Medical Microbiology

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Replication 385

Hepadnaviruses:thegenomeconsistsofanssDNAantisensestrandanda
shortsensestrand(Fig. 7 .5e).TheinfectedcelltranscribesanRNAsense
strand(“templatestrand”)fromtheantisensestrand.Thistemplatestrand
isintegratedinviruscapsidstogetherwithanRTDNApolymerase.Thepoly-
merasesynthesizesacomplementaryantisenseDNAand,to“sealoff”the
endsofthegenome,ashortsenseDNAfromthetemplatestrand.
— RNAviruses:sinceeukaryoticcellspossessnoenzymesforRNArepli-
cation,thevirusmustsupplytheRNA-dependentRNApolymerase(s)
(“replicase”).Theseenzymesarethusinanycasevirus-codedproteins,
andinsomecasesareactuallycomponentsofthevirusparticle.
Single-strandedRNA:insense-strandviruses,theRNAfunctionsasmRNA“as
is,”meaningtheinformationcanbereadoff,andthereplicasesynthesized
immediately.Antisense-strandvirusesmustfirsttranscribetheirgenomeinto
acomplementarystrandthatcanthenactasmRNA.Inthiscase,thepoly-
meraseforthefirsttranscriptioniscontainedinthematurevirionandde-
liveredintothecell.InssRNAviruses,whethersenseorantisensestrands,
complementarystrandsofthegenomeareproducedfirst(Fig. 7 .5a,b),
thentranscribedintodaughterstrands.Theythereforeonceagainshow
thesamepolarityastheviralgenomeandareusedinassemblyofthe
newviralprogeny.
Double-strandedRNA:atranslatablesense-strandRNAisproducedfrom
thegenome,whichconsistsofseveraldsRNAsegments(segmentedgenome).
Thisstrandfunctions,atfirst,asmRNAandlaterasamatrixforsynthesisof
antisense-strandRNA(Fig. 7 .5c).Hereaswell,anRNA-dependentRNApoly-
meraseispartofthevirusparticle.
Retrovirusesalsopossessasense-orientedRNAgenome,althoughitsre-
plicationdiffersfromthatofotherRNAviruses.Thegenomeconsistsoftwo
single-strandedRNAsegmentswithsensepolarityandistranscribedbyan
enzymeinthevirion(reversetranscriptase[RT])intocomplementaryDNA.
TheDNAiscomplementedtomakedsDNAandintegratedinthecellgenome.
Transcriptionintosense-strandRNAisthebasisforbothviralmRNAandthe
genomicRNAintheviralprogeny(Fig. 7 .5d).

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Kayser, Medical Microbiology © 2005 Thieme

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