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假基因类似于基因但不表达的DNA序列.ppt

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Chapter4Geneclustersareformedbyduplicationanddivergence Sequencedivergenceisthebasisfortheevolutionaryclock Pseudogenesaredeadendsofevolution Unequalcrossing-overrearrangesgeneclusters GenesforrRNAformtandemrepeats (TherepeatedgenesforrRNAmaintainconstantsequence) Crossoverfixationcouldmaintainidenticalrepeats SatelliteDNAsoftenlieinheterochromatin Arthropodsatelliteshaveveryshortidenticalrepeats Mammaliansatellitesconsistofhierarchicalrepeats MinisatellitesareusefulforgeneticmappingGeneclusterisagroupofadjacentgenesthatareidenticalorrelated.Genefamilyconsistsofasetofgeneswhoseexonsarerelated;thememberswerederivedbyduplicationandvariationfromsomeancestralgene.SatelliteDNAconsistsofmanytandemrepeats(identicalorrelated)ofashortbasicrepeatingunit.Unequalcrossing-overdescribesarecombinationeventinwhichthetworecombiningsiteslieatnonidenticallocationsinthetwoparentalDNAmolecules.Figure1.23Chiasmaformationisresponsibleforgeneratingrecombinants.Figure1.24RecombinationinvolvespairingbetweencomplementarystrandsofthetwoparentalduplexDNAs.Pseudogenesareinactivebutstablecomponentsofthegenomederivedbymutationofanancestralactivegene.假基因：类似于基因但不表达的DNA序列。不表现任何功能，是基因的退化形式。假基因在基因组中形成稳定的和无活性的拷贝，由活化的原始基因突变而来，这是因为存在着在某个阶段伤及基因表达的一种或多种缺陷（入启动子错误、有缺陷的剪接信号、框架中有终止信号等）之故。一旦不能产生正常的基因产物，就失去了对发生进一步突变的选择性屏障作用，因此典型的假基因都有很多缺陷。某些假基因有3‘-多聚A尾巴及准确地切掉了内含子，因而与mRNA类似，被认为是源自插入基因组的逆转录体（可能由某些病毒携带）。Figure2.13Allfunctionalglobingeneshaveaninterruptedstructurewiththreeexons.Thelengthsindicatedinthefigureapplytothemammalianb-globingenes.Figure4.1Eachofthea-likeandb-likeglobingenefamiliesisorganizedintoasingleclusterthatincludesfunctionalgenesandpseudogenes.Figure4.2Clustersofb-globingenesandpseudogenesarefoundinvertebrates.Sevenmousegenesinclude2earlyembryonic,1lateembryonic,2adultgenes,and2pseudogenes.Rabbitandchickeachhavefourgenes.Figure4.3Allglobingeneshaveevolvedbyaseriesofduplications,transpositions,andmutationsfromasingleancestralgene.Figure4.3Allglobingeneshaveevolvedbyaseriesofduplications,transpo