Virus Classification Overview

Baltimore Classification of Viruses

• The Baltimore classification system classifies viruses based on their genome structure and the method of mRNA synthesis.
• Group II in the Baltimore classification contains viruses with a DNA intermediate in their life cycle.
• The Baltimore classification system categorizes viruses into seven groups.
• The International Committee on Taxonomy of Viruses (ICTV) is responsible for naming and classifying viruses.
• Group V of the Baltimore classification encompasses retroviruses. These viruses use reverse transcriptase to generate DNA from their RNA genome.

Virus Classification and Properties

• Holmes Classification: Divides viruses into three groups: Phaginae (infecting bacteria), Phytophaginae (infecting plants), and Zoophaginae (infecting animals).
• LHT (Lwoff, Horne, and Tournier) System: Organizes viruses based on structural properties.
  • Phylum: Vira
  • Subphyla: Deoxyvira (DNA viruses) and Ribovira (RNA viruses)
  • Classes: Deoxybinala, Deoxyhelica, Deoxycubica, Ribocubica, and Ribohelica
• ICTV (International Committee on Taxonomy of Viruses) System: A hierarchical classification, focusing on structural properties and viral lineages.
  • Orders: "…virales"
  • Suborders: "…viridales"
  • Families: "…viridae"
  • Subfamilies: "…virinae"
  • Genera: "…virus"
  • Subgenera: "…virus"
• Baltimore Classification: Groups viruses based on their mRNA synthesis strategy.
  • Group I: Double-stranded DNA viruses (e.g., Herperviridae, Iridoviridae, Poxviridae, Adenoviridae, Asfaviridae, Papillomaviridae)
  • Group II: Single-stranded DNA viruses (e.g., Parvoviridae, Circoviridae)
  • Group III: Double-stranded RNA viruses (e.g., Birnaviridae, Reoviridae)
  • Group IV: Single-stranded RNA viruses (Positive sense) (e.g., Arteriviridae, Coronaviridae, Picornaviridae, Flaviviridae, Togaviridae, Astroviridae)
  • Group V: Single-stranded RNA viruses (Negative sense) (e.g., Arenaviridae, Bunyaviridae, Orthomyxoviridae, Paramyxoviridae, Bornaviridae, Rhabdoviridae, Filoviridae)
  • Group VI: Single-stranded RNA viruses with a DNA intermediate (e.g., Retroviridae)
  • Group VII: Double-stranded DNA viruses with an RNA intermediate (e.g., Hepadnaviridae)

Virus Structure

• Size: Varies drastically, ranging from 18-20 nm (Parvovirus) to 130-375 nm (Poxvirus). Some filamentous Filoviruses can reach up to 14,000 nm.
• Capsid: The protein coat of a virus.
• Nucleocapsid: The capsid enclosing the nucleic acid genome.
• Capsomers: Structural subunits of the capsid.
• Envelope: A lipid bilayer membrane surrounding some viruses outside of the capsid.
• Peplomers: Spikes found on the envelope of some viruses.
• Viroids: Complete viral particles with all structural components.
• Virus Symmetry: The shape of the capsid/nucleocapsid.
  • Icosahedral Viruses: Have a polyhedral shape with 20 equilateral triangular faces.
    • Hexons: Capsomers on the faces of icosahedral viruses, bonding to six neighbors.
    • Pentons: Capsomers at the vertices of icosahedral viruses, bonding to five neighbors.
  • Helical Viruses: Have a cylindrical or rod-like shape.
    • Pitch: The distance covered by each complete turn of the helix.
    • Example: Tobacco mosaic virus (TMV) has a pitch of 2.28 Å.
  • Complex Viruses: Exhibit more intricate structures, combining elements from both icosahedral and helical symmetries.
    • Example: Rhabdovirus particles (e.g., vesicular stomatitis virus).
• Caspar-Klug Principle: Explains how capsomers are organized quasi-equivalently to minimize free energy during virion formation.
• Triangulation number (T): A mathematical formula used to describe the complexity of icosahedral structures.
  • Formula: T=a2+ab+b2 where a and b are integers.
  • Regular icosahedra: T=1 or 3.
• Enveloped Viruses: Acquire their envelopes by budding through a host cell membrane, acquiring a lipid bilayer derived from the host cell.