Medical Microbiology

MorphologyandStructure 377

componentsofnewvirusesinaccordancewiththegeneticinformationit

contains.Onethusmightcallviruses“vagabondgenes.”

Virusesinfectbacteria(so-calledbacteriophages),plants,animals,and

humans.Thefollowingpagescovermainlythehumanpathogenviruses

(seeFig. 7. 1 ).Table 7. 1 liststheessentialcharacteristicsofviruses.

MorphologyandStructure

&Amaturevirusparticleisalsoknownasavirion.Itconsistsofeithertwo

orthreebasiccomponents:

— AgenomeofDNAorRNA,double-strandedorsingle-stranded,linearor

circular,andinsomecasessegmented.Asingle-strandednucleicacidcan

haveplusorminuspolarity.

— Thecapsid,virus-codedproteinsenclosingthenucleicacidofthevirus

anddeterminingitsantigenicity;thecapsidcanhaveacubic(rotational),

helicalorcomplexsymmetryandismadeupofsubunitscalledcap-

somers.

— Insomecasesanenvelope(Fig. 7. 2 )thatsurroundsthecapsidandis

alwaysderivedfromcellularmembranes. &

Genome.TheviralgenomeiseitherDNAorRNA,andvirusesarehencecat-

egorizedasDNAorRNAviruses(seealsop.380).ThenucleicacidofDNA

virusesisusuallydouble-stranded(ds)andlinearorcirculardepending

ComparativeSizesofVirusesandBacteria

Herpes virus

Adenovirus

Poliovirus

Influenza virus Smallpox virus

1 00 nm

Escherichia coli

Fig. 7. 1 DifferentvirusspeciesareshownheretoscaleinsideanE.colibacterium.

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Kayser, Medical Microbiology © 2005 Thieme