virus 1

• viruses need...

  a host cell to survive as they lack organelles
• virus classification

  family name ends in -virdae. genus name ends in -virus.
• Enveloped viruses

  envelope surrounds capsid. polymorphic (meaning no distinctive symmetry)
• Uneveloped viruses

  capsid only. proteins bind to host cell receptors to facilitate entry of the genome into the cell
• viruses are classified via

  Baltimore classification
• Baltimore classification


  classifies viruses based on their type of genome. DNA or RNA. double strand or single strand. (for ss RNA if it is positive or negative). and their method of replication.
• features of DNA genome

  monoparticle, mostly double stranded, few circular, little structural diversity, linear DNA can adopt circular structure.
• features of RNA genome

  mostly single stranded, linear genomes, can have multiple segments.
• what do RNA viruses need?

  RNA polymerase to copy their RNA in the host cell.
• RNA polymerases are error prone meaning...


  RNA viruses are more variable and can evolve rapidly if needed e.g. zoonotics. reassortment leads to rapid diversification
• what are the 3 capsid types?

  icosahedral, helical, complex
• pentons and hexons


  penton found at vertex of capsomer, 12, 5 neighbouring capsomers. hexons are found between pentons, 6, number can vary.
• icosahedral


  12 vertices, 20 triangular sides, capsomers (penton or hexon)

  
• Parvoviridae characteristics

  12 capsomers, 60 copies of single protein. T=1 symmetry. non enveloped
• Adenoviridae characteristics

  252 capsomers, 240 hexavalent and 12 pentavalent. T=25. non enveloped. capsomers contain 1-4 different proteins
• helical capsids


  spiral with single axis and helix arranged around genome.

  
• paramyxoviridae characteristics

  ssRNA, helical, sphere shape,
• rhabdoviridae characteristics


  ssRNA, helical capsid, bullet shape, spikes, nucleoproteins contain RNA genome.
• complex capsid


  larger viruses, asymmetrical, can have extra structures such as tails

  
• what capsids do few viruses have an envelope for?

  icosahedral
• Where do enveloped viruses get their envelope?

  Budding off from the host cell membrane. viral proteins contain receptors needed for virus entry.
• how are unenveloped viruses released?

  lysis from infected cell. receptors are on capsid surface.
• properties of enveloped viruses (unenveloped are opposite)

  more fragile, easily destroyed by detergent and disinfectants, must stay wet, can't survive in GI tract.
• naked viruses can be transmitted...

  inside vesicles. so sometimes enveloped. rotavirus and noroviruses. if vesicles remain intact they can pass through GI tract
• non structural viral proteins

  no structural components of virus particle. made in infected cell. enzymes involved in replication. can help virus avoid immune detection. targets T cell epitopes
• viral envelope proteins are often..

  integral membrane proteins, external receptors that interact with Abs and receptors. internal domains that interact with capsid
• RNA viruses:

  • Require RNA polymerase to replicate their RNA genome.
  • Lack proofreading capability, leading to variability and rapid evolution.
  • Can readily adapt to new environments or species jumps, often causing zoonotic infections.
  • Have more subtypes/serotypes within a species.
  • Segmentation allows rapid diversity increase through reassortment.
  • Code for RNA-dependent RNA polymerase due to host cell enzyme absence.
• • RNA viruses have diverse structures, mostly single-stranded.
  • Majority are single-stranded with positive or negative sense.
  • Double-stranded RNA viruses are all segmented, while only some single-stranded negative sense RNA viruses are segmented.
  • Families like Reoviridae and Orthomyxoviridae have segmented genomes where each segment codes for a single gene.
  • Most RNA viruses have linear genomes, except Reo and Birna viruses.
• • Majority of DNA viruses have double-stranded DNA genomes.
  • Exceptions include two single-stranded DNA viruses.
  • Monopartite: All viral genes on a single segment.
  • Most are double-stranded, except parvo and circo viruses.
  • Few are circular, while many linear DNA viruses can adopt a circular configuration.
  • DNA viruses generally have their DNA on a single segment.
  • Little diversity in structure among DNA viruses.
• Viruses can be classified based on:

  • Genomic composition (DNA or RNA) and structure.
  • Nucleic acid structure: single or double-stranded DNA or RNA.
  • RNA viruses have different polarities: positive or negative sense.
  • Positive sense RNA viruses can start making proteins upon entering a cell, while negative sense RNA viruses need to replicate first.
  • Some RNA viruses can have segmented genomes, allowing for increased genetic capacity and minimizing genomic degradation risks.

  
• Virus classification is based on several criteria:

  • Genomic composition (DNA or RNA) and its structure.
  • Capsid structure, which can be icosahedral, helical, or complex.
  • Presence or absence of an envelope.
  • Morphology.
  • Replication strategy, classified using the Baltimore classification system.
• Naked (Unenveloped) Virus:

  • Structure: Consists of a protein shell called a capsid.
  • Capsid: Encloses the viral nucleic acid (DNA or RNA).
  • Function: Protects the viral genome and aids in its transmission.
  • Examples: Adenovirus, Norovirus.
• Enveloped Virus:

  • Structure:
  • Host-derived lipid bilayer envelope (blue).
  • Embedded viral glycoproteins (pink), often forming spikes on the surface.
  • Function: Allows the virus to enter host cells by interacting with cell receptors.
  • Examples: Influenza virus, Herpes virus.
• Family Name:

  • Ends in "-viridae."
  • Represents a group of related viruses sharing common characteristics.
  • Example: Herpesviridae.
• Genus Name:

  • Ends in "-virus."
  • Represents a subgroup within a family sharing more specific traits.
  • Example: Alphavirus.
• irus Species:

  • A group of viruses with identical genetic information and ecological niche.
  • Further divided into types and subtypes based on variations.
  • Example: Human herpesvirus 1 (HSV-1) and Human herpesvirus 2 (HSV-2).
• Viruses can be classified based on their capsid structure, which encloses the nucleic acid:

  • Icosahedral: These viruses have a highly organized, nearly spherical capsid structure.
  • Helical: Viruses with a helical capsid structure appear as cylindrical coils around the nucleic acid.
  • Complex: Complex viruses, such as poxviruses, have a capsid structure that does not fit into the icosahedral or helical categories.
• Icosahedral capsids have the following characteristics:

  • They have twelve vertices or points.
  • The capsid is composed of 20 triangular sides, known as facets.
  • Capsids are constructed from capsomers, which are the basic structural building blocks of the capsid.
  • There are two types of capsomers:
  • Penton capsomers
  • Hexon capsomers
• In icosahedral capsids, there are two types of capsomers: Penton and Hexon.

  • Penton capsomers are located at the vertices of the capsid.
  • Hexon capsomers fill the space between the penton capsomers as the capsid size increases.

  Smaller capsids have penton capsomers at each vertex, while larger capsids have hexon capsomers filling the spaces between them.
• • Penton capsomers: Found at the vertices of the capsid, one at each vertex, surrounded by five neighboring capsomers.
  • Hexon capsomers: Present between the penton capsomers, with six neighboring capsomers. The number of hexon capsomers can vary.