Klasifikace
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 atd. Bakterie mají také malou RNA (50~500 nt).
MessengerRNA
MessengerRNA(mRNA)wasfirstdiscoveredin1960.Itisresponsiblefortransmittinggeneticinformationanddirectlyguidingproteinsynthesisduringproteinsynthesis.Ithasthefollowingcharacteristics.
1. Nízký obsah, představující 1 % až 5 % celkové buněčné RNA.
2.Therearemanytypes,upto105species.DifferentgenesexpressdifferentmRNAs.
3.Thelifespanisshort,anddifferentmRNAguidesthesynthesisofdifferentproteins,whichwillbedegradedaftercompletingthemission.Theaveragehalf-lifeofbacterialmRNAisabout1.5minutes.Thehalf-lifeofvertebratemRNAvariesgreatly,withanaverageofabout3hours.
4.LargedifferencesinlengthmammalianmRNAlengthis5×102~1×105ntthemRNAofprokaryotesandeukaryotesalthoughTherearedifferencesinstructure,butthesamefunction,theyaretemplatesforguidingproteinsynthesis.
TransferRNA
TransferRNA(tRNA) je zodpovědná za transport aminokyselin a interpretaci genetického kódu RNA během syntézy proteinů. tRNA představuje 10 % až 15 % celkové buněčné RNA, z nichž většina je umístěna v cytoplazmě.
1.Primární struktura RNA
Má následující vlastnosti:
①Je to typ jednovláknové malé molekuly RNA, délka 73~95 nt (konsensuální sekvence76 nt), sedimentační koeficient4S.
②Je to RNA s nejzákladnějšími bázemi, obsahující 7-15 vzácných základen (připadajících na 15 % až 20 % pádových základen), umístěných v nepárové oblasti.
③5′terminální báze je softenguanin.
④Ten 3'končí CCA sekvenci, ve které se adenylová kyselina změkne nazývá A76 a její 3'-OH je vazebným místem pro aminokyseliny.
2.tRNAsekundární struktura
About50%basepairing,formingafour-segmentdoublehelix,formingaclover-shapedstructurewithfiveunpairedsequences.Therearefourarmsandfourloopsinthisstructure:
①Aminoacidarmy.
②Dihydrouracilarm (DHUarm,Darm) a dihydrouracilring (DHUring,Dring), vyznačující se tím, že obsahuje dihydrouracil (DHU,D).
③Theanticodonarmandanticodonlooparecharacterizedbythefactthattheanticodonloopcontainsanticodons.The5'endoftheanticodonisconnectedtouridineacid,andthe3'endisconnectedtopurinenucleotides.TheTΨCarm(Tarm)andtheTΨCring(Ψring)arecharacterizedbytheTΨCringcontainingthymineribonucleotideT54pseudouridineΨ55cytidineC56.
④Extraloop3~21nt.
3.Terciální struktura RNA
ItisL-shaped,withtheaminoacidbindingsiteatoneend,theanticodonloopattheotherend,andtheDHUloopAlthoughtheandTΨCringsarelocatedonbothsidesinthesecondarystructure,theyareadjacentinthetertiarystructure.AlthoughthelengthandsequenceofvarioustRNAsarenotthesame,theirtertiarystructureissimilar,suggestingthatthetertiarystructureiscloselyrelatedtoitsfunction.
Ribozomální RNA
Ribozomální RNA(rRNA)andribosomalproteinformakindofnucleoproteinparticlescalledribosomes.Thereareabout15,000ribosomesinanE.coli.
1.Složení a struktura ribozomu
Theribosomesofprokaryotesandeukaryotesarecomposedofalargesubunitandasmallsubunit.BothsubunitsarecomposedofrRNAandribosomalprotein.Thesizeofribosomes,ribosomalsubunitsandrRNAisgenerallyexpressedbythesedimentationcoefficient.
2.Ribosomální RNAcharakteristiky
(1) Vysoký obsah, rRNA je nejvyšší obsah RNA v buňce, tvoří 80 % celkové RNA buňky ~ 85 %.
(2) Dlouhá životnost, pomalá aktualizace RNA a dlouhá životnost.
(3)Therearefewtypes.Prokaryoteshave5S,16S,and23srRNAs,whichaccountfor66%ofthemassofribosomes(5Sand23SrRNAsaccountfor70%ofthelargeribosomalsubunits,and16SrRNAItaccountsfor60%ofthesmallribosomalsubunits);eukaryotesmainlyhave5S,5.8S,18S,28SrRNA,andasmallamountofmitochondrialrRNAandchloroplastrRNA.Escherichiacoli16SrRNAhasaconservedsequenceACCUCCUatthe3'end,whichcancomplementallybindtotheSDsequenceinthemRNA.5TwoconservedsequencesofSrRNAhavealsobeenidentified:
①CGAAC,whichcancomplementtheGTCGoftheTΨCloopoftRNA.
②GCGCCGAAUGGUAGU může být komplementární k sekvenci 23SrRNA.
3. Typy ribozomů
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.Objev ribozymů
RibozymeswerefirstdiscoveredbyCechandAltman(theNobelPrizewinnerinChemistryin1989).In1967,Woese,Crick,andOrgelproposedthatitmayhavecatalyticactivitybasedonthecomplexityoftheRNAsecondarystructure;in1982,Cechdiscoveredthatitsintronhasself-splicingactivitywhenstudyingthesplicingoftheTetrahymenarRNAprecursor;in1983,AltmanfoundthattheMRNAinribonucleasePisinvolvedinthepost-transcriptionalprocessingoftRNAprecursorswhenstudyingbacterialtRNAprecursors;in1982,Krugeretal.suggestedthatthecatalyticallyactiveRNAbenamed"ribozyme(ribozyme)".
2.Ribozymecharakteristika
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)Obsah mostribozymů v buňkách je extrémně nízký.
3.Význam ribozymů
①ThediscoveryandresearchofribozymeshasgivenusafurtherunderstandingofthephysiologicalfunctionsofRNA,thatis,itisbothgeneticThecarrierofinformationisalsoabiocatalyst,whichhasthefunctionsoftwotypesofbiologicalmacromolecules,DNAandprotein.
②Objev ribozymu otřásl tradičním pojetím, že všechny biokatalyzátory jsou proteiny.
③Thediscoveryofribozymesisofgreatsignificanceforunderstandingtheevolutionoflife,andRNAmaybethefirstbiologicalmacromoleculetoappear.
4. Aplikace ribozymu
①Geneterapie;②Specifická RNAdegradace;③Biosensor;④Funkční genomika;⑤Genediscovery.
Distribuce v buňkách
90%ofeukaryoticRNAisdistributedinthecytoplasm,andasmallamountisfoundinmitochondria,chloroplastsandnucleoli.
RNA prokaryotézy je distribuována v cytoplazmě.
Struktura složení
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.
Mechanismus rušení
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.
Funkce
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.