Classification
Acellinthehumanbodycontainsabout10pgofRNA(about7pgofDNA).ComparedwithDNA,RNAhasawidevarietyoftypes,smallermolecularweights,andlargechangesincontent.RNAcanbedividedintomessengerRNAandnon-codingRNAaccordingtoitsstructureandfunction.Non-codingRNAisdividedintonon-codinglargeRNAandnon-codingsmallRNA.Largenon-codingRNAincludesribosomalRNAandlongnon-codingRNA.Non-codingsmallRNAsincludetransferRNA,ribozymes,smallmoleculeRNAsandsoon.SmallRNA(20~300nt)includesmiRNA,SiRNA,piRNA,scRNA,snRNA,snoRNA,etc.BacteriaalsohavesmallRNA(50~500nt).
MessengerRNA
MessengerRNA(mRNA)wasfirstdiscoveredin1960.Itisresponsiblefortransmittinggeneticinformationanddirectlyguidingproteinsynthesisduringproteinsynthesis.Ithasthefollowingcharacteristics.
1.Lowcontent,accountingfor1%to5%oftotalcellRNA.
2.Therearemanytypes,upto105species.DifferentgenesexpressdifferentmRNAs.
3.Thelifespanisshort,anddifferentmRNAguidesthesynthesisofdifferentproteins,whichwillbedegradedaftercompletingthemission.Theaveragehalf-lifeofbacterialmRNAisabout1.5minutes.Thehalf-lifeofvertebratemRNAvariesgreatly,withanaverageofabout3hours.
4.LargedifferencesinlengthmammalianmRNAlengthis5×102~1×105ntthemRNAofprokaryotesandeukaryotesalthoughTherearedifferencesinstructure,butthesamefunction,theyaretemplatesforguidingproteinsynthesis.
TransferRNA
TransferRNA(tRNA)isresponsiblefortransportingaminoacidsandinterpretingmRNAgeneticcodeduringproteinsynthesis.tRNAaccountsfor10%to15%oftotalcellRNA,mostofwhicharelocatedinthecytoplasm.TheexistenceoftRNAwasproposedbyCrickin1955,anditwasidentifiedbyZamecnikandHoaglandin1957.
1.TheprimarystructureoftRNA
Ithasthefollowingcharacteristics:
①Itisatypeofsingle-strandedsmallmoleculeRNA,length73~95nt(consensussequence76nt),sedimentationcoefficient4S.
②ItistheRNAwiththemostrarebases,containing7-15rarebases(accountingfor15%-20%ofallbases),locatedintheunpairedarea.
③The5′terminalbaseisoftenguanine.
④The3'endistheCCAsequence,inwhichadenylicacidisoftencalledA76,andits3'-OHistheaminoacidbindingsite.
2.tRNAsecondarystructure
About50%basepairing,formingafour-segmentdoublehelix,formingaclover-shapedstructurewithfiveunpairedsequences.Therearefourarmsandfourloopsinthisstructure:
①Aminoacidarms.
②Dihydrouracilarm(DHUarm,Darm)anddihydrouracilring(DHUring,Dring),characterizedbycontainingdihydrouracil(DHU,D).
③Theanticodonarmandanticodonlooparecharacterizedbythefactthattheanticodonloopcontainsanticodons.The5'endoftheanticodonisconnectedtouridineacid,andthe3'endisconnectedtopurinenucleotides.TheTΨCarm(Tarm)andtheTΨCring(Ψring)arecharacterizedbytheTΨCringcontainingthymineribonucleotideT54pseudouridineΨ55cytidineC56.
④Extraloop3~21nt.
3.ThetertiarystructureoftRNA
ItisL-shaped,withtheaminoacidbindingsiteatoneend,theanticodonloopattheotherend,andtheDHUloopAlthoughtheandTΨCringsarelocatedonbothsidesinthesecondarystructure,theyareadjacentinthetertiarystructure.AlthoughthelengthandsequenceofvarioustRNAsarenotthesame,theirtertiarystructureissimilar,suggestingthatthetertiarystructureiscloselyrelatedtoitsfunction.
RibosomalRNA
RibosomalRNA(rRNA)andribosomalproteinformakindofnucleoproteinparticlescalledribosomes.Thereareabout15,000ribosomesinanE.coli.
1.Ribosomecompositionandstructure
Theribosomesofprokaryotesandeukaryotesarecomposedofalargesubunitandasmallsubunit.BothsubunitsarecomposedofrRNAandribosomalprotein.Thesizeofribosomes,ribosomalsubunitsandrRNAisgenerallyexpressedbythesedimentationcoefficient.
2.RibosomalRNAcharacteristics
(1)Highcontent,rRNAisthehighestcontentofRNAinthecell,accountingfor80%ofthetotalRNAofthecell~85%.
(2)Longlifespan,slowrRNAupdate,andlonglifespan.
(3)Therearefewtypes.Prokaryoteshave5S,16S,and23srRNAs,whichaccountfor66%ofthemassofribosomes(5Sand23SrRNAsaccountfor70%ofthelargeribosomalsubunits,and16SrRNAItaccountsfor60%ofthesmallribosomalsubunits);eukaryotesmainlyhave5S,5.8S,18S,28SrRNA,andasmallamountofmitochondrialrRNAandchloroplastrRNA.Escherichiacoli16SrRNAhasaconservedsequenceACCUCCUatthe3'end,whichcancomplementallybindtotheSDsequenceinthemRNA.5TwoconservedsequencesofSrRNAhavealsobeenidentified:
①CGAAC,whichcancomplementtheGTCGoftheTΨCloopoftRNA.
②GCGCCGAAUGGUAGUcanbecomplementarytoasequenceof23SrRNA.
3.Typesofribosomes
Prokaryoteshaveonlyonetypeofribosomes,whileeukaryoteshavethefollowingtypeslocatedindifferentpartsofthecell:ribosomes,Freeribosomes,endoplasmicreticulumribosomes(alsocalledattachmentribosomes),mitochondrialribosomesandchloroplastribosomes(plants).Freeribosomesandendoplasmicreticulumribosomesareactuallythesametypeofribosomes.TheyarelargerthanprokaryoticribosomesandcontainmorerRNAandprotein.Mitochondrialribosomesandchloroplastribosomesaresmallerthanprokaryoticribosomes.However,thebasicstructureandfunctionoftheseribosomesarethesame.
Ribozyme
WhenscientistsarestudyingRNApost-transcriptionalprocessing,theyhavefoundthatcertainRNAshavecatalyticactivityandcancatalyzethesplicingofRNA.TheseRNAsaresynthesizedbylivingcellsandplayacatalyticrole.Knownasribozymes.ThesubstrateofmanyribozymesisalsoRNA,evenitsown,anditscatalyticreactionisalsospecific.
Thenaturalribozymesthathavebeenelucidatedincludehammerheadribozyme,hairpinribozyme,typeIintron,typeIIintron,hepatitisDvirusribozyme,ribonucleaseP,Peptidyltransferaseandsoon.Howtoevaluatethetheoreticalandpracticalsignificanceofribozymes,andhowtotreatthestatusofribozymesandtraditionalenzymesinmetabolism,allneedtobefurtherstudied.
1.Discoveryofribozymes
RibozymeswerefirstdiscoveredbyCechandAltman(theNobelPrizewinnerinChemistryin1989).In1967,Woese,Crick,andOrgelproposedthatitmayhavecatalyticactivitybasedonthecomplexityoftheRNAsecondarystructure;in1982,Cechdiscoveredthatitsintronhasself-splicingactivitywhenstudyingthesplicingoftheTetrahymenarRNAprecursor;in1983,AltmanfoundthattheMRNAinribonucleasePisinvolvedinthepost-transcriptionalprocessingoftRNAprecursorswhenstudyingbacterialtRNAprecursors;in1982,Krugeretal.suggestedthatthecatalyticallyactiveRNAbenamed"ribozyme(ribozyme)".
2.Ribozymecharacteristics
Thevariousribozymesdiscoveredsofarhavethefollowingcharacteristics.
(1)ThechemicalnatureofribozymesisRNAorRNAfragments.Someribonucleoproteinsalsohaveacatalyticeffect,buttheactivecenterislocatedontheirproteincomponentsandisnotaribozyme,suchastelomerase.However,iftheRNAoftheribonucleoproteincontainsanactivecenter,theRNAcomponentistheribozyme,suchastheM1RNAintheribonucleasePmolecule.
(2)Ribozymeshaverelativelyfewtypesofsubstrates,mostofwhichareself-RNAorotherRNAmolecules.Therefore,theyaredividedintotwotypes:autocatalysisandheterocatalysis.Inaddition,thereareothersubstrates.Forexample,thesubstratesofpeptidyltransferaseareaminoacyltRNAandpeptidyltRNA.
(3)Thecatalyticefficiencyofribozymesismuchlowerthanthatofenzymes.
(4)Ribozymesarealsospecific.Forexample,M1RNAonlycutstheextranucleotidesatthe5'endoftheRNAprecursor,butdoesnotcuttheextranucleotidesandothersequencesatthe3'end.
(5)Thereactionscatalyzedbyribozymesareirreversible.
(6)Mg2+isrequiredfortheribozymetocatalyzethereaction.Mg3+notonlymaintainstheactiveconformationoftheribozyme,butalsoparticipatesinthecatalyticreaction.
(7)Thecontentofmostribozymesincellsisextremelylow.
3.Significanceofribozymes
①ThediscoveryandresearchofribozymeshasgivenusafurtherunderstandingofthephysiologicalfunctionsofRNA,thatis,itisbothgeneticThecarrierofinformationisalsoabiocatalyst,whichhasthefunctionsoftwotypesofbiologicalmacromolecules,DNAandprotein.
②Thediscoveryofribozymeshasshakenthetraditionalconceptthatallbiocatalystsareproteins.
③Thediscoveryofribozymesisofgreatsignificanceforunderstandingtheevolutionoflife,andRNAmaybethefirstbiologicalmacromoleculetoappear.
4.Ribozymeapplication
①Genetherapy;②SpecificRNAdegradation;③Biosensor;④Functionalgenomics;⑤Genediscovery.
Distributionincells
90%ofeukaryoticRNAisdistributedinthecytoplasm,andasmallamountisfoundinmitochondria,chloroplastsandnucleoli.
TheRNAofprokaryotesisdistributedinthecytoplasm.
Compositionstructure
LikeDNA,RNAisalsoapolynucleotidechaincomposedofvariousnucleotidesconnectedby3′,5′-phosphodiesterbonds,butwithDNAThereareaseriesofdifferences.
1.Intermsofchemicalcomposition,RNAcontainsribosebutnotdeoxyribose.Containsuracilbutdoesnotcontainthymidine.TheexceptionisthateachtNAmoleculecontainsathymine,whichismethylatedbyuracilaftertheRNAstrandissynthesized.Inaddition,asmentionedearlier,asmallnumberofDNAcontainsasmallamountofribose,buttheseindividualexceptionscannotbeusedThisnegatesthedifferenceinthecompositionofthetwotypesofnucleicacids.
2.AlthoughtheconceptofRNAprimarystructureisthesameasDNA.Butitsbasicstructuralunitisribonucleotideinsteadofdeoxyribonucleotide.Inaddition,partofRNAhasaspecialnucleotidesequenceatthe5'endor3'end,andthereisnocomplicatedsequenceorganizationlikeDNAintheRNAprimarystructure.
3.MostRNAsaresingle-strandedmolecules,whichcanfoldthemselvestoformahairpin-likestructureandhavethecharacteristicsofalocaldoublehelixstructure.ThisisthecommonfeatureofvariousRANspatialstructures.feature.TheruleofbasecomplementarypairinginthelocaldoublehelixstructureofRNAisAtoUandGtoC.SincethebasepairingcannotbefullyformedinsidetheRNAmolecule,thebasemolarratioAisnotequaltoU,GisnotequaltoC,andthereisnoChargafflawofDNAbaseratio.
Mechanismofinterference
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,andtheseinstructionsaretransmittedthroughmRNA.TheyfoundawaytodegrademRNAwithspecificgenes.InthisRNAinterferencephenomenon,double-strandedRNAinhibitsgeneexpressioninaveryclearway.Thistechnologyisusedinlaboratoriesaroundtheworldtodeterminewhichgenesplayanimportantroleinvariousdiseases.
RNAinterferenceexistsinplants,animals,andhumans,whichisofgreatsignificanceforthemanagementofgeneexpression,participationintheprotectionofviralinfections,andcontrolofactivegenes.RNAinterferenceisabiologicalprocessinwhichdouble-strandedRNAinhibitsgeneexpressioninaveryclearway.Sinceitsdiscoveryin1998,RNAinterferencehasemergedasapowerful"genesilencing"technology.RNAinterferencehasbeenwidelyusedinbasicscienceasaresearchmethodforstudyinggeneoperation,anditmayproducemorenewertreatmentmethodsinthefuture.ScientistsbelievethatRNAinterferencetechnologyisnotonlyapowerfultoolforstudyinggenefunctions.Inthenearfuture,thistechnologymaybeusedtodirectly“silence”disease-causinggenesfromthesourcetotreatcancerandevenAIDS.Itisalsousedinagriculture.Thereisalottobedone.
Function
mRNA
mRNAcontainsfournucleotidesofA,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:⑨IntheentiremRNAmolecule,fromthestartsignaltothestopsignal,thetripletofthecodeiscontinuous,andthereisnointervalbetweenthecodeandthecode;②thestartsignalAUGisnotthestart(5'end)ofthemRNA,butcanbeseparatedfromthe5'endbyseveralnucleotides;andtheterminationsignalisnotatthe3'endofthemRNA.
tRNA
TherearemanykindsoftRNAsas"transportationtools".The20aminoacidsinthebodyhavetheirownuniquetRNAs.Therefore,therearenolessthan20typesoftRNAs.UndertheactionofATPtosupplyenergyandenzymes,tRNAcanbindtospecificaminoacidsrespectively.EachtRNAhasan"anti-code"madeupofthreenucleotides.Thisanti-codecanbepairedwiththecorrespondingcodeonthemRNAaccordingtotheprincipleofbasepairing,andonlywhentheanti-codecorrespondstothecodeonthemRNAcanitbematched,otherwiseitwillbe"outoffit".Therefore,duringtranslation,eachtRNAwithdifferentaminoacidscanbeaccurately"checked"onthemRNAmolecule,andinturnconformstothecanonicalcode,whichensuresthattheaminoacidscanbearrangedinacertainorder.
Ofcourse,theanti-codeonthetRNAshouldbeabletorecognizethecorrespondingandcomplementarycodeonthemRNAandpairwithit.However,whenexperimentingwithpurifiedtRNA,itwasfoundthatonetRNAcanrecognizeseveralcodes.Forexample,alaninetRNA,whoseanti-codeisIGC(5'>3'),canrecognizethreekindsofcodes.
rRNA
RRNAandavarietyofproteinmoleculestogetherformaribosome.Theribosomeisequivalenttoan"assemblymachine",whichcanpromotethecondensationofaminoacylgroupscarriedbytRNAintopeptides.TheribosomeattachestothemRNAandmovesalongthestartsignaltothestopsignalofthelongmRNAchain.AsforthespecificroleofrRNAinproteinbiosynthesis,itisunclear.