Viral Characteristics And Structures

Chapters 24, 25: Viruses Part 1 - General Characteristics

Virology

  • The study of viruses is known as VIROLOGY.
  • Virus in Latin means “poison”.

North American Viral Examples

  • HIV/AIDS
  • Hantavirus pulmonary syndrome (HPS)
  • West Nile encephalitis (WNE)
  • Severe acute respiratory syndrome (SARS)
  • Monkey pox
  • Human metapneumovirus
  • 2009 H1N1 pandemic influenza

Virus Associations and Impact

  • Viruses are associated with certain diseases: Chronic fatigue, Prostate cancer, Multiple sclerosis, Diabetes
  • Viral illnesses in the US account for the most cases of illness in the US each year.
  • Some viral illnesses are asymptomatic!

Usefulness of Viruses

  • Vaccine development
  • Gene therapy
  • Tools to investigate host cells

Definition of a Virus

  • "A piece of bad news wrapped up in a protein"

Characteristics of Viruses

  • Nucleic acid genome:
    • DNA or RNA (never both)
  • Protein coat:
    • Protection
    • Entry
  • Small size: 20 – 1400 nm
  • Obligate intracellular parasites:
    • Cannot multiply outside the host cell
    • Use the host cell enzymes, protein synthesis machinery, nutrients, etc. to replicate

Virus Sizes and Shapes

  • Viruses exhibit a wide range of sizes and shapes. Examples include:
    • HPV1
    • Influenza
    • Hep. C
    • Herpes
    • HIV
    • Rubella
    • West Nile
    • Rotavirus
    • Coronavirus
    • Adenovirus
    • Bacteriophage T4 (170 x 70 nm)
    • Rhinovirus (30 nm)
    • Bacteriophage M13 (800 x 10 nm)
    • Chlamydia elementary body (300 nm)
    • Bacteriophages f2, MS2 (24 nm)
    • Tobacco mosaic virus (250 x 18 nm)
    • Prion (200 x 20 nm)
    • Poliovirus (30 nm)
    • Vaccinia virus (300 x 200 x 100 nm)
    • Viroid (300 x 10 nm)
    • Ebola virus (970 nm)
  • For scale:
    • E. coli (a bacterium) is 3000 x 1000 nm
    • Human red blood cell is 10,000 nm in diameter
    • Plasma membrane of red blood cell is 10 nm thick

Consequences of Virus Specificity

  • No broad-range drugs are available.
  • Need to look for weak links in viral structure or processes.

Viral Structure

  • Virion – complete, fully infectious viral particle
  • Capsid – protein coat surrounding genome
    • Capsid is usually symmetrical
  • Envelope – if present, lipid (bilayer), protein, carbohydrate structure that surrounds capsid

Virion components

  • A virion is composed of nucleic acid (viral genome) surrounded by a capsid.

Types of Capsids

  • Helical: Structure of Tobacco mosaic virus
  • Icosahedral: Example shown is West Nile Virus

Enveloped Virus

  • Enveloped virus with helical capsid
  • Components include: Glycoproteins, Envelope, Helical capsid