Medical Microbiology

Replication 381

Theoriginsandevolutionofthevirusesarestilllargelyinthedark.Incontrast

tothetaxonomicsystemsusedtoclassifythehigherformsoflife,weare

thereforeunabletoclassifyvirusesinsuchevolutionarysystems.Aninterna-

tionalnomenclaturecommitteegroupsvirusesaccordingtovariouscriteria

anddesignatesthesegroups,analogouslytothehigherforms,asfamilies,

genera,andspecies.Despitethiselementof“artificiality”inthesystem

nowinuse,thegroupsappeartomakebiologicalsenseandtoestablishorder

intheenormousvarietyofknownviruses(seeTable 7. 2 ,basedonpublica-

tionsbytheInternationalCommitteeonTaxonomyofViruses). &

Replication

&Thestepsinviralreplicationareasfollows:

— Adsorptionofthevirustospecificreceptorsonthecellsurface.

— Penetrationbythevirusandintracellularreleaseofnucleicacid.

— Proliferationoftheviralcomponents:virus-codedsynthesisofcapsid

andnoncapsidproteins,replicationofnucleicacidbyviralandcellular

enzymes.

— Assemblyofreplicatednucleicacidandnewcapsidprotein.

— Releaseofvirusprogenyfromthecell. &

Asshownonp. 37 6,virusesreplicateonlyinlivinghostcells.Thedetailed

stepsinvolvedintheirreplicationareshownbelow(Fig. 7. 4 ).Thereactions

oftheinfectedcell(cytopathology,tumortransformation,etc.)aredescribed

onp.392.

VirusReplication

Virion Nucleic acid mRNA Protein Capsid Virions

Adsorption,

penetration, uncoating Protein synthesis Maturation Release

(Replicase)

Nucleic acid replication

ExtracellularIntracellular Extracellular

Fig. 7. 4 Seetextfordetailsofeachstep.

7

Kayser, Medical Microbiology © 2005 Thieme