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lecture 14

Viruses - General Principles

  • Non-living obligate intracellular parasites.
  • Contain nucleic acid (DNA or RNA) and a protein coat (capsid).
  • Sometimes have an envelope (lipids, carbohydrates, and proteins).
  • Specific regarding the types of cells they can infect (host range).
  • Can be seen by electron microscopy only (20 to 1,000 nM).

Viral Structure

  • A virion is a mature, infective particle.
  • Genomes: RNA or DNA (never both), single or double-stranded, linear or circular.
  • Capsid: protein coat, consists of capsomeres.
  • Envelope: may have spikes (carbohydrate-lipid complexes).

General Morphology

  • Helical viruses: capsomeres in a helical configuration (long rods).
  • Polyhedral viruses: many-sided, capsid is an icosahedron (20 triangular faces).
  • Enveloped viruses: helical or polyhedral, acquire envelope from host cell membrane. Naked viruses lack an envelope.
  • Complex viruses: polyhedral head with a complex tail (bacteriophages).

Taxonomy of Viruses

  • Based on nucleic acid type, life-cycle strategy, and morphology.
  • Grouped into families, genera, and species.
  • A viral species shares the same genetic information and ecological niche.
  • Viral species names are not binomial (e.g., influenza virus, HIV).

Taxonomy Examples

  • RNA Viruses
    • HIV (Human Immunodeficiency Virus): Family Retroviridae, Genus Lentivirus
    • Poliovirus: Family Picornaviridae, Genus Enteroviridae
    • Rabies Virus: Family Rhabdoviridae, Genus Lyssavirus
  • DNA Viruses
    • HPV (Human papillomavirus): Family Papovaviridae, Genus Papillomavirus
    • HSV (Herpes simplex virus): Family Herpesviridae, Genus Simplexvirus

Viral Cultures

  • Host cells must be present.
  • Bacteriophages: incubated on a lawn of bacteria, plaques develop where they multiply.
  • Animal viruses: may require a living host or cell cultures.

Viral Replication Strategies

  • Viruses use host cell machinery to produce viral components.
  • Viral infection turns a cell into a virus production factory.

Bacteriophages

  • Viruses that infect bacteria.
  • Almost all carry their genomes as double-stranded DNA.
  • Almost always have a complex morphology.
  • Most widely studied are viruses of E. coli (T-even phages and phage lambda).

Bacteriophages: Lytic and Lysogenic Cycles

  • Lytic cycle: phage causes lysis and death of host bacterium.
  • Lysogenic cycle: phage incorporates its nucleic acid into the host chromosome and remains dormant (temperate phages).

Stages in the Lytic Cycle

  • Attachment: tail fibers attach to receptor sites on the bacterium.
  • Penetration: tail sheath contracts, phage DNA enters the cell.
  • Biosynthesis: transcription, translation, and replication of viral DNA occurs.
  • Maturation: new phage DNA and capsids assembled into virions.
  • Release: phage lysozyme destroys the cell wall, cell bursts, and new virions are released.

Stages in the Lysogenic Cycle

  • Attachment/Penetration: same as in the lytic cycle.
  • Viral DNA recombines with bacterial chromosome to form a prophage.
  • Prophage replicates with bacterial DNA until triggered to excise itself.
  • Then continues with the steps of the lytic cycle.