Medical Microbiology

378 7 GeneralVirology

onthefamily;thenucleicacidofRNAvirusesisusuallysingle-stranded(ss),

withtheexceptionofthereoviruses,andisalsosegmentedinanumberof

virusfamilies.ViruseswithssRNAaredividedintotwogroups:iftheRNAof

thegenomehasthesamepolarityastheviralmRNAandcanthusfunction

directlyasmessengerRNAitiscalledaplus-strand(orpositive-strand)or

“sense”RNAstrandandthesevirusesaresenseorplus-strandviruses.If

thegenomeRNAhasthepolarityoppositetothatofthemRNA,andtherefore

cannotbetranslatedintoproteinsuntilithasfirstbeentranscribedintoa

complementarystrand,itiscalledaminus-strand(ornegative-strand)or

“antisense”RNAstrandandthevirusesareantisenseorminus-strandviruses.

Capsid.Thecapsid(Fig. 7. 2 )isthe“shell”ofvirus-codedproteinthatencloses

thenucleicacidandismoreorlesscloselyassociatedwithit.Thecombina-

tionofthesetwocomponentsisoftentermedthenucleocapsid,especiallyif

theyarecloselyassociatedasinthemyxoviruses.Thecapsidismadeupof

subunits,thecapsomers,thenumberofwhichvariesbutisspecificandcon-

stantforeachviralspecies.Thesearesphericalorcylindricalstructurescom-

posedofseveralpolypeptides.Thecapsidprotectsthenucleicacidfromde-

gradation.Inallexceptenvelopedviruses,itisresponsiblefortheattachment

ofthevirusestothehostcell(“adsorption,”seethechapteronreplication,p.

384)anddeterminesspecificviralantigenicity.

Envelope.Theenvelope(Fig. 7. 2 ),whichsurroundsthecapsidinseveralvirus

families,isalwaysdependentoncellularmembranes(nuclearorcellmem-

brane,lessfrequentlyendoplasmicreticulum).Bothcell-codedandviralpro-

teinsareintegratedinthemembranewhentheseelementsaretransformed

intotheenvelope,frequentlyintheformof“spikes”(orpeplomers,Fig. 7. 3 ).

Envelopedvirusesdonotadsorbtothehostcellwiththecapsid,butrather

withtheirenvelope.Removingitwithorganicsolventsordetergentsreduces

theinfectivityoftheviruses(“ethersensitivity”).

VirusParticleStructure

Capsid, made up

of capsomers

Nucleic acid

Envelope

(not present in

all virus species)

Fig. 7. 2

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Kayser, Medical Microbiology © 2005 Thieme