Gene-X-ppt-(Gene10-基因十)--Chapter02

Chapter2GenesCodeforProteins2.1Introduction•allele–Oneofseveralalternativeformsofageneoccupyingagivenlocusonachromosome.•locus–Thepositiononachromosomeatwhichthegeneforaparticulartraitresides;itmaybeoccupiedbyanyoneoftheallelesforthegene.•geneticrecombination–AprocessbywhichseparateDNAmoleculesarejoinedintoasinglemolecule,duetosuchprocessesascrossing-overortransposition.FIGURE01:EachchromosomehasasinglelongmoleculeofDNAwithinwhicharethesequencesofindividualgenes2.2AGeneCodesforaSinglePolypeptide•Theonegene:oneenzymehypothesissummarizesthebasisofmoderngenetics:thatageneisastretchofDNAcodingforoneormoreisoformsofasinglepolypeptide.•onegene:onepolypeptidehypothesis–Amodifiedversionofthenotgenerallycorrectonegene:oneenzymehypothesis;thehypothesisthatageneisresponsiblefortheproductionofasinglepolypeptide.2.2AGeneCodesforaSinglePolypeptide•heteromultimer–Amolecularcomplex(suchasaprotein)composedofdifferentsubunits.•homomultimer–Amolecularcomplex(suchasaprotein)inwhichthesubunitsareidentical.•Somegenesdonotencodepolypeptides,butencodestructuralorregulatoryRNAs.2.2AGeneCodesforaSinglePolypeptide•Mostmutationsdamagegenefunctionandarerecessivetothewild-typeallele.•Amutationinageneaffectsonlytheproduct(proteinorRNA)codedbythemutantcopyofthegeneanddoesnotaffecttheproductcodedbyanyotherallele.FIGURE02:Genescodeforproteins;dominanceisexplainedbythepropertiesofmutantproteins2.3MutationsintheSameGeneCannotComplement•complementationtest=Atestthatdetermineswhethertwomutationsareallelesofthesamegene.•Crosstwodifferentrecessivemutationsthathavethesamephenotype•Determinewhetherthewild-typephenotypecanbeproducedFIGURE03:Thecistronisdefinedbythecomplementationtest2.3MutationsintheSameGeneCannotComplement•Failureoftwomutationstocomplement(producewildphenotype)whentheyarepresentintransconfigurationinaheterozygotemeansthattheyarepartofthesamegene.•cistron–Thegeneticunitdefinedbythecomplementationtest;itisequivalenttoagene.2.4MutationsMayCauseLoss-of-FunctionorGain-of-Function•Recessivemutationsareduetoloss-of-functionbytheproteinproduct.•Dominantmutationsresultfromagain-of-function.•Testingwhetherageneisessentialrequiresanullmutation(onethatcompletelyeliminatesitsfunction).FIGURE04:Mutationsthatdonotaffectproteinsequenceorfunctionaresilent2.4MutationsMayCauseLoss-of-FunctionorGain-of-Function•Silentmutationshavenoeffect,eitherbecausethebasechangedoesnotchangethesequenceoramountofprotein,orbecausethechangeinproteinsequencehasnoeffect.•neutralsubstitutions–Substitutionsinaproteinthatcausechangesinaminoacidsthatdonotaffectactivity.2.5ALocusMayHaveManyDifferentMutantAlleles•Theexistenceofmultipleallelesallowsheterozygotesthatrepresentanypairwisecombinationofallelestoexist.FIGURE05:Thewlocushasanextensiveseriesofalleleswhosephenotypesextendfromwild-type(red)colortocompletelackofpigment2.6ALocusMayHaveMoreThanOneWild-typeAllele•Alocusmayhaveapolymorphicdistributionofalleleswithnoindividualallelethatcanbeconsideredtobethesolewild-type.FIGURE06:TheABObloodgrouplocuscodesforagalactosyltransferasewhosespecificitydeterminesthebloodgroup2.7RecombinationOccursbyPhysicalExchangeofDNA•Recombinationistheresultofcrossing-overthatoccursatchiasmata(sing.chiasma)andinvolvestwoofthefourchromatids.FIGURE07:Chiasmaformationisresponsibleforgeneratingrecombinants2.7RecombinationOccursbyPhysicalExchangeofDNA•RecombinationoccursbyabreakageandreunionthatproceedsviaanintermediateofhybridDNAthatdependsonthecomplementarityofthetwostrandsofDNA.FIGURE08:RecombinationinvolvespairingbetweencomplementarystrandsofthetwoparentalduplexDNAs2.7RecombinationOccursbyPhysicalExchangeofDNA•Thefrequencyofrecombinationbetweentwogenesisproportionaltotheirphysicaldistance.–Recombinationbetweengenesthatareverycloselylinkedisrare.FIGURE09:Recombinationbetweengenesdependsontheirdistanceapart2.7RecombinationOccursbyPhysicalExchangeofDNA•Forgenesthatareveryfarapartonasinglechromosome,thefrequencyofrecombinationisnotproportionaltotheirphysicaldistancebecauserecombinationhappenssofrequently.2.8TheGeneticCodeIsTriplet•Thegeneticcodeisreadintripletnucleotidescalledcodons.•Thetripletsarenonoverlappingandarereadfromafixedstartingpoint.•acridines–MutagensthatactonDNAtocausetheinsertionordeletionofasinglebasepair.–Theywereusefulindefiningthetripletnatureofthegeneticcode.2.8TheGeneticCodeIsTriplet•Mutationsthatinsertordeleteindividualbasescauseaframeshiftinthetripletsetsafterthesiteofmutation.•Combinationsofmutationsthattogetherinsertordeletethreebases(ormultiplesofthree)insertordeleteaminoacids,butdonotchangethereadingofthetripletsbeyondthelastsiteofmutation.FIGURE10:Frameshiftmutationsshowthatthegeneticcodeisreadintripletsfromafixedstartingpoint2.9EverySequenceHasThreePossibleReadingFrames•Ingeneral,onlyonereadingframeistranslated,andtheothertwoareblockedbyfrequentterminationsignals.•openreadingframe(ORF)–AsequenceofDNAconsistingoftripletsthatcanbetranslatedintoaminoacidsstartingwithaninitiationcodonandendingwithaterminationcodon.FIGURE11:AnopenreadingframestartswithAUGandcontinuesintripletst