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1、MethodsinMolecularBiologyMethodsinMolecularBiologyTMVOLUME235E.coliE.coliPlasmidVectorsPlasmidVectorsMethodsandApplicationsMethodsandApplicationsEditedbyEditedbyNicolaCasaliNicolaCasaliAndrewPrestonAndrewPrestonPlasmidOrganization11TheFunctionandOrganizationofPlasmidsFinbarrHayes1
2、.IntroductionIn1952,JoshuaLederbergcoinedthetermplasmidtodescribeanybacterialgeneticelementthatexistsinanextrachromosomalstateforatleastpartofitsreplicationcycle(1).Asthisdescriptionincludedbacterialviruses,thedefinitionofwhatconsti-tutesaplasmidwassubsequentlyrefinedtodescribeexc
3、lusivelyorpredominantlyextrachromosomalgeneticelementsthatreplicateautonomously.PlasmidsarenowknowntobepresentinmostspeciesofEubacteriathathavebeenexamined,aswellasinArchaeaandlowerEukarya(2).Althoughmostofthegeneticmaterialthatdirectsthestructureandfunctionofabacterialcellisconta
4、inedwithinthechromosome,plasmidscontributesignificantlytobacterialgeneticdiversityandplasticitybyencodingfunctionsthatmightnotbespecifiedbythechromosome(3)(seeSubheading3).Forexample,antibioticresis-tancegenesareoftenplasmid-encoded,whichallowsthebacteriumtopersistinanantibiotic-c
5、ontainingenvironment,therebyprovidingthebacteriumwithacompeti-tiveadvantageoverantibiotic-sensitivespecies.Underlaboratoryconditions,plasmidsaregenerallynotessentialforthesurvivalofthehostbacteriumandtheyhaveservedasinvaluablemodelsystemsforthestudyofprocessessuchasDNAreplication,
6、segregation,conjugation,andevolution(3).More-over,eversincetheirutilitywasevincedbythefirstgene-cloningexperimentsintheearly1970s,plasmidshavebeenpivotaltomodernrecombinantDNAtechnologyasgene-cloningandgene-expressionvehicles,amongotheruses(4,5).2.BasicPlasmidCharacteristics2.1.Si
7、zeandCopyNumberNaturallyoccurringplasmidsvarygreatlyintheirphysicalproperties,afewexamplesofwhichareshowninTable1.Theyrangeinsizefrom<2-kilobasepair(kbp)plas-mids,whichcanbeconsideredtobeelementssimplycapableofreplication,toFrom:MethodsinMolecularBiology,Vol.235:E.coliPlasmidVecto
8、rsEditedby:N.CasaliandA.Preston©H
