Virology: Baltimore classifications

Question 1

DNA viruses

Answer 1

• Enveloped (dsDNA)
  
  • Pox
  • Herpes
  • Hepadna
• Naked capsid (dsDNA)
  
  • polyoma
  • papilloma
  • adeno
  • parvo

Question 2

RNA viruses

Answer 2

• • RNA
    
    • Naked
      
      • Picorna
      • Calici
    • Enveloped
      
      • Toga
      • Flavi
      • Corona
• • RNA
    
    • Enveloped
      
      • Rhabdo
      • Filo
      • Orthomyxo
      • Paramyxo
      • Bunya
      • Arena
• +/- RNA
  
  • double capsid
  • reo
• • RNA via DNA
    
    • Retro (enveloped)

Question 3

Baltimore classification of viruses: Class I

Answer 3

• Class I: dsDNA
  
  • Papilloma, polyoma, herpes, adeno, pox
  • some can transform cells
  • host enzymes transcrobe IE or E mRNA
  • E-proteins: replicate parental DNA & late (L) mRNA;
  • L-proteins: assemble into capsids
  • Capsids: incorporate the viral DNA to make mature virus

Question 4

Class II

Answer 4

Class II: ssDNA

• very small (parvum=small), circo- or parvo-virus (linear so susceptible to DNAses)
• replication occurs in nucleus
• form a (-) strand -> template for (+) strand
• require cell enzymes
• mRNA translated into viral proteins
• NOTE: no immediate early or early proteins
• viral proteins: make up capsids
• viral DNA is encapsidated in progeny virus
• Parvovirus can only replicate when cells are actively replicating
• icosahedral, non-enveloped

Question 5

Class III

Answer 5

Class III: ds RNA Virus

• dsRNA: cannot function as mRNA; class III must package an RNAP to make mRNA after infection of host cell
• genomes segmented
• RNA-dependent RNAP in virion
• Ex. Reo-, Birnaviruses, rotavirus
• ds RNA -> transcribed to viral mRNA (v-mRNA) by virus-associated RNA-dependent RNAP
• v-mRNA: translated to structural and non-structural proteins:
  
  • viral proteins to form immature capsid
  • mRNA packaged into immature capsid, then copied w/in capsid to form ds RNA
• Rotaviruses, members of Reoviridae
  
  • acute gastro in infants and young children
  • icosahedral
  • 11 dsRNA segments
  • RNA replication: concurrent w packaging of genome segments

Question 6

Class IV

Answer 6

Class IV: (+) sense RNA viruses

• Divided into IVa & IVb
• always infectious w/o viral proteins
• ends of genomes often modified w caps
• Ex. Picorna-, toga-, corona-, flaviviruses, dengue

IVa

• polyprotein: post-translational cleavage make multiple viral proteins
• non-structural proteins: transcribe parental (+) RNA into (-) RNA whih in turn is transcribed to (+) RNA
• Structural proteins: make up capsids, incorporate viral RNA - progeny virus

IVb

• stuctural proteins (not one polypeptide)
  
  • parental RNA transcribed into (-) RNA by viral non-structural proteins
  • ss (-) RNA -> transcribed into (+) RNA (many copies)
• Capsid incorporates the (+) RNA, other viral proteins -> progency virus

IV: Picornaviruses

• naked, small, icosahedral
• genome is mRNA (naked genome is suffcient for infection)
• virus replicates in cytoplasm, most are cytolytic
• v-RNA: translated into polyprotein; cleaved into enzymatic & structural proteins
• include poliovirus, hepA, rhino- and enteroviruses

Question 7

Class V

Answer 7

Class V: ss (-) sense RNA viruses

• virus-associated, premade polymerase reads (-) RNA
• segmentation common
• genome by virus-specfic polymerase (premade); reads (-) RNA -> forms (+) RNA
• assembly of capsid -> incorporation of genomic RNA and viral proteins, including the virus RNAP -> progeny virus
• Ex. Filiviruses, Orthomyxo-, Bunya- (Hantavirus), Paramyxo-viruses

Question 8

Class VI

Answer 8

Class VI: ss (+) sense RNA viruses (RT)

• 2 copies of +ve strand RNA (not ds); premade RT generates ds provirus
• retroviruses (HIV)
• Provirus: integrates into host genome, template for synthesis of viral mRNA
• Viral mRNA: viral non-structural and viral structural proteins
• not directly infectious on its own
• Non-structural proteins: may be RT and other polymerases
• Structural proteins: assemble into the capsid
• Mature virion: the capsid incorporates viral genome AND the nucleoproteins including RT -> progeny virus

Question 9

Class VII

Answer 9

Class VII: ds DNA viruses, ssRNA intermediate, RT

• Ex. Hepadnaviruses including HBV
• VII: partially ds DNA virus comes in w its own RT
• core is stripped of nucleic acids
• fills in gaps
• ds supercoiled DNA
• pre-genomic RNA
• structural proteins made and then assemble into a capsid
• pre-genomic RNA goes into capsid; this is ss
• maturation: RT makes a RNA-DNA hybrid in capsid
• RNAase activity digests RNA
• DNA polymerase activity makes the partially dsDNA
• comes out of Golgi and goes out of cell
• VII: HBV
  
  • enveloped virion w partially ds circular DNA genome
  • replication is thru a circular RNA intermediate
  • virus encodes and carries RT
  • HBV has a strict tissue tropism to liver
  • HBV genome can integrate into host chromosome