viruses bio 3

• Viruses


  Not included in the five kingdoms, on the boundary between what we regard as living and non-living. Consist of only nucleic acid surrounded by a protein coat, can only survive and reproduce inside a living cell
• Viruses


  • Smallest organisms, 20 to 300 nm, 50 times smaller than bacteria, cannot be seen with light microscope, pass through filters which retain bacteria

  
• Poliovirus


  20 nm in diameter
• Poxvirus


  400 nm long and 200 nm wide, causes smallpox
• Viruses reproduce, but only within infected living cells. They use the host cell's genetic information, replication mechanisms, and translational and transcriptional mechanisms
• Viruses cannot independently perform metabolic activities, they do not have the components necessary to carry out cellular respiration or to synthesize proteins and other molecules
• Viruses are highly specific to their hosts, each virus will recognise and infect only certain types of cell
• Viral structure


  Nucleic acid core (genetic material, either RNA or DNA), surrounded by a protein or lipoprotein coat called capsid, some viruses also have an outer membranous envelope

  
• Capsid


  Highly symmetrical protein coat, built up of identical repeating subunits called capsomeres, plays an important role in viral infection and provides protection
• Capsid shapes


  • Helical
  • Polyhedral
  • Complex combination of both
• Envelope


  Additional lipoprotein layer around the capsid, rich in proteins, lipids, and glycoprotein molecules, derived from the host cell's membrane and viral genes, plays a role in viral infection
• Viral genetic material


  Single type of nucleic acid, either DNA or RNA, may be linear, circular, single-stranded or double-stranded
• Bacteriophages


  Viruses that infect bacteria
• Bacteriophage morphology


  • T3 and T7 phages: icosahedral with short tails
  • T-even phages (T2, T4, T6): icosahedral head, connecting neck, long tail, complex base plate
  • Some tails can be contractile/non-contractile

  
• Bacteriophage lytic life cycle


  1. Attachment

  2. Penetration

  3. Replication

  4. Construction

  5. Release
• Bacteriophage lysogenic life cycle


  1. Viral DNA integrated into bacterial chromosome, host lives normally

  2. Viral DNA replicated during host cell division

  3. Viral DNA released from bacterial chromosome by UV radiation or chemicals, begins lytic cycle
• Diseases caused by DNA viruses

  • Poxviruses: smallpox, cowpox
  • Herpesviruses: genital herpes, chickenpox
  • Adenoviruses: respiratory tract disorders such as tonsillitis
  • Papoviruses: degenerative brain diseases, some cancers
  • Parvoviruses: infections in dogs
• Diseases caused by RNA viruses


  • Picornaviruses: polio, hepatitis A, aseptic meningitis
  • Togaviruses: rubella (german measles)
  • Orthomyxoviruses: influenza
  • Rhabdoviruses: rabies
  • Coronaviruses: upper respiratory infections, SARS
  • Flaviviruses: yellow fever, JE
  • Retroviruses (e.g. HIV): AIDS, some types of cancer