Medical Microbiology

168 3 GeneralBacteriology

Manyofthemcarrygenesthatcodeforcertainphenotypiccharacteristicsof

thehostcell.Thefollowingplasmidtypesaremedicallyrelevant:

&Virulenceplasmids.Carrydeterminantsofbacterialvirulence,e.g.,ente-

rotoxingenesorhemolysingenes.

&Resistanceplasmids.Carrygeneticinformationbearingonresistanceto

anti-infectiveagents.RplasmidsmaycarryseveralRgenesatonce(seealso

Fig.3. 23 ,p. 17 6).Plasmidshavealsobeendescribedthatcarrybothvirulence

andresistancegenes.

DNAReplication...........................................

TheidenticalduplicationprocessofDNAistermedsemiconservativebecause

thedoublestrandofDNAisopenedupduringreplication,whereuponeach

strandservesasthematrixforsynthesisofacomplementarystrand.Thus

eachofthetwonewdoublestrands“conserves”oneoldstrand.Thedoubling

ofeachDNAmolecule(replicon)beginsatagivenstartingpoint,theso-called

originofreplication.Thisprocesscontinuesthroughouttheentirefission

cycle.

TranscriptionandTranslation.............................

&Transcription.CopyingofthesensestrandoftheDNAintomRNA.The

continuous geneticnucleotide sequenceistranscribed“colinearly” into

mRNA.Thisprincipleofcolinearityapplieswithveryfewexceptions.The

transcriptionprocesscanbebrokendownintothethreephasespromoterre-

cognition,elongation,andtermination.Thepromoterregionisthesitewhere

theRNApolymerasebeginsreadingtheDNAsequence.Asigmafactorisre-

quiredforbindingtothepromoter.Sigmafactorsareproteinsthatassociate

temporarilywiththeRNApolymerase(coreenzyme)toformaholoenzyme,

thendissociatethemselvesoncethetranscriptionprocesshasbegun,making

themavailabletoassociateonceagain.Specificsigmafactorsrecognizethe

standardpromotersofmostgenes.Additionalsigmafactors,theexpressionof

whichdependsonthephysiologicalstatusofthecell,facilitatethetranscrip-

tionofspecialdeterminants.Genesthatcodeforfunctionallyrelatedpro-

teins,forexampleproteinsthatacttogethertocatalyzeacertainmetabolic

step,areoftenarrangedsequentiallyatspecificlocationsonthechromosome

orplasmid.SuchDNAsequencesareknownasoperons(Fig.3. 18 ).ThemRNA

synthesizedbythetranscriptionofanoperonispolycistronic,i.e.,itcontains

theinformationsequencesofseveralgenes.Theinformationsequencesare

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Kayser, Medical Microbiology © 2005 Thieme