VIRUS
Virusesarenucleoprotein particlesthat can multiply only in living cellsof animals, plants, or bacteria. The
nameisfromaLatin word meaning “slimy liquid” or “poison”.
Virusesoccupy aspecial taxonomicposition: they arenot plants, animals, or prokaryoticand they are
generally placed in their own kingdom. In fact, virusesshould not even beconsidered organisms, in the
strictest sense, becausethey arenot free-living; i.e., they cannot reproduceand carry on metabolic
processeswithout ahost cell.
GENERALCHARACTERSTICS
a. Smallest Living Entities:
o Thesizeof virusesrangesfrom20-300 nm
o Thesizeis1/100th to 1/1000th of thesizeof acell of any other common living
organism.
o Thestructureof viruswasstudied in detail only after thedevelopment of electron
microscopy
b. Dual Properties:
o Virushasno independent existenceoutsidethehost and behaveslikeabsolutenon-
living particle.
o Virusparticlescan becrystallized, without thelossof any pathogenicity.
o When insidethebody of host, it can reproduceusing thehost machinery.
c. Nucleo-protein particles:
o Viruseshaveavery simplestructure.
o They contain nucleicacid, either DNA(deoxyribonucleicacid)or RNA(ribonucleic
acid), and protein.
o Thenucleicacid encodesthegeneticinformation uniquefor each virus.
o Theprotein coat iscalled capsid.
o In rarecases, theremay beasurrounding lipid membranewhich isderived fromthe
host
d. Obligateendo-parasites:
o Viruscan reproduceonly when they arein association with their host
PM| Hislop . Botany
, o Each typeof viruscan recognizeand infect only acertain typeof living cell. They
showabsolutehost specificity.
STRUCTURE
i. Theintact virusunit or infectiousparticleiscalled aviron. Each viron consistsof;
o Nucleicacid core
o Protein coat
Thisnucleocapsid may benaked or sometimessurrounded by aloosemembranous
envelopederived fromthehost.
ii. Thenucleicacid may be
o DNA– Theviruscontaining DNAcoreiscalled aDeoxyribo-virus. It may beadouble
stranded (e.g. T4 bacteriophage)or asinglestranded DNA(e.g. Øx174 virus).
o RNA– Theviruscontaining RNAcoreiscalled Ribo-virus. . It may beadoublestranded
(e.g. Ø6 bacteriophage, Reterovirus)or asinglestranded RNA(e.g. Tobacco Mosaic
Virus).
iii. Thecapsid ismadeup of repeating unitscalled capsomere. Thesecapsomeresarecomposed
of several typesof proteinsthat havebeen identified and used to characterizeviruses. For
example, thefoot-and-mouth diseaseviruscapsid hasfacesconsisting of threeproteins
named VP-1to 3; InfluenzaviruscontainsproteinsH(Haemagluttinin)and N(Neuraminidase)
iv. Capsomeresremain associated with thecorenucleicacid forming thenucleocapsid particle.
v. Thearrangement of capsomeresformstwo typesof capsid symmetry –
o HELICAL: Thehelical capsid consistsof monomersarranged in ahelix around asingle
rotational axis. Themonomerscurveinto ahelix becausethey arethicker at oneend
than theother. Thesizeand shapeof themonomer determinestheshapeof thevirus.
Helical capsidsmay benaked (e.g. Tobacco MosaicVirus)or surrounded by an
envelope(e.g. Influenzavirus)
PM| Hislop . Botany
Virusesarenucleoprotein particlesthat can multiply only in living cellsof animals, plants, or bacteria. The
nameisfromaLatin word meaning “slimy liquid” or “poison”.
Virusesoccupy aspecial taxonomicposition: they arenot plants, animals, or prokaryoticand they are
generally placed in their own kingdom. In fact, virusesshould not even beconsidered organisms, in the
strictest sense, becausethey arenot free-living; i.e., they cannot reproduceand carry on metabolic
processeswithout ahost cell.
GENERALCHARACTERSTICS
a. Smallest Living Entities:
o Thesizeof virusesrangesfrom20-300 nm
o Thesizeis1/100th to 1/1000th of thesizeof acell of any other common living
organism.
o Thestructureof viruswasstudied in detail only after thedevelopment of electron
microscopy
b. Dual Properties:
o Virushasno independent existenceoutsidethehost and behaveslikeabsolutenon-
living particle.
o Virusparticlescan becrystallized, without thelossof any pathogenicity.
o When insidethebody of host, it can reproduceusing thehost machinery.
c. Nucleo-protein particles:
o Viruseshaveavery simplestructure.
o They contain nucleicacid, either DNA(deoxyribonucleicacid)or RNA(ribonucleic
acid), and protein.
o Thenucleicacid encodesthegeneticinformation uniquefor each virus.
o Theprotein coat iscalled capsid.
o In rarecases, theremay beasurrounding lipid membranewhich isderived fromthe
host
d. Obligateendo-parasites:
o Viruscan reproduceonly when they arein association with their host
PM| Hislop . Botany
, o Each typeof viruscan recognizeand infect only acertain typeof living cell. They
showabsolutehost specificity.
STRUCTURE
i. Theintact virusunit or infectiousparticleiscalled aviron. Each viron consistsof;
o Nucleicacid core
o Protein coat
Thisnucleocapsid may benaked or sometimessurrounded by aloosemembranous
envelopederived fromthehost.
ii. Thenucleicacid may be
o DNA– Theviruscontaining DNAcoreiscalled aDeoxyribo-virus. It may beadouble
stranded (e.g. T4 bacteriophage)or asinglestranded DNA(e.g. Øx174 virus).
o RNA– Theviruscontaining RNAcoreiscalled Ribo-virus. . It may beadoublestranded
(e.g. Ø6 bacteriophage, Reterovirus)or asinglestranded RNA(e.g. Tobacco Mosaic
Virus).
iii. Thecapsid ismadeup of repeating unitscalled capsomere. Thesecapsomeresarecomposed
of several typesof proteinsthat havebeen identified and used to characterizeviruses. For
example, thefoot-and-mouth diseaseviruscapsid hasfacesconsisting of threeproteins
named VP-1to 3; InfluenzaviruscontainsproteinsH(Haemagluttinin)and N(Neuraminidase)
iv. Capsomeresremain associated with thecorenucleicacid forming thenucleocapsid particle.
v. Thearrangement of capsomeresformstwo typesof capsid symmetry –
o HELICAL: Thehelical capsid consistsof monomersarranged in ahelix around asingle
rotational axis. Themonomerscurveinto ahelix becausethey arethicker at oneend
than theother. Thesizeand shapeof themonomer determinestheshapeof thevirus.
Helical capsidsmay benaked (e.g. Tobacco MosaicVirus)or surrounded by an
envelope(e.g. Influenzavirus)
PM| Hislop . Botany
