viral classification

7/30/2019 Viral Classification

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Virology


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Classification of Virus

Two main schemes:

ICTV (International Committee on Taxonomy of

Viruses )

Baltimore : (Developed by David Baltimore)


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ICTV

It is a universal code for viral classification

The essential principles of virus nomenclature

are:-

To aim for stability.

To avoid or reject the use of names which might

cause error or confusion.

To avoid the unnecessary creation of names


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Virus classification system shall employ the hierarchicallevels of Order: A group of families sharing certain common characters.

An order name must be a single word ending in virales

Family: A group of genera or subfamilies, sharing certaincommon characters. A family name must be a single wordending in viridae

Subfamily: A group of genera sharing certain commoncharacters. The taxon shall be used only when it is needed to

solve a complex hierarchical problem. A subfamily name mustbe a single word ending in virinae.

Genus: A group of related species that share some significantproperties and often only differ in host range and virulence. Agenus name must be a single word ending in virus.

Species: A virus species is defined as a polythetic class of virusesthat constitutes a replicating lineage and occupies a particularecological niche.

According to ICTV 2011 report there are 6+1 orders, 94families, 22 subfamily, 395 genera and 2480 species.


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Caudovirales : Tailed dsDNA (group I) bacteriophages.

Herpesvirales: Large eukaryotic dsDNA viruses.

Mononegavirales : Nonsegmented (-) strand ssRNA plantand animal viruses.

Nidovirales : (+) strand ssRNA viruses with vertebratehosts.

Picornavirales: Small (+) strand ssRNA viruses that infecta variety of plant, insect and animal hosts.

Tymovirales: Monopartite (+) ssRNA viruses that infectplants.

Ligamenvirales: infecting archaea has been proposed.


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Baltimore Baltimore system classify virus based on type of genome

and strandedness

There are 7 groups according to Baltimore classification

I: dsDNA viruses (e.g. Adenoviruses, Herpesviruses,Poxviruses)

II: ssDNA viruses (+)sense DNA (e.g. Parvoviruses)

III: dsRNA viruses (e.g. Reoviruses)

IV: (+)ssRNA viruses (+)sense RNA (e.g. Picornaviruses,Togaviruss)

V: ()ssRNA viruses()sense RNA (e.g. Orthomyxoviruses,Rhabdoviruses)

VI: ssRNA-RT viruses (+)sense RNA with DNA intermediatein life-cycle (e.g. Retroviruses)

VII: dsDNA-RT viruses (e.g. Hepadnaviruses)


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Fine structure of Virus

Capsid with(out) Envelope

Protect virus from physical, chemical and bilogical

damage

Helps virus to attach on the host

Mostly Capable of self assembly

Nucleic acid


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Helical Capsid

Fraenkel-Conrat & Williams showed that

when mixtures of purified tobacco mosaic

virus (TMV) RNA & coat protein wereincubated together, virus particles formed

The fact that helical symmetry is a useful

way of arranging a single protein subunit to

form a particle is confirmed by the largenumber of different types of virus which

have evolved with this capsid arrangement.


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Icosahedral (isometric) capsids

Simplest known icosahedaral virus are

bacteriophage X174


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Enveloped viruses

Matrix Proteins: These are internal virion

proteins whose function is effectively to linkthe internal nucleocapsid assembly

Glycoproteins: These are transmembrane

proteins, anchored to the membrane by a

hydrophobic domain

External Glycoproteins

Transport Channels


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Complex Virus Structures

Pox virus Tailed phages


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Life cycle of VIrus


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Attachment

Penetration & Release

Biosynthesis Immediate Early Genes : Early phase proteins

Delayed Early Genes : Middle phase

Late Genes : Late phase

Assembly

Release


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PRION (INfectious PROtein)

Prions are mis-folded proteins (PrpSC)

Responsible for the transmissiblespongiform encephalopathies

They are formed from a normalproteins called PrPc

PrP play important roles in cell-celladhesion and intracellular signaling invivo, and may therefore be involvedin cell-cell communication in thebrain

Completely digested by Proteinase K


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Prion Infection

PrPsc is an isoform of PrPc

Infection of PrPsc from another animal

Genetic factor (PRPN gene mutation)


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Growth of Prions

two models

1. Heterodimer

2. fibril model


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Characteristics of Prion

Highly resistant to chemical and physical

treatments

Can aggregate by simply transforming the

normal proteins


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Diseases due to prion

Initiated when a prion agent aenter in the

system through digestive tract.

Then they move to lymph system and multiply

Slowly it starts accumulating in the neurons

Eg. Find out by yourself

Assignment: Protein only hypothesis


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viral classification

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