基因簇和基因重复.ppt

Part1 GenesandgenomesChapter4Clustersandrepeats基因簇和重复序列 Agenefamilyconsistsofasetofgeneswhoseexonsarerelated thememberswerederivedbyduplicationandvariationfromsomeancestralgene Atranslocationisarearrangementinwhichpartofachromosomeisdetachedbybreakageoraberrantrecombinationandthenbecomesattachedtosomeotherchromosome Ageneclusterisagroupofadjacentgenesthatareidenticalorrelated 4 1Introduction Nonreciprocalrecombination不等交换 unequalcrossing over resultsfromanerrorinpairingandcrossing overinwhichnonequivalentsitesareinvolvedinarecombinationevent Itproducesonerecombinantwithadeletionofmaterialandonewithaduplicatio造成一个缺失一个重复 SatelliteDNA Simple sequenceDNA 卫星DNAconsistsofmanytandemrepeats identicalorrelated ofashortbasicrepeatingunit MinisatelliteDNAs小卫星DNAconsistof 10copiesofashortrepeatingsequence thelengthoftherepeatingunitismeasuredin10sofbasepairs Thenumberofrepeatsvariesbetweenindividualgenomes 4 1Introduction Figure6 1Unequalcrossing overresultsfrompairingbetweennon equivalentrepeatsinregionsofDNAconsistingofrepeatingunits 4 1Introduction 4 2Geneduplicationisamajorforceinevolution Figure6 2Afteragenehasbeenduplicated differencesmayaccumulatebetweenthecopies Thegenesmayacquiredifferentfunctionsoroneofthecopiesmaybecomeinactive Duplicatedgenesmaydivergetogeneratedifferentgenesoronecopymaybecomeinactive 4 3Globinclustersareformedbyduplicationanddivergence Figure6 3Eachofthe likeand likeglobingenefamiliesisorganizedintoasingleclusterthatincludesfunctionalgenesandpseudogenes Thebestcharacterizedexampleofageneclusterispresentedbytheglobin球蛋白genes whichconstituteanancientgenefamily concernedwithafunctionthatiscentraltotheanimalkingdom thetransportofoxygenthroughthebloodstream 4 3Globinclustersareformedbyduplicationanddivergence Figure6 4Differenthemoglobingenesareexpressedduringembyonic fetal andadultperiodsofhumandevelopment Figure6 5Clustersof globingenesandpseudogenesarefoundinvertebrates Sevenmousegenesinclude2earlyembryonic 1lateembryonic 2adultgenes and2pseudogenes Rabbitandchickeachhavefourgenes 4 3Globinclustersareformedbyduplicationanddivergence Figure6 6Allglobingeneshaveevolvedbyaseriesofduplications transpositions andmutationsfromasingleancestralgene 4 3Globinclustersareformedbyduplicationanddivergence DivergenceisthepercentdifferenceinnucleotidesequencebetweentworelatedDNAsequencesorinaminoacidsequencesbetweentwoproteins Theevolutionaryclockisdefinedbytherateatwhichmutationsaccumulateinagivengene 4 4Sequencedivergenceisthebasisfortheevolutionaryclock 4 6Pseudogenesaredeadendsofevolution Pseudogeneshavenocodingfunction buttheycanberecognizedbysequencesimilaritieswithexistingfunctionalgenes Theyarisebytheaccumulationofmutationsinfunctionalgenes 4 7Unequalcrossing overrearrangesgeneclusters Figure6 10Genenumbercanbechangedbyunequalcrossing over Ifgene1ofonechromosomepairswithgene2oftheotherchromosome theothergenecopiesareexcludedfrompairing Recombinationbetweenthemispairedgenesproducesonechromosomewithasinglecopyofthegeneandonechromosomewiththreecopiesofthegene 4 7Unequalcrossing overrearrangesgeneclusters Thalassemia地中海贫血isdiseaseofredbloodcellsresultingfromlackofeither or globin Figure6 11Thalassemiasresultfromvariousdeletionsinthe globingenecluster RibosomalRNAisthepredominantproductoftranscription constitutingsome80 90 ofthetotalmassofcellularRNAinbotheukaryotesandprokaryotes ThenumberofmajorrRNAgenesvariesfrom7inE coli 100 200inlowereukaryotes toseveralhundredinhighereukaryotes Inbacteria themultiplerRNAgenepairsaredispersed Inmosteukaryoticnuclei therRNAgenesarecontainedinatandemclusterorclusters SometimestheseregionsarecalledrDNA 4 8GenesforrRNAformtandemrepeats Figure6 12Atandemgeneclusterhasanalternationoftranscriptionunitandnontranscribedspacerandgeneratesacircularrestrictionmap 4 8GenesforrRNAformtandemrepeats ThepairofmajorrRNAsistranscribedasasingleprecursorinbothbacteriaandeukaryoticnuclei Followingtranscription theprecursoriscleavedtoreleasetheindividualrRNAmolecules Figure6 14TranscriptionofrDNAclustersgeneratesaseriesofmatrices eachcorrespondingtoonetranscriptionunitandseparatedfromthenextbythenontranscribedspacer AnrDNAclustercontainsmanytranscriptionunits eachseparatedfromthenextbyanontranscribedspacer 4 8GenesforrRNAformtandemrepeats 4 9SatelliteDNAs卫星oftenlieinheterochromatin异染色质 Figure6 17MouseDNAisseparatedintoamainbandandasatellitebycentrifugationthroughadensitygradientofCsCl HighlyrepetitiveDNAconsistsofveryshortsequencesrepeatedmanytimesintandem串inlargeclusters Becauseofitsshortrepeatingunit itissometimesdescribedassimplesequenceDNA ThetermsatelliteDNAisessentiallysynonymouswithsimplesequenceDNA 4 9SatelliteDNAsoftenlieinheterochromatin Figure6 18CytologicalhybridizationshowsthatmousesatelliteDNAislocatedatthecentromeres SatelliteDNAsareoftenfoundinregionsofconstitutiveheterochromatin Heterochromatinisthetermusedtodescriberegionsofchromosomesthataretightlycoiledupandinert高度螺旋化 incontrastwiththeeuchromatin常染色质thatrepresentsmostofthegenome 4 10Arthropodsatelliteshaveveryshortidenticalrepeats Figure6 19SatelliteDNAsofD virilisarerelated Morethan95 ofeachsatelliteconsistsofatandemrepetitionofthepredominantsequence Inthearthropods astypifiedbyinsectsandcrabs eachsatelliteDNAappearstoberatherhomogeneous Usually asingleveryshortrepeatingunitaccountsfor 90 ofthesatellite 4 11Mammaliansatellitesconsistofhierarchicalrepeats Figure6 21Thealignmentofquarter repeatsidentifieshomologiesbetweenthefirstandsecondhalfofeachhalf repeat Positionsthatarethesameinall4quarter repeatsareshownincolor identitiesthatextendonlythrough3quarter repeatsareindicatedbygreylettersinthepinkarea 4 11Mammaliansatellitesconsistofhierarchicalrepeats Figure6 22Thealignmentofeighth repeatsshowsthateachquarter repeatconsistsofan anda half Theconsensussequencegivesthemostcommonbaseateachposition The ancestral sequenceshowsasequenceverycloselyrelatedtotheconsensussequence whichcouldhavebeenthepredecessortothe and units 4 11Mammaliansatellitesconsistofhierarchicalrepeats Figure6 23Theexistenceofanoverallconsensussequenceisshownbywritingthesatellitesequenceintermsofa9bprepeat 4 11Mammaliansatellitesconsistofhierarchicalrepeats 4 12Minisatellitesareusefulforgeneticmapping Thenamemicrosatelliteisusuallyusedwhenthelengthoftherepeatingunitis 10bp andthenameminisatelliteisusedwhenthelengthoftherepeatingunitis 10 100bp buttheterminologyisnotpreciselydefined ThesetypesofsequencesarealsocalledVNTR variablenumbertandemrepeat regions 4 12Minisatellitesareusefulforgeneticmapping Figure6 24Allelesmaydifferinthenumberofrepeatsataminisatellitelocus sothatcleavageoneithersidegeneratesrestrictionfragmentsthatdifferin