueтРНКs.Therefore,therearenolessthan20typesofтРНКs.UndertheactionofATPtosupplyenergyandenzymes,тРНКcanbindtospecificaminoacidsrespectively.EachтРНКhasan"anti-code"madeupofthreenucleotides.Thisanti-codecanbepairedwiththecorrespondingcodeontheтРНКaccordingtotheprincipleofbasepairing,andonlywhentheanti-codecorrespondstothecodeontheтРНКcanitbematched,otherwiseitwillbe"outoffit".Therefore,duringtranslation,eachтРНКwithdifferentaminoacidscanbeaccurately"checked"ontheтРНКmolecule,andinturnconformstothecanonicalcode,whichensuresthattheaminoacidscanbearrangedinacertainorder.
Ofcourse,theanti-codeontheтРНКshouldbeabletorecognizethecorrespondingandcomplementarycodeontheтРНКandpairwithit.However,whenexperimentingwithpurifiedтРНК,itwasfoundthatoneтРНКcanrecognizeseveralcodes.Forexample,alanineтРНК,whoseanti-codeisIGC(5'>3'),canrecognizethreekindsofcodes.
рРНК
RRNAandavarietyofproteinmoleculestogetherformaribosome.Theribosomeisequivalenttoan"assemblymachine",whichcanpromotethecondensationofaminoacylgroupscarriedbyтРНКintopeptides.TheribosomeattachestotheтРНКandmovesalongthestartsignaltothestopsignalofthelongтРНКchain.AsforthespecificroleofрРНКinproteinbiosynthesis,itisunclear.