《分子生物学》双语-名词解释

《分子生物学》名词解释Thegenomeisthecompletesetofsequencesinthegeneticmaterialofanorganism.ItincludesthesequenceofeachchromosomeplusanyDNAinorganelles.Agene(cistron)isthesegmentofDNAspecifyingproductionofapolypeptidechain;itincludesregionsprecedingandfollowingthecodingregion(leaderandtrailer)aswellasinterveningsequences(introns)betweenindividualcodingsegments(exons).Semiconservativereplicationisaccomplishedbyseparationofthestrandsofaparentalduplex,eachthenactingasatemplateforsynthesisofacomplementarystrand.Areplicationfork(Growingpoint)isthepointatwhichstrandsofparentalduplexDNAareseparatedsothatreplicationcanproceed.AcomplexofproteinsincludingDNApolymeraseisfoundatthefork.ADNApolymeraseisanenzymethatsynthesizesadaughterstrand(s)ofDNA(underdirectionfromaDNAtemplate).Anyparticularenzymemaybeinvolvedinrepairorreplication(orboth).RNApolymerasesareenzymesthatsynthesizeRNAusingaDNAtemplate(formallydescribedasDNA-dependentRNApolymerases).Adeoxyribonuclease(DNAase)isanenzymethatattacksbondsinDNA.Itmaycutonlyonestrandorbothstrands.Ribonucleases(RNAase)areenzymesthatcleaveRNA.Theymaybespecificforsingle-strandedorfordouble-strandedRNA,andmaybeeitherendonucleasesorexonucleases.Exonucleasescleavenucleotidesoneatatimefromtheendofapolynucleotidechain;theymaybespecificforeitherthe5or3endofDNAorRNA.Endonucleasescleavebondswithinanucleicacidchain;theymaybespecificforRNAorforsingle-strandedordouble-strandedDNA.Denaturationofproteindescribesitsconversionfromthephysiologicalconformationtosomeother(inactive)conformation.RenaturationdescribesthereassociationofdenaturedcomplementarysinglestrandsofaDNAdoublehelix.AnnealingofDNAdescribestherenaturationofaduplexstructurefromsinglestrandsthatwereobtainedbydenaturingduplexDNA.HybridizationdescribesthepairingofcomplementaryRNAandDNAstrandstogiveanRNA-DNAhybrid.TheTmisthemidpointofthetemperaturerangefordenaturation.Spontaneousmutationsoccurintheabsenceofanyaddedreagenttoincreasethemutationrate,astheresultoferrorsinreplication(orothereventsinvolvedinthereproductionofDNA)orbyenvironmentaldamage.Thebackgroundlevelofmutationdescribestherateatwhichsequencechangesaccumulateinthegenomeofanorganism.Itreflectsthebalancebetweentheoccurrenceofspontaneousmutationsandtheirremovalbyrepairsystems,andischaracteristicforanyspecies.MutagensincreasetherateofmutationbyinducingchangesinDNAsequence,directlyorindirectly.Inducedmutationsresultfromtheactionofamutagen.ThemutagenmayactdirectlyonthebasesinDNAoritmayactindirectlytotriggerapathwaythatleadstoachangeinDNAsequence.AbortiveinitiationdescribesaprocessinwhichRNApolymerasestartstranscriptionbutterminatesbeforeithasleftthepromoter.Itthenreinitiates.Severalcyclesmayoccurbeforetheelongationstagebegins.ApointmutationisachangeinthesequenceofDNAinvolvingasinglebasepair.Atransitionisamutationinwhichonepyrimidineisreplacedbytheotherand/orinwhichonepurineisreplacedbytheother.Atransversionisamutationinwhichapurineisreplacedbyapyrimidineorviceversa.BasemispairingisacouplingbetweentwobasesthatdoesnotconformtotheWatson-Crickrule,e.g.,adeninewithcytosine,thyminewithguanine.AninsertionistheadditionofastretchofbasepairsinDNA.Duplicationsareaspecialclassofinsertions.Atransposon(transposableelement)isaDNAsequenceabletoinsertitself(oracopyofitself)atanewlocationinthegenome,withouthavinganysequencerelationshipwiththetargetlocus.AdeletionistheremovalofasequenceofDNA,theregionsoneithersidebeingjoinedtogetherexceptinthecaseofaterminaldeletionattheendofachromosome.Revertantsarederivedbyreversionofamutantcellororganismtothewild-typephenotype.Forwardmutationsinactivateawild-typegene.Abackmutationreversestheeffectofamutationthathadinactivatedagene;thusitrestoreswildtype.AtruereversionisamutationthatrestorestheoriginalsequenceoftheDNA.Second-sitereversionoccurswhenasecondmutationsuppressestheeffectofafirstmutation.SuppressionoccurswhenasecondeventeliminatestheeffectsofamutationwithoutreversingtheoriginalchangeinDNA.Asuppressorisasecondmutationthatcompensatesfororalterstheeffectsofaprimarymutation.Theonegene:oneenzymehypothesissummarizesthebasisofmoderngenetics:thatageneisastretchofDNAcodingforasinglepolypeptidechain.Frameshiftmutationsarisebydeletionsorinsertionsthatarenotamultipleof3basepairsandchangetheframeinwhichtripletsaretranslatedintoprotein.Thetermisinappropriateoutsideofcodingsequences.Areadingframeisoneofthethreepossiblewaysofreadinganucleotidesequence.Eachreadingframedividesthesequenceintoaseriesofsuccessivetriplets.Therearethreepossiblereadingframesinanysequence,dependingonthestartingpoint.Ifthefirstframestartsatposition1,thesecondframestartsatposition2,andthethirdframestartsatposition3.Anopenreadingframe(ORF)isasequenceofDNAconsistingoftripletsthatcanbetranslatedintoaminoacidsstartingwithaninitiationcodonandendingwithaterminationcodon.ProcessingofRNAdescribeschangesthatoccurafteritstranscription,includingmodificationofthe5and3ends,internalmethylation,splicing,orcleavage.RNAsplicingistheprocessofexcisingthesequencesinRNAthatcorrespondtointrons,sothatthesequencescorrespondingtoexonsa