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Flashcards in Viruses III Deck (17)
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describe the classification tree of DNA viruses


describe the classification tree of RNA viruses


describe the 3 ways of producing viral proteins with a single function

  • transcription of individual monocistronic mRNA molecules from the genome
  • segmented genome where each molecule gives single monocistronic mRNA
  • production of a single long polyprotein that is later cleaved into individual functional peptides


when viruses replicate in the cytoplasm, they can either make their own _____ or be fitted with a 3D RNA structure known as _____

when viruses replicate in the cytoplasm, they can either make their own 5' cap or be fitted with a 3D RNA structure known as IRES


describe the class I viral genome replication

e.g. Herpesvirus

  • all DNA viruses (except poxviruses) replicates in the nucleus
  • immediate early proteins take over the cell command
  • early proteins target genome replication: like viral DNA polymerase
  • late proteins are structural: translocate back into the nucleus to form a capsid


describe class II viral genome replication

e.g. Parvovirus

  • class II is a ssDNA genome
    • most viruses are either (+ve) or (-ve) ssDNA
    • ssDNA is converted to dsDNA by host factors and DNA polymerases
    • ssDNA viruses will produce both (+ve) or (-ve) virions


describe viral classes III, IV and V

  • they all code RNA-dependent RNA-polymerase because they do not go through DNA intermediates during growth
  • class III and V carry viral RNA replicase within their virion
  • class IV is (+ve) ssRNA which is used directly as mRNA:
    • can be translated by the host machinery
    • eventually required to produce viral RNA-replicase for coping to (-ve) ssRNA and then generating (+ve) ssRNA genomes


describe class III viral genome replication (e.g. Rotavirus)


describe class IV viral genome replication

e.g. poliovirus

  • these are some of the smallest viruses
    • e.g. Hepatitis A virus, poliovirus
  • simple replication cycle
  • picornavirus mRNA does not have a cap and instead has a IRES element
    • viral RNA is translated as a polyprotein which is proteolytically cleaved to give rise to several polypeptides


describe class V viral genome replication

e.g. Rabies virus

  • rhabdoviruses, like rabies virus, and paromyxoviruses are representative of (-ve) ssRNA
  • rhabdoviruses must carry their own RNA-dependent RNA polymerase for initial mRNA production
  • rabies virus give rise to five mRNA molecules and a single long (+ve) ssRNA which serves as a template for genome replication (-ve) ssRNA


describe class VI viral genome replication

retroviruses (e.g. HIV)

  • enveloped viruses with complex replication that involves reverse transcriptase (RNA-dependent DNA polymerase)
  • contain 2 identical (+ve) ssRNA
  • the viral genome becomes permanently integrated anywhere in the host genome
    • known as a provirus and similar to bacteriophages without the excision part
  • 8% of the human genome is from retroviral origin
  • can be tumorigenic


describe class VII viral genome replication

e.g. hepatitis B virus

  • this class of dsDNA genomes has a partial single stranded DNA overhang
  • the DNA replication requires processing through RNA intermediate e.g. hepatitis B virus (HBV)
  • once inside the host cell, the partial dsDNA is completed and further transcribed into mRNA and pre-genomic (+ve) ssRNA
  • new partial dsDNA genomes are generated from pre-genomic ssRNA intermediates by reverse transcriptase (RNA-dependent DNA-polymerase)


describe the replication of hepatitis D virus (class V: (-ve) ssRNA)

  • HDV RNA genome is translocated from the cytoplasm to the nucleolus (steps 1-3)
  • the (-ve) ssRNA is transcribed by cellular RNA polymerase II and mRNA is subsequently transferred to the cytoplasm (steps 4-7)
  • hepatitis D antigen produced from HDV mRNA is translocated from the cytoplasm to the nucleolus
  • hepatitis B surface antigen is produced from mRNA of the HBV helper virus (steps 7-9)


describe recombination (interaction of viruses inside cells)

  • recombination between 2 viruses infecting the same host is possibly but usually happens with low frequency, e.g. HSV 1 and HSV 2


describe complementation (interaction of viruses inside cells)

  • occurs when one or both viruses which are simultaneously infecting the host cell have a defective function but at different gene locations
    • they can successfully propagate because of complementing and rescuing each others defective function(s) e.g. the relationship between helper HBV and defective HDV


describe antigenic drift vs. antigenic shift

  • antigenic drift: evolution due to small changes in the genome sequence within a define strain which usually occurs at a slower rate compared to antigenic shift 
    • e.g. influenza virus, rhinovirus
  • antigenic shift: usually occurs in segmented viruses, notably influenza virus, resulting in exchange (reassortment) of RNA segments between 2 different influenza viruses that subsequently gives rise to a highly pathogenic (reassorted) strain virus


describe phenotypic mixing

association of a genotype with a heterologous phenotype

  • phenotypic mixing or transcapsidation: genome of one virus randomly incorporated in capsid composed of proteins from a different, or both, viruses
  • pseudotypes: formed when the nucleocapsid of one virus acquires envelope from another type of virus