Gene-X-ppt(Gene10-基因十)--Chapter07

Chapter7ClustersandRepeats7.1Introduction•genefamily–AsetofgeneswithinagenomethatcodeforrelatedoridenticalproteinsorRNAs.–Thememberswerederivedbyduplicationofanancestralgenefollowedbyaccumulationofchangesinsequencebetweenthecopies.–Mostoftenthemembersarerelatedbutnotidentical.7.1Introduction•pseudogenes–Inactivebutstablecomponentsofthegenomederivedbymutationofanancestralactivegene.–Usuallytheyareinactivebecauseofmutationsthatblocktranscriptionortranslationorboth.•genecluster–Agroupofadjacentgenesthatareidenticalorrelated.7.1IntroductionFIGURE01:Chiasmaformationrepresentsthegenerationofrecombinants7.1IntroductionFIGURE02:RecombinationinvolvespairingbetweencomplementarystrandsofthetwoparentalduplexDNAs7.1Introduction•unequalcrossing-over(nonreciprocalrecombination)–Unequalcrossing-overresultsfromanerrorinpairingandcrossing-overinwhichnonequivalentsitesareinvolvedinarecombinationevent.–Itproducesonerecombinantwithadeletionofmaterialandonewithaduplication.FIGURE03:Unequalcrossing-overresultsfrompairingbetweennonequivalentrepeatsinregionsofDNAconsistingofrepeatingunits7.1Introduction•satelliteDNA–DNAthatconsistsofmanytandemrepeats(identicalorrelated)ofashortbasicrepeatingunit.7.1Introduction•minisatellite–DNAsconsistingoftandemlyrepeatedcopiesofashortrepeatingsequence,withmorerepeatcopiesthanamicrosatellitebutfewerthanasatellite.–Thelengthoftherepeatingunitismeasuredintensofbasepairs.–Thenumberofrepeatsvariesbetweenindividualgenomes.7.2UnequalCrossing-overRearrangesGeneClusters•Whenagenomecontainsaclusterofgeneswithrelatedsequences,mispairingbetweennonalleliclocicancauseunequalcrossing-over.–Thisproducesadeletioninonerecombinantchromosomeandacorrespondingduplicationintheother.FIGURE04:Genenumbercanbechangedbyunequalcrossing-over7.2UnequalCrossing-overRearrangesGeneClusters•Differentthalassemiasarecausedbyvariousdeletionsthateliminateα-orβ-globingenes.–Theseverityofthediseasedependsontheindividualdeletion.FIGURE05:athalassemiasresultfromvariousdeletionsinthea-globingenecluster7.2UnequalCrossing-overRearrangesGeneClustersFIGURE06:Deletionsintheb-globingeneclustercauseseveraltypesofthalassemia7.2UnequalCrossing-overRearrangesGeneClusters•HbHdisease–Aconditioninwhichthereisadisproportionateamountoftheabnormaltetramerβ4relativetotheamountofnormalhemoglobin(α2β2).•hydropsfetalis–Afataldiseaseresultingfromtheabsenceofthehemoglobinαgene.7.2UnequalCrossing-overRearrangesGeneClusters•HbLepore–Anunusualglobinproteinthatresultsfromunequalcrossing-overbetweentheβandδgenes.–Thegenesbecomefusedtogethertoproduceasingleβ-likechainthatconsistsoftheN-terminalsequenceofδjoinedtotheC-terminalsequenceofβ.7.2UnequalCrossing-overRearrangesGeneClusters•Hbanti-Lepore–Afusiongeneproducedbyunequalcrossing-overthathastheN-terminalpartofβglobinandtheC-terminalpartofδglobin.•HbKenya–Afusiongeneproducedbyunequalcrossing-overbetweentheAγ-andβ-globingenes.7.3GenesforrRNAFormTandemRepeatsIncludinganInvariantTranscriptionUnit•RibosomalRNAiscodedbyalargenumberofidenticalgenesthataretandemlyrepeatedtoformoneormoreclusters.•EachrDNAclusterisorganizedsothattranscriptionunitsgivingajointprecursortothemajorrRNAsalternatewithnontranscribedspacers.•ThegenesinanrDNAclusterallhaveanidenticalsequence.7.3GenesforrRNAFormTandemRepeatsIncludinganInvariantTranscriptionUnit•Thenontranscribedspacersconsistofshorterrepeatingunitswhosenumbervariessothatthelengthsofindividualspacersaredifferent.FIGURE07:Atandemgeneclusterhasanalternationoftranscriptionunitandnontranscribedspacerandgeneratesacircularrestrictionmap7.3GenesforrRNAFormTandemRepeatsIncludinganInvariantTranscriptionUnit•nucleolus–Adiscreteregionofthenucleuswhereribosomesareproduced.•nucleolarorganizer–TheregionofachromosomecarryinggenescodingforrRNA.7.3GenesforrRNAFormTandemRepeatsIncludinganInvariantTranscriptionUnit•Bamislands–AseriesofshortrepeatedsequencesfoundinthenontranscribedspacerofXenopusrDNAgenes.FIGURE10:ThenontranscribedspacerofX.laevisrDNAhasaninternallyrepetitiousstructurethatisresponsibleforitsvariationinlength7.4CrossoverFixationCouldMaintainIdenticalRepeats•Unequalcrossing-overchangesthesizeofaclusteroftandemrepeats.•Individualrepeatingunitscanbeeliminatedorcanspreadthroughthecluster.•concertedevolution(coincidentalevolution)–Theabilityoftwoormorerelatedgenestoevolvetogetherasthoughconstitutingasinglelocus.7.4CrossoverFixationCouldMaintainIdenticalRepeats•geneconversion–ThealterationofonestrandofaheteroduplexDNAtomakeitcomplementarywiththeotherstrandatanyposition(s)wherethereweremispairedbases.•crossoverfixation–Apossibleconsequenceofunequalcrossing-overthatallowsamutationinonememberofatandemclustertospreadthroughthewholecluster(ortobeeliminated).7.4CrossoverFixationCouldMaintainIdenticalRepeatsFIGURE11:Unequalrecombinationallowsoneparticularrepeatingunittooccupytheentirecluster7.5SatelliteDNAsOftenLieinHeterochromatin•HighlyrepetitiveDNA(orsatelliteDNA)hasaveryshortrepeatingsequenceandnocodingfunction.•simplesequenceDNA–ShortrepeatingunitsofDNAsequence.•SatelliteDNAoccursinlargeblockstha