Viruses

• Viruses


  Obligate intracellular parasites of living but non-cellular nature

  
• Viruses


  • Outside of the cell, they are inert
  • Recognized by the diseases they cause in plants, animals and prokaryotes

  
• Types of nucleic acid in viruses

  • Single-stranded DNA (sDNA)
  • Double-stranded DNA (dDNA)
  • Single-stranded RNA (sRNA)
  • Double-stranded RNA (dRNA)
• Viruses


  • Contain a protein coat (some are enclosed by an envelope) around the nucleic acid
  • Some have spikes
  • Infect only specific types of cells in one host
  • Multiply inside living cells by using the synthesizing machinery of the host cell

  
• Host range


  Spectrum of host cells the virus can infect, determined by specific host attachment sites and cellular factors
• Bacteriophages or phages


  Viruses that infect bacteria

  
• Virion


  A complete, fully developed, infectious viral particle composed of nucleic acid and surrounded by a protein coat outside a host cell

  
• Viral structure


  • Nucleic acid is protected by a capsid
  • Protein subunits called capsomeres make up the capsid
  • Capsid may be covered by an envelope
  • Some have spikes covering the envelope used to attach to host cells

  
• Growing viruses


  1. Viruses must be grown in living cells, never in culture media

  2. Bacteriophages form plaques on a lawn of bacteria

  3. Animal viruses may be grown in living animals or in embryonated eggs

  
• Criteria for viral taxonomy

  • Nucleic acid
  • Morphology
  • Strategy for replication
  • Symptoms

  
• Viral species


  A group of viruses sharing the same genetic information and ecological niche (host). Common names are used for species
• Viral subspecies


  Designated by a number e.g. HHVII
• Viral multiplication


  1. Virus must invade a host cell and take over the host's metabolic machinery

  2. This process can drastically change the host cell and usually causes its death

  
• Lysogenic cycle


  • Lysogenic cells become immune to reinfection by the same phage
  • Phage conversion - host cell exhibits new properties (e.g. C. botulinum toxin, E. coli Shiga toxins encoded by prophage)
  • Specialized transduction - prophage carries bacterial DNA with it when excised

  
• Oncogenes


  Mutated and/or overexpressed version of a normal gene of animal cells; can convert a cell into a tumor cell
• Oncoviruses


  Viruses capable of inducing tumors in animals

  
• Oncoviruses


  • After transforming cells, they have tumor-specific transplantation antigen (TSTA) on the cell surface or T antigen in the nucleus
  • Oncolytic viruses infect and lyse cancer cells

  
• Examples of DNA viruses

  • Adenoviridae
  • Herpesviridae
  • Poxviridae
  • Papovaviridae
  • Hepadnaviridae

  
• Examples of RNA viruses


  • Retroviridae (viral RNA is transcribed to DNA which can integrate into host DNA, e.g. Human T cell leukemia viruses HTLV 1, HTLV 2)

  
• Latent viral infections


  Virus remains in asymptomatic host cell for long periods (e.g. cold sores, shingles)

  
• Persistent viral infections


  Disease processes occur over a long period, generally fatal (e.g. subacute sclerosing panencephalitis caused by measles virus)
• Prions


  Proteinaceous infectious particles that cause mad cow disease and Creutzfeldt-Jakob disease

  
• Plant viruses


  • Resemble animal cells
  • Can possibly infect insects
  • Can infect economically important crops
  • Enter plant cells through wounds as plant cells are generally protected by cell walls

  
• Viroids


  Short pieces of naked RNA that can cause plant diseases