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.