资源描述：
《分子生物学Chapter6MobileGeneticElements》由会员上传分享，免费在线阅读，更多相关内容在教育资源-天天文库。
1、Chapter6MobileGeneticElementsIntroductionMuchofthediscussionthroughoutthisbookdealswiththefunctionsofthegeneproductsexpressedfromproteincodinggenesandRNAgenes.However,noncodingrepetitiousDNA,consistingofsimple-sequenceDNAandmoderatelyrepeatedDNA,constitu
2、tesasignificantfractionofthegenomicDNAinhighereukaryotes.Inthissection,wefocusonmoderatelyrepeatedDNAsequences,ormobileDNAelements,whichareinterspersedthroughoutthegenomesofhigherplantsandanimals.AlthoughmobileDNAelements,rangingfromhundredstoafewthousan
3、dbasepairsinlength,originallywerediscoveredineukaryotes,theyalsoarefoundinprokaryotes.Theprocessbywhichthesesequencesarecopiedandinsertedintoanewsiteinthegenomeiscalledtransposition.MobileDNAelements(orsimplymobileelements)areessentiallymolecularparasite
4、s,whichappeartohavenospecificfunctioninthebiologyoftheirhostorganisms,butexistonlytomaintainthemselves.Forthisreason,FrancisCrickreferredtothesesequencesas“selfishDNA.”ThetranspositionofmobileDNAelementsisbelievedtohaveresultedintheirslowaccumulationineu
5、karyoticgenomesoverevolutionarytime.TheseelementsalsoarelostataveryslowratebydeletionofsegmentsofDNAcontainingthemandbyaccumulationofmutationsuntiltheycannolongerberecognizedtoberelatedtotheoriginalmobileDNAelement.Sincemobileelementsareeliminatedfromeuk
6、aryoticgenomessoslowly,theyhaveaccumulatedtothepointwheretheynowconstituteasignificantportionofthegenomesofmanyeukaryotes.Asresearchonmobileelementsprogressed,theywerefoundtofallintotwocategories:(1)thosethattransposedirectlyasDNAand(2)thosethattranspose
7、viaanRNAintermediatetranscribedfromthemobileelementbyanRNApolymeraseandthenconvertedbackintodouble-strandedDNAbyareversetranscriptase.MobileelementsthattransposethroughaDNAintermediatearegenerallyreferredtoastransposons.(Asdiscussedbelow,thistermhasamore
8、specificmeaninginreferencetobacterialmobileelements.)MobileelementsthattransposetonewsitesinthegenomeviaanRNAintermediatearecalledretrotransposonsbecausetheirmovementisanalogoustotheinfectiousprocessofretroviruses.Indeed,r
