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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