Virus, its types, classification and replication of virus

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Virus, its types, classification and replication
of virus
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Overview
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Virus

• Definition
• an infective agent that typically consists of a
  nucleic acid molecule in a protein coat, is too
  small to be seen by light microscopy, and is able
  to multiply only within the living cells of a
  host.

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Virus

• Size Comparison
• Cant be seen by eye or light micro-scope

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Virus

• Size Comparison
• Their effects can be observed
• Cytopathic effects
• Enlarged nuclei in tissues

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Structure

• Genome or coreDNA or RNA
• Capsid protects genome
• Also have virally encoded proteins that help in
  replication
• Some virus have envelopes on outer Virion surface
  that interact with surface of cell

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Viral Genomes

• RNA or DNA SS or DS (strands) linear or
  circular
• Used to classify viruses
• DNA viruses replicate in host nucleus, share a
  common patterns as host cells
• RNA viruses must have specific enzymes that are
  not in cells so can make RNA for new viruses
• Viral and Cellular proteins are not similar
• Virus encodes enzymes and proteins that modify
  the cells in which virus replicate

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Viral capsids

• Complex
• Many identical subunits of viral proteins
• 1 subunit 1 capsomer
• Protects viral genome in 2 stable shapes for a
  particle of nucleic acid and protein-
  nucleoprotein
• Helix --capsomer associates with helical genome
• Icosahedron regular, solid shape that enfolds
  viral genome

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Capsids have symmetry

• Capsids around genome is unique for each type of
  virus
• They have symmetry which is criteria for
  classification
• Capsid structure is defined by X-ray
  Crystallography
• Capsids have various complexity of structures
  based on viral genomes

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Viral membrane envelopes

• Some viruses have a lipid envelope that surrounds
  the capsid
• Lipid bilayer is from the cell in which the virus
  has previously replicated
• This virally encoded proteins are also used to
  characterize the viruses
• Usually is amorphous because shape is defined by
  the capsid

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Classification

• International Committee on Taxonomy of Viruses

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Classification scheme

• Based on basic properties of virus and specific
  features of replication cycle
• What type of genome
• RNA/DNA, SS/DS, linear/circular
• How is capsid arranged
• Symmetry and dimension
• What other components?
• Envelope
• Proteins in virion responsible for infection

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Baltimore Scheme

• Used mainly for RNA viruses and based on how mRNA
  is synthesized
• Limited by type of genome
• Sense RNA virusRNA serves as mRNA template
• - Sense RNA virusRNA serves as complimentary
  sense of mRNA
• Retrovirus-- Sense RNA but use DNA in
  replication cycle

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Baltimore Classification System

• Class 1 Double-stranded DNA viruses
• Class 2 Single-stranded DNA viruses
• Class 3 Double-stranded RNA viruses
• Classes 4 5 Single-stranded RNA viruses
• Class 4 Single-stranded RNA viruses -
  positive-sense
• Class 5 Single-stranded RNA viruses -
  negative-sense
• Class 6 Positive-sense single-stranded RNA
  viruses that replicate through a DNA intermediate
• Class 7 Double-stranded DNA viruses that
  replicate through a single-stranded RNA
  intermediate

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Taxonomy
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Replication

• The virus replication occurs in seven stages,
  namely
• Adsorption,
• Entry
• Uncoating
• Transcription / mRNA production
• Synthesis of virus components
• Virion assembly
• Release (Liberation Stage)

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Replication
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Replication

• Lysogenic cycle
• Lytic cycle

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Replication
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