VIROLOGY

• Dmitri Iwanowski tried to filter the sap of infiltrated tobacco plants


  1892
• Martin Beijerik ruled out the filterable toxin conclusion because the filtered sap is capable of causing undiluted infection

  1899
• Wendell Meredith Stanley discovered this agent after crystallization

  1935
• Tobacco Mosaic Virus (TMV)


  Tobacco mosaic virus viewed with transmission electron microscope
• Ivanovski


  Source of TMV (small bacteria or spore)
• Beijerinck


  Not bacteria "poison"
• Transmission electron micrographs

  Virus dwarfed by the bacterial cell it infects, E. coli cells dwarfed by cultured colon cells
• Frederick Twort and Felix d'Herelle

  Early 20th century discovery of bacteriophage - a virus that infects and replicates in bacteria
• Bacteriophage


  • Virus that infects bacteria
• Naked or Enveloped viruses

  Naked atadenovirus uses spikes made of glycoproteins from its capsid to bind to host cells, Enveloped human immunodeficiency virus uses spikes made of glycoproteins embedded in its envelope to bind to host cells
• Wendell Stanley electron microscope, TMV & Flu B Nobel Peace Prize

  1935
• Demonstration of "filterable agent" infectivity, for Tobacco mosaic virus and Foot and mouth disease virus


  1886-1898
• The first human virus described was the agent which causes yellow fever

  1901
• Rinderpest, Vaccinia, Rabies, and Cassava mosaic all shown to be filterable viruses


  1902-1906
• Avian leukosis, poliomyelitis and chicken sarcomas were shown to be caused by viruses


  1908-1911
• Bacterial viruses discovered


  1915-1917
• The "Spanish Flu" pandemic kills 50 – 100 million people


  1918-1922
• More than 65 viruses had been described including Yellow fever virus


  1920-1927
• Relative Size


  The size of a virus is small relative to the size of most bacterial and eukaryotic cells and their organelles
• Characteristics of Viruses


  • Small infectious agents (20nm to 300nm in diameter), Infections, acellular pathogens, Obligate intracellular parasites with host and cell-type specificity, DNA or RNA genome (never both), They lack genes for products needed for reproduction, Infect all types of cells – humans, animals, plants, bacteria, yeast, archaea and protozoa
• Viral Characteristics


  DNA or RNA genome is surrounded by a protein coat called a capsid, A capsid is composed of small structural unit called capsomers, Capsid + genome = nucleocapsid, Protects/coats the viral genome = capsid, Virus particle = virion, Capsid usually symmetrical, May have an envelope acquired from the nuclear or plasma membrane, modified by viral protein = peplomers
• Virus characteristics


  • Non-Living, Acellular, with no cell nucleus, organelles or cytoplasm, Cannot move and reproduce, Living, Can only reproduce in living cells, Obligate intracellular parasite, They can evolve, Genetic material
• Envelope


  Outer thin loose covering composed of proteins from the virus, lipids, and carbohydrates from the host, Present in HIV, Herpes virus, Many small projections called spikes present on the virus envelope
• Capsid


  Made up of small subunits of proteins called capsomeres, Outer covering for the protection of the genetic material of a virus
• Nucleic acid


  Contains the viral genetic information encoded as DNA or RNA, DNA-containing viruses are called deoxy viruses, RNA-containing viruses are called ribo viruses, A complete virus that consists of the genetic material, coat protein, and an envelope is called the virion
• Envelope


  Acquired from the nuclear or plasma membrane of the infected host cell, and then modified with viral proteins called peplomers, Viruses that do not have an envelope are referred to as nonenveloped viruses (naked viruses)
• DNA viruses


  • Papillomaviridae, Adenoriridae, Pavoviride, Polymaviridae
• RNA viruses


  • Caliciviridae, Piconaviridae, Reoviriade, Hepeviridae
• RNA viruses


  eg., hepatitis, polio, West Nile fever, influenza, SARS, and measles, Ss/sd RNA, High mutation rate, All plant viruses, Positive /Negative sense, +sense – mRNA, -sense – RNA polymerase
• DNA viruses


  DNA dependent and they replicate using DNA polymerase, ds DNA but in some cases, they can either be ssDNA, Eg., Bacteriophages, cyanophages, and most animal
• Types of viruses


  • Animal viruses: infect and live inside animal cells including humans, contain DNA or RNA as genetic material, eg.,rabies virus, influenza virus, poliovirus, mumps virus, etc.

  Plant viruses: viruses that infect plants, contain RNA as a genetic material, which remains enclosed in the protein coat, eg., potato virus, tobacco mosaic virus, beet yellow virus, turnip yellow virus etc.

  Bacteriophage: viruses that invade and infect bacterial cells, DNA as genetic material, Infect only one species or only one strain of bacteria
• Host and Viral Transmission

  Viruses can infect every type of host cell, including those of plants, animals, fungi, protists, bacteria, and archaea, Most viruses have a host range, Viruses can be transmitted through direct contact, indirect contact with fomites, or through a vector: mechanical or biological, Viruses lie in the threshold of life and nonliving, Zoonoses, Reverse Zoonoses
• Viral shapes


  • Helical – elongated tube-like structure, the nucleic acid is spiral, Capsomers are arranged helically around the coil

  Icosahedral – has a spherical shape with nucleic acid inside the the capsid, Composed of 20 triangular faces

  Spherical – round and some have spikes

  Complex – do not confirm into any category i.e., bacteriophages and poxvirus
• Viral classification/Nomenclature


  International Committee of Taxonomy of Viruses (ICTV)<|>Species - population of viruses that share a distinct pool of genes<|>Genus - a group of related viral species<|>Subfamily - group of related viral genera<|>Family - group of a related subfamily<|>Order - a group of family
• Criteria for Classification


  • Morphology - size and shape of the virion, presence of an envelope
  • Structural Protein - Contributes to the molecular weight of the virion
  • Nucleic acid properties - Double/single-stranded, positive/ negative sense
• DNA viruses


  • Double-stranded DNA genomes (dsDNA): most DNA viruses
  • Single-stranded DNA genomes (ssDNA): e.g., Parvoviridae
  • Linear: most DNA viruses
  • Circular: e.g., Papillomaviridae, Polymaviridae (supercoiled), Hepadnaviridae (incomplete)
• RNA viruses

  • Double-stranded RNA genomes (ds-RNA): Reoviridae
  • Single-stranded RNA genome (ssRNA): most RNA viruses
  • Positive-sense RNA virus (+ssRNA): e.g. Retroviridae, Togaviridae, Flaviviridae, Coronaviridae
  • Negative-sense-RNA (-ssRNA): e.g. Arenaviridae, Bunyaviridae, Paramyxoviridae, Orthomyxoviridae, Filoviridae, Rhabdoviridae
  • Linear: most RNA viruses
  • Circular: e.g., Arenaviridae, Deltaviridae
  • Segmented RNA viruses: Bunyaviruses: 3 segments, Arenavirus: 2 segments, Orthomoxyvirus: 8 segments, Reovirus: 10 -12 segments
• Virus life cycle for different genomes

  1. Make viral proteins

  2. Make copies of the viral genome
• Baltimore Scheme

  • Class I: Double-stranded DNA - e.g., Adenovirus, Herpes Simplex virus
  • Class II: Single-stranded DNA - e.g., Canine Parvovirus - dogs
  • Class III: Double-stranded (+/-) RNA - e.g., Rotavirus & Coronavirus
  • Class IV: Single stranded +RNA - e.g., Poliovirus
  • Class V: Single-stranded -RNA - e.g., Influenza viruses, Ebola viruses, Rabies virus
  • Class VI: single stranded +RNA- retroviruses
  • Class VII: dsDNA viruses that use reverse transcriptase - e.g. Hepatitis B virus
• Hepatitis B virus

  Class VII: dsDNA viruses that use reverse transcriptase<|>Gapped genomic dsDNA<|>mRNA transcription: host RNA pol<|>Translation: host ribosomes, tRNA<|>Coat protein + RT + RNA forms provirion<|>Genomic ssRNA + genomic RNA/DNA virion<|>DNA pol<|>With LTRs<|>In provirion<|>Matures to virion<|>Regulatory protein helps bias transcription