L10-12 viruses

• viruses infect all living things
• eat and breathe billions of virions regularly
• viruses are molecular fossils
• Virome is 


  viruses the have infected human across life, not causing infection as low level
• once infected, it is for life
• virus definition 


  simple, miniscule, infectious, obligate intracellular parasite. Comprises of genetic material surrounded by protein coat and or envelope derived from a host cell membrane
• genetic material can be

  DNA or RNA
• protein coat is capsid
• viruses can contain DNA or RNA as 


  single stranded or double stranded
• viruses have small number of genes, as rely on host for other proteins
• viruses are very small ranging from

  20-300 nm in diameter
• viruses can be seen with electron microscopes
• giant viruses can be seen under

  light microscope
• example of giant virus

  Pandora virus (infect Amoeba)
• viruses exist in 2 states

  extracellular virion and intracellular virus
• virion is for transmission (outside host cell)
• virus is for replication (after infecting host cell)
• capsid 


  protein coat that surrounds nucleic acid
• nucleocapsid 


  nucleic acid plus the capsid, may be enclosed inside an envelope containing proteins encoded by the virus
• naked viruses lack 


  envelope
• structural units are the smallest equivalent building units of the capsid
• capsomere is 


  structural protein units that make up the capsid
• viral replication

  • bursting of cell with lots of replicates of the virus
• bacterial multiplication

  • asexual reproduction into 2 daughter cells (binary fission)
• to determine structure

  • electron microscopy
  • X-ray crystallography
  • cryo-electron microscopy
  • nuclear magnetic resonance spectroscopy
• structure

  labels

  A) capsid

  B) capsomers

  C) nucleocapsid

  D) nucleic acid

  
• shapes of viruses

  • helical (spiral)
  • polyhedral (roughly spherical)
  • binal (irregular shapes/complex structure)
• capsid function

  • protect nucleic acid
  • sites to attach to host cell
  • proteins to be able to penetrate host cell membrane
• naked virion example 


  polio virus
• enveloped virion example 


  coronavirus
• hosts 


  can be any organism
• bacteria host 


  infected by T4 bacteriophages
• virophage infect other viruses
• specificity 


  each virus only infects specific host/group of hosts
• bacteriophage T4 infects 


  E.coli
• bacteriophages

  • most have dsDNA
  • complex
  • only few have envelopes
• replication of lytic bacteriophage T4

  • adsorption
  • penetration
  • replication
  • maturation (assembly and packaging of new virions)
  • release
  • reinfection
• lytic phage 


  kills host
• temperate phage's 


  don't kill host
• temperate phage

  • after infection, genome integrated into host chromosome
  • prophage produces repressor protein (blocks lytic genes)
  • prophage replicates in host