viral structure

• Viruses


  A unique group of pathogens with a simple a cellular organization and a distinct pattern of multiplication
• Despite their simple structure, viruses are a major cause of disease
• Viruses


  • Have no cytoplasmic membrane, cytosol or functional organelles
  • Can infect all types of cells
  • Can also infect bacteria, which are called bacteriophages
• Viruses and bacteriophages are not capable of metabolic activity on their own
• Virus replication


  1. Invade other cells

  2. Use host's metabolic machinery

  3. Produce more viral molecules, nucleic acid and proteins

  4. Assemble into new viruses
• Virion


  The virus in the extracellular state, not capable of reproducing
• Virion


  • Consists of a protein coat called a capsid surrounding a nucleic acid core containing the genetic material
  • Some variants have a phospholipid membrane derived from the host cell called an envelope, surrounding the nucleocapsid
• Enveloped viruses


  Viruses that have an envelope, e.g. herpes viruses and HIV
• Non-enveloped or naked viruses


  Viruses that lack the envelope, e.g. poliovirus
• Virus in the intracellular state


  1. Capsid is removed

  2. Virus becomes active

  3. Virus exists solely as nucleic acids

  4. Induces host to synthesize viral components

  5. Viral components assemble into new virions

  6. Virions are eventually released
• Capsid


  • Protects the viral genome
  • Aids in transfer between host cells
• Virus shapes


  • Helical
  • Icosahedral
  • Complex
• Helical viruses


  • Have a capsid with a central cavity or hollow tube, made by proteins arranged in a circular fashion
  • Disc shapes are attached helically, creating a tube with room for the nucleic acid
• Helical virus


  • Tobacco mosaic virus
• Icosahedral viruses


  • Made up of equilateral triangles fused together in a spherical shape that fully encloses the genetic material
  • Released into the environment when the cell dies, breaks down and lyses
• Icosahedral viruses


  • Poliovirus, Rhinovirus, Adenovirus
• Complex viruses


  • Have a combination of icosahedral and helical shape
  • May have a complex outer wall or head-tail morphology
• Head-tail morphology


  • Unique to viruses that infect bacteria (bacteriophages)
  • Have an icosahedral shaped head and a helical shaped tail
  • Use the tail to attach to the bacterium, create a hole in the cell wall, and insert their DNA into the cell
• Viral genome


  The total genetic content of the virus, usually small and containing only a few genes encoding proteins needed by the virus but not present in the host cell
• Viral genomes


  • Can be DNA or RNA, single-stranded or double-stranded, linear or circular
• Replication of DNA viruses


  1. Viral DNA enters the nucleus

  2. Uses host's DNA polymerase to create more copies of itself and viral mRNA

  3. Viral mRNA is translated into viral proteins
• Replication of RNA viruses


  1. Stay in the cytoplasm

  2. Viral RNA is used by host's ribosomes to create viral RNA polymerase proteins

  3. RNA polymerases help replicate the viral genome
• RNA polymerases are more likely to make copying errors than DNA polymerases, leading to more frequent mutations in RNA viruses