Viruses

• Viruses


  • Acellular infectious agents
  • Lack nucleus, cytoplasm, organelles, plasma membrane
  • No mitochondria and no ATP generation
  • Lack ribosomes and transfer RNA
  • Too small (20 and 500 nm in diameter) to be seen by light microscope

  
• Viruses


  • Smallest and simplest forms of life on Earth
  • Obligatory intracellular parasites
• Components of viruses


  • Genetic material (DNA or RNA)
  • Capsid (protein coat)
  • Envelope (lipid-bilayer that surrounds the capsid)
  • Peplomer (glycoprotein spike on a viral capsid or viral envelope)

  
• Genetic material of viruses

  • All viruses contain genomes made of one and only one type of nucleic acid
  • The genome can be either DNA or RNA, linear or circular, single-stranded or double-stranded
  • Possess genes to invade and regulate the host metabolic activity
  • No viral metabolic genes (it uses Host's metabolic machinery)

  
• Capsid


  • Protein shell of virus that protects nucleic acid
  • Composed of capsomeres (building blocks of the capsid)
  • Capsomere spikes (used for binding to target cell receptors)

  
• Shapes of viruses


  • Helical
  • Icosahedral
  • Complex

  
• Envelope


  • Derived from host cell membranes (phospholipids and proteins) + some viral-encoded glycoproteins
  • Glycoproteins (spikes) on the surface of the envelope serve to identify and bind to receptor sites on the host's cell membrane
• Reasons to study viruses

  • Viruses are important disease-causing agents
  • Viruses can infect all forms of life
  • Viruses are the most abundant form of life on Earth
  • Study of viruses has led to numerous discoveries in molecular and cell biology

  
• There are 10–50 million bacteriophages on average per mL of seawater, and even more in many soils
• There are 10^31 bacteriophages in the world
• The 10^31 phages, if lined up head-to-tail, would stretch some 200 million light years into space—that is, far into the universe beyond many of our known neighboring galaxies
• Classifications of viruses


  • Based on disease
  • Based on host organism
  • Based on virus particle morphology
  • Baltimore classification

  
• Baltimore classification


  • Double stranded DNA (dsDNA) viruses
  • Single stranded DNA (ssDNA) viruses
  • Double stranded RNA (dsRNA) viruses
  • Single stranded RNA + (SsRNA) viruses
  • Single stranded RNA - (SsRNA) viruses
  • Positive-sense ssRNA reverse transcriptase viruses
  • Double stranded DNA (dsDNA) reverse transcriptase viruses