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1、Chapter5MobileGeneticElementsMobilegeneticelementsaresegmentsofDNAabletomovefromsitetositeinthegenome,orbetweengenomesinthesamecell.Foundinbothprokaryotesandeukaryotes,theyareadiversegroupdifferinginstructure,mechanismofmobilization,distribution,freedomofmovementandlevelofauton
2、omy.Mobileelementscanbeplacedintothreebroadcategoriesaccordingtotheirlevelofindependence:Episomes:Beabletoexistintwostateseitherintegratedintothehostgenomeorasautonomousextrachromosomalreplicons-aretermedepisomes(e.g.bacteriophageλ,theFplasmid).Cassettes:Elementswhosemobilityis
3、controlledbythecelltomediatespecificgenomicrearrangementsaretermedcassettes(e.g.yeastmatingtypecassettes,trypanosomeVSGcassettes).Transposableelements:Thoseelementswhichcannotreplicateoutsidethehostgenome,butwhichcontroltheirownmobilitywithinit,aretermedtransposableelements(e.g
4、.P-elements,retroviruses)Transposableelementsfallintotwomajorclassesbasedontheiractivetranspositionmechanism.ClassItransposableelements(retroelements)utilizeanRNAintermediateduringthetranspositionprocess-anintegratedelementistranscribed,thenreversetranscribedtogenerateacDNAcopy
5、whichisintegratedatanewsite.ClassIItransposableelements(transposons)transposedirectlyasDNA,eitherbyexcisionandintegrationorbyareplicativeprocesswherebyonecopyoftheelementisleftattheoriginalsiteandanotherisintegratedatanewsite.5.1MechanismsoftranspositionOverviewoftranspositionm
6、echanisms.Transposableelementscanbedividedintotwofamiliesaccordingtotheirgeneralmechanismoftransposition.TransposonsmobilizedirectlyasDNA,whereasretroelementsemployanRNAintermediate.Withineachfamily,severalspecifictypesoftranspositionmechanismcanbeused.(1)Proliferation.Increase
7、thenumberofcopiesofthetransposableelementinthegenome;thiscanbeaccomplishedactivelyorpassively.(2)Stagesoftransposition.Transposonsactivelymobilizeinthreestages:(i)thedonorcutstage,anendonucleolyticcleavagereactioncatalyzedbytransposasewhichseparatesthetransposableelementfromthe
8、hostDNA;(ii)thestrandexchangestage,apairoftransesterif