virus

• Viruses


  Smallest structures of infectious agents that pass through filters and cannot provide energy/synthesize proteins on their own
• Viruses


  • Obligate intracellular pathogens, acellular infectious agents that require the presence of a host cell in order to multiply
• Viruses


  Minuscule, acellular, infectious agents having either DNA or RNA that infect all types of cells - humans, animals, plants, bacteria, yeast, archaea, protozoa
• Viruses cause most of the diseases that plague the industrialized world
• Viruses


  • Cannot carry out any metabolic pathway, neither grow nor respond to the environment, cannot reproduce independently
• Viruses


  Recruit the cell's metabolic pathways to increase their numbers, have extracellular and intracellular state
• Viruses


  • Most viruses infect only particular host's cells due to affinity of viral surface proteins for complementary proteins on host cell surface, may be so specific they only infect particular kind of cell in a particular host
• Viruses


  • HIV - Immune cells (T cells), West Nile virus - most bird, several mammals
• Virion


  The entire infectious unit, the extracellular state of a virus
• Virion


  The main function is to deliver its genome into the host cell so that the genome can be expressed (mRNA and protein expression) and the virus is replicated within the host cell
• Components of a simple virus (virion)

  • Nucleic acid (single- or double-stranded RNA or DNA)
  • Protein coat (capsid made up of capsomeres)
• Nucleocapsid


  The nucleic acid genome plus the protective protein coat
• Enveloped viruses


  Obtain their envelope by budding through a host cell membrane
• Naked virus


  Has only a protein capsid covering it
• Capsid


  • Provides protection for viral nucleic acid, means of attachment to host's cells
• Capsomeres


  Proteinaceous subunits that make up the capsid
• Viral structure


  • Naked
  • Enveloped
• Naked viruses are resistant to heat, acid, proteases, detergents, dryness and are easily transmitted through objects, surfaces, and hand-to-hand</b>
• Naked viruses can be transmitted via fecal-oral route
• Enveloped viruses are fragile, labile to treatment with acid and detergents, and must remain wet and spread in respiratory droplets, blood, mucus, saliva and semen, by injection, or organ transplants
• Genome structure


  • DNA or RNA, Single-stranded or double-stranded, Segmented or nonsegmented
• Nonsegmented genome


  All on one piece of RNA or DNA
• Segmented genome


  Several fragments of DNA/RNA that make a complete virus genome
• The segmented nature enables different strains of influenza virus to exchange their genes upon co-infection of a single host, leading to progeny carrying genetic information of different parental viruses
• Pigs are considered as reservoirs for human and avian flu gene collections as they have receptors for both avian and swine/human, and are susceptible for all these viruses
• DNA Viruses


  • Papillomavirus and Polyomavirus
  • Human Herpesviruses
  • Adenovirus
  • Parvovirus
• RNA Viruses


  • Picornavirus
  • Noroviruses
  • Astrovirus
  • Reoviruses
  • Rhabdoviruses, Filoviruses, Bornaviruses
  • Togaviruses and Flaviviruses
  • Bunyaviruses and Arenoviruses
  • Retroviruses
• Enveloped Viruses


  • Coronavirus
  • Paramyxovirus
  • Orthomyxovirus
  • Hepatitis viruses (HCV, HDV)
• Naked Viruses


  • Picornavirus
  • Noroviruses
  • Parvovirus
  • Hepatitis viruses (HAV, HEV)
• Capsid symmetry


  • Icosahedral / cubic/ spherical, helical (sarmal) / pleomorphic, Complex
• Icosahedral capsids


  Formed independently of nucleic acid, contain some "empty" particles devoid of nucleic acids
• Helical symmetry


  Protein subunits bound in a periodic way to the viral nucleic acid, winding it into helix, length determined by length of nucleic acid, no "empty" helical particles
• Complex virus structures


  • Poxviruses - brick-shaped, with ridges on the external surface and a core and lateral bodies inside, Phages - have head structure which can vary in size and shape, some icosahedral, others filamentous, some have tails attached to the phage head
• Infectious route


  • Arboviruses (via vectors)
• Diseases caused


  • Encephalitis, hepatitis viruses
• Biochemical properties


  • Structure and replication mechanism: based on DNA or RNA
• Baltimore classification


  Clusters viruses into families depending on their type of genome and the mode of replication and transcription
• Baltimore virus classes


  • Class I - dsDNA viruses
  • Class II - ss DNA viruses
  • Class III - ds RNA viruses
  • Class IV - ss (+)sense RNA viruses
  • Class V - ss (-)sense RNA viruses
  • Class VI - ss (+)sense RNA viruses with DNA intermediate
  • Class VII - ds DNA viruses with RNA intermediate
• (+) sense RNA viral genome

  Can directly act as cellular mRNA, can bind to ribosomes, can be used directly in protein synthesis, sufficient to start infection itself i.e. infectious
• (+) sense RNA viruses

  • Picornaviruses, Coronaviruses, Flaviviruses, Togaviruses, Retroviruses