Lesson 4.2

• Viral Taxonomy

  • International Committee on Taxonomy of Disease
  • Viruses of medical importance - orders end with "-virales", families end with "-viridae" based on: genome structure, morphology, replication "strategies", genera ends with "virus", based on: biologic, genomic, physicochemical, and serologic characteristics
• Basis of Virus Classification

  • Virion morphology - size, shape, symmetry
  • Virus genome - RNA/DNA, ss/ds, (+)/(-) sense genome size
  • Genome organization and replication (gene order)
  • Virus protein properties - number, size, amino acid sequence
  • Antigenic properties
  • Physicochemical properties - pH and temperature stability
  • Biologic properties
• DNA viruses

  • dsDNA

  -enveloped

  1. Icosahedral: Herpes, human herpes viruses
  2. Complex coats: Pox, Variola, Hepadna, Hepa B

  -naked

  1. Icosahedral: Papova, Adeno, Papilloma polyoma, adenovirus

  • ssDNA

  -naked

  1. Icosahedral: Parvo, Adenovirus, B19
• RNA viruses

  • dsDNA

  -naked

  1. Icosahedral: Reo, Rotavirus

  • Ambisense (-)

  -enveloped

  1. Complex: Arena, Lassa virus
  2. Helical: Bunya, Hantavirus
• RNA viruses

  • (-)ssRNA

  -enveloped

  1. Helical: Paramyxo, Orthomyxo, Rhabdo, Mumps disease, Influenza, Rabies
  2. Complex: Filo, Ebola virus

  • (+)ssRNA

  -enveloped: Toga, Flavi, Corona, Retro, Rubella, Dengue Hepa C, SARS 1&2, HIV and HTLV

  -naked: Calici, Picorna, Astro, Norovirus, Polio, HAV, Coxsackie, Rhino, Astrovirus
• DNA viruses classification

  1. DS-DNA except Parvovirus
  2. Enveloped except Parvovirus, Adenovirus, Pavovavirus
  3. Icosahedral except Poxvirus
  4. Multiply in the nucleus except Poxvirus (cytoplasm)
• RNA viruses classification

  1. SS-RNA except Reovirus
  2. Enveloped except Picornavirus, Calicivirus, Reovirus, Astrovirus (PCRA)
  3. Non-segmented except Reovirus, Orthomyxovirus, Bunyavirus, Arenavirus (ROBA)
  4. Icosahedral - (+) sense; Helical - (-) sense
• Baltimore Classification

  1. dsDNA
  2. ssDNA
  3. dsRNA
  4. (+) ssRNA
  5. (-) ssRNA
  6. (+) ssRNA with DNA intermediate
  7. dsDNA with RNA intermediate
• Type I: dsDNA Viruses

  • dsDNA - mRNA - protein
  • dsDNA - dsDNA

  -own DNA polymerase: Papovaviridae (Papillomaviruses and Polyomaviruses)

  -host DNA polymerase: Poxviridae, Adenoviridae, Herpesviridae
• Type II: ssDNA viruses

  • ssDNA - dsDNA - mRNA - protein
  • dsDNA - dsDNA
  • Example: Parvoviridae
• Type VII: dsDNA with RNA intermediate

  • aka gapped DNA viruses
  • portions: ds, ss, RNA, protein
  • cannot be transcribed to mRNA as is

  -repair gapped genome first

  -gapped -dsDNA - mRNA - protein

  -mRNA -RT - ssDNA - gapped dsDNA

  • example: Hepadnaviridae
• RNA viruses

  • no pathogens have RNA genomes except viruses
  • cells are incapable of replicating RNA
  • RNA viruses can replicate RNA by: encoding RNA-dependent RNA polymerase (RdRp) and carrying RdRp with the virus particles
  • From RNA, RdRp can produce: RNA replicates and mRNA
• RNA viruses and mRNA

  • mRNAs, known as the (+) strand, are read from 5' - 3'
  • RNAs that are complimentary to the mRNA/ (+) strand are termed as (-) stranded RNAs
  • RNA viruses carry: (+)ssRNA, (-)ssRNA, and dsDNA
• Type III: dsDNA viruses

  • has both (+) and (-) strands
  • ribosomes cannot translate despite the (+) strand

  -dsRNA - RdRp - mRNA - protein

  -mRNA - dsRNA

  • RdRp is carried in the virion itself
  • Example: Reoviridae
• Type IV: (+) ssRNA viruses

  • readily "translatable"

  -(+)ssRNA - protein

  -(+)ssRNA -RdRp - (-)ssRNA - (+)ssRNA

  • example: Picornaviridae, Astroviridae, Caliciviridae, Togaviridae, Flaviviridae, Coronaviridae
• Type V: (-ssRNA) Viruses

  • Ribosomes cannot translate and cell cannot make mRNA from it

  -(-)ssRNA -RdRp - (+)ssRNA (mRNA) - protein

  -(+)ssRNA - (-)ssRNA

  • RdRp is carried in the virion itself
  • example: Orthomyxoviridae, Paramyxoviridae, Rhabdoviridae, Filoviridae, Arenaviridae, Bunyaviridae
• Type VI: ssRNA with DNA intermediate

  • (+)ssRNA "translatable" but does not occur

  -(+) ssRNA -RT- (-)ssDNA - dsDNA - mRNA - protein

  -dsDNA-(+)ssRNA

  • RT is carried in the virion itself
  • Example: Retroviridae