Viral structure &Viral structure &propertiespropertiesViral structure &Viral structure &propertiespropertiesRiboNik

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Dr. Nikhil Kirtipal

What is a virus

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Dr. Nikhil Kirtipal

VirusesVirus is defined as a nucleoprotein

complex which infects cells anduses their metabolic processes toreplicateSmallest known infective agents• Metabolically inert - no metabolic

activity outside host cell; mustenter host cell to replicate

Virus is defined as a nucleoproteincomplex which infects cells anduses their metabolic processes toreplicateSmallest known infective agents• Metabolically inert - no metabolic

activity outside host cell; mustenter host cell to replicate

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Dr. Nikhil Kirtipal

What is Life?Can grow, i.e. increase in size.

Can reproduce.

Metabolism: can acquire andutilize energy.

What is Life?Can grow, i.e. increase in size.

Can reproduce.

Metabolism: can acquire andutilize energy.RiboNik

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Dr. Nikhil Kirtipal

Kingdoms of LifeAnemalia

Plantae

ProtistaProtista

Fungi

Bacteria

Archaea

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Dr. Nikhil Kirtipal

Discovery of VirusesDiscovery of VirusesBeijerinck (1897) coinedthe Latin name “virus”meaning poison

He studied filtered plantjuices & found they causedhealthy plants to becomesick

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Beijerinck (1897) coinedthe Latin name “virus”meaning poison

He studied filtered plantjuices & found they causedhealthy plants to becomesick

copyright cmassengale

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Dr. Nikhil Kirtipal

UNCONVENTIONAL AGENTSVirion: viral particleVIROIDSRNA only

viroids infect plants

Hepatitis D virus (HDV) is a viroid

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Virion: viral particleVIROIDSRNA only

viroids infect plants

Hepatitis D virus (HDV) is a viroidRiboNik

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Dr. Nikhil Kirtipal

PRIONSprotein only

do not contain any nucleic acid

Cause animal diseases e.g.spongiform encephalopathy

Cause human diseases e.g.Creutzfeldt-Jakob disease.

protein only

do not contain any nucleic acid

Cause animal diseases e.g.spongiform encephalopathy

Cause human diseases e.g.Creutzfeldt-Jakob disease.

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Dr. Nikhil Kirtipal

Why are some viruses harmful?Virus invadescell

When your cellsmake virusesinstead ofoperatingnormally, YOUget sickVirus forces cell to make

copies of virus

Eventually the cell explodes,releasing all of the new viruses

When your cellsmake virusesinstead ofoperatingnormally, YOUget sick

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Dr. Nikhil Kirtipal

Who do viruses infect?Viruses infect BacteriaThese viruses are called bacteriophagesViruses infect PlantsOne example is the Tobacco Mosaic Virus

Viruses infect AnimalsOne example is Influenza virus

Viruses infect humans

Viruses infect BacteriaThese viruses are called bacteriophagesViruses infect PlantsOne example is the Tobacco Mosaic Virus

Viruses infect AnimalsOne example is Influenza virus

Viruses infect humans

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Dr. Nikhil Kirtipal

BacteriophageBacteriophage

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Dr. Nikhil Kirtipal

How small are the viruses

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Dr. Nikhil Kirtipal

The smallest viruses (parvoviruses)are ≈ 20 nm (nm=10-9 m)

The largest (poxviruses) are ≈ 300nm

The smallest viruses (parvoviruses)are ≈ 20 nm (nm=10-9 m)

The largest (poxviruses) are ≈ 300nm RiboNik

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Dr. Nikhil Kirtipal

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Dr. Nikhil Kirtipal

Viral Structure

Nucleic acid: either RNA or DNAnever both.

Capsid: Protein shell surround thenucleic acid.

Envelope: lipid membrane acquiredfrom the host cell. Not all viruseshave it.

Nucleic acid: either RNA or DNAnever both.

Capsid: Protein shell surround thenucleic acid.

Envelope: lipid membrane acquiredfrom the host cell. Not all viruseshave it.

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Dr. Nikhil Kirtipal

Nucleic Acid:-DNA or RNA- RNA can be –ve or +ve sense.- single - or double - stranded- linear or circular- segmented or not- encodes proteins

-DNA or RNA- RNA can be –ve or +ve sense.- single - or double - stranded- linear or circular- segmented or not- encodes proteins

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Dr. Nikhil Kirtipal

proteins:

a)capsid: made of capsomeres

b) enzymes:

- differ from host cell enzymes.

- targets of antiviral therapy

proteins:

a)capsid: made of capsomeres

b) enzymes:

- differ from host cell enzymes.

- targets of antiviral therapy

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Dr. Nikhil Kirtipal

Envelope: layer made of lipid and protein

Surrounds the capsid

Not found in all viruses

If destroyed the virus become noninfectious.

Some enveloped viruses have a matrixSome have protrusions made of

glycoprotein and called spikes.

layer made of lipid and protein

Surrounds the capsid

Not found in all viruses

If destroyed the virus become noninfectious.

Some enveloped viruses have a matrixSome have protrusions made of

glycoprotein and called spikes.

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Dr. Nikhil Kirtipal

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Dr. Nikhil Kirtipal

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Dr. Nikhil Kirtipal

Virus particles exhibit 3 types ofcapsid symmetry: helical : most helical viruses possess

an outer envelope

icosahedral or cubic: may or maynot possess an outer envelope

Complex: does not conform to eitherof above (e.g. Pox viruses)

Virus particles exhibit 3 types ofcapsid symmetry: helical : most helical viruses possess

an outer envelope

icosahedral or cubic: may or maynot possess an outer envelope

Complex: does not conform to eitherof above (e.g. Pox viruses)

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Dr. Nikhil Kirtipal

Helical CapsidHelical capsid isrod-like structurewith the RNA inthe center of thehelix. A helix ismade by stackingrepeating units ina spiral.

RNA

Helical capsid isrod-like structurewith the RNA inthe center of thehelix. A helix ismade by stackingrepeating units ina spiral. protein

coat

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Dr. Nikhil Kirtipal

ICOSAHEDRAL SYMMETRY

20 faces12 vertices20 faces12 vertices

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Dr. Nikhil Kirtipal

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Dr. Nikhil Kirtipal

COMPLEX SYMMETRY

cross section

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POXVIRUS FAMILY

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Dr. Nikhil Kirtipal

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Dr. Nikhil Kirtipal

Capsid symmetryIcosahedral Helical

Naked capsid

Enveloped

Lipid

Glycoprotein

Matrix

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Dr. Nikhil Kirtipal

nveloped virusesEEnvelope is sensitive toDrying, heat, detergents and acidConsequencesMust stay wet during transmissionTransmission in large droplets and

secretionsCannot survive in the gastrointestinal tractMay not need to kill cells in order to spread

Envelope is sensitive toDrying, heat, detergents and acidConsequencesMust stay wet during transmissionTransmission in large droplets and

secretionsCannot survive in the gastrointestinal tractMay not need to kill cells in order to spread

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Dr. Nikhil Kirtipal

Naked VirusesCapsid is resistant toDrying, heat, detergents and acids.

ConsequencesCan survive in the gastrointestinal tractSurvive well on environmental surfacesSpread easily via fomitesMust kill host cells to release mature

virus particles

Capsid is resistant toDrying, heat, detergents and acids.

ConsequencesCan survive in the gastrointestinal tractSurvive well on environmental surfacesSpread easily via fomitesMust kill host cells to release mature

virus particles

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Dr. Nikhil Kirtipal

Differences betweenbacteria and virusesDifferences betweenbacteria and virusesRiboNik

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Dr. Nikhil Kirtipal

VirusesBacteriaNoOftenGrowth on artificial media

NoYesDivision by binary fission

NoYesContain DNA and RNA NoYesContain DNA and RNA

NoYesprotein synthesismachinery

NoYesSensitive to antibiotics

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Dr. Nikhil Kirtipal