Viruses, Viroids, and Prions

Viruses

Differences Between Viruses and Cells

  • non-living biological particles
  • cannot exist independently
    • dependent on host cells to reproduce
  • can invade cells then remain dormant
    • if unable to infect, they eventually “die”
  • do not synthesize proteins
  • do not produce or use energy or waste
  • have capsids
    • protective protein coat
  • contain genetic material
    • DNA or RNA
  • can undergo replication
    • only with host cells

Virus Structure

  • capsid
    • covering made up of repeating protein molecules
    • gives a virus their specific 3D shape
  • core
    • contains RNA or DNA genetic material
    • may contain proteins
  • spikes
    • protein molecules on outer surface
    • used to attach to host cells that viruses invade
  • can be helical, spherical, complex, or polyhedral
  • 20-450 nanometres
    • smaller than a cell

Naked vs Enveloped Virus

  • naked virus contains only capsid and nucleic acid
  • enveloped virus contains envelope
    • when virus leaves host cell, cell membrane wraps around it

Virus Replication

  • undergo replication within a host cell
    • host can be prokaryotic or eukaryotic
  • host cell is hijacked and used to replicate viral DNA

Viral Life Cycles

Lytic Cycle

  1. Attachment

    1. virus attaches to cell

  2. Penetration

    1. cell engulfs virus

  3. Uncoating

    1. viral contents are released

  4. Synthesis

    1. viral RNA enters cell nucleus

  5. Assembly

    1. new phage particles are assembled

  6. Release

    1. viral particles are made and released

Lysogenic Cycle

  1. Attachment

  2. Provirus Formation

    1. viral DNA becomes part of host cells chromosomes

  3. Cell Division

    1. provirus replicates

  4. Departure

    1. provirus leaves

Viral Spread

  • spread differently depending on virus
  • spread vertically (from mother to unborn child)
  • spread horizontally (from person to person)
    • simple contact
    • coughing
    • sneezing
    • saliva
    • sexual contact
    • spread through insects
    • malaria

Virus Classification and Phylogeny

  • order, familia, genera, species
  • features
    • shape, size, genetic material
  • type of host cell, organ system, or species
  • method of replication through host
  • 80 known virus families
    • 22 infect humans

Viroids

  • small, infectious pieces of single stranded RNA
  • smaller than viruses
    • no capsid
    • less than 460 nucleotides long
  • RNA does not code for any proteins
  • mainly impact plants
  • interfere with normal RNA in host cell

Prions

  • proteins that are normally found within the body
    • convert from normal form to harmful particles
  • lacks RNA and DNA
  • found in brain and nervous tissue of infected animal
  • eating infectious tissue allows prions to travel via bloodstream to the brain
  • trigger normal proteins to fold abnormally
    • aggregates accumulate and leave gaps in brain tissue

Treatments for Viral Diseases

  • antiviral agents, vaccines

Anti Viral Agents

  • any chemicals used to treat viral diseases
  • developed largely for AIDS
  • do not destroy pathogens
    • inhibit development
  • specific antivirals for specific viruses
  • difficult to design safe and effective antiviral drugs
    • viruses use host cell to replicate
  • disable parts of viral proteins
    • should be unlike any proteins in humans
    • help to reduce side effects
  • can attack at any stage of virus cycle