Viruses

• Viruses

  • Not cells and not classified as living organisms (debated to be non-living)
  • Much smaller than cells
  • Have some type of genetic material in the form of DNA or RNA
  • Usually have some kind of protein coat, also known as a capsid
  • Can protect the DNA or RNA genetic material
  • Some have special enzymes or an outer envelope (these additions can be useful to virus replication)
  • Require a host cell to grow and reproduce (virus replication)
  • Very specific for the host that they're going to go infect
  • Anything alive can be a host because cells have the machinery viruses need in order to replicate
  • Feed off healthy cells, sometimes killing them
• Antiviral medications


  Can treat certain viral infections
• HIV

  • A virus that can lead to AIDS
  • Binds into a CD4 glycoprotein that is found on the surface of specific cells such as Helper T Cells
  • Play an important role in protecting the body in the immune system
  • A person with HIV can be made vulnerable to other infections
  • Medications have been developed to help keep HIV from replicating as quickly
  • Research looking into ways to keep the virus from attaching
  • When a virus cannot attach, it cannot insert its genetic material
  • Can mutate (treatment developed may not work on a mutated form)
• Viruses are much smaller than bacteria and visible only under an electron microscope
• Shapes of Viruses

  • Helical Viruses
  • Polyhedral (Icosahedral) or Isometric Viruses
  • Enveloped or Spherical Viruses
  • Complex Viruses (Bacteriophages)
• Helical Viruses

  • Consist of nucleic acid (either RNA or DNA) surrounded by a hollow protein cylinder or capsid
  • Structure resembles a helix or spiral
• Polyhedral (Icosahedral) or Isometric Viruses


  • Have nucleic acid surrounded by a polyhedral (many-sided) shell or capsid
  • Often forms an icosahedron shape (20 triangular faces)
• Enveloped or Spherical Viruses

  • Have nucleic acid surrounded by either a helical or polyhedral core
  • Covered by an outer envelope made of lipids
• Complex Viruses (Bacteriophages)


  • Have a more intricate structure
  • Bacteriophages, specifically, are viruses that infect/attack bacteria
  • Consist of a head, sheath, and tail
• Despite its negative effects, some viruses are useful in gene therapy and some, having insects as its target, can be an alternative to chemical pesticide (for pest insects, though it can still have harmful effects to the ecosystem)
• Useful viruses

  • Nuclear Polyhydrosis Virus
• Lytic Cycle

  1. Virus (selectively) attaches to host cell

  2. Virus injects DNA/RNA into cell

  3. Host cell takes in virus genetic material

  4. Host cell uses own resources to make copies of virus

  5. Host cell membrane ruptures, releasing new viral copies to infect other cells
• Lysogenic Cycle

  1. Virus does same as lytic cycle initially

  2. Viral genetic material stays hidden in host genetic material

  3. When host replicates, viral genetic material also replicates

  4. Virus can later get triggered into lytic cycle
• Triggerfor the Lysogenic Cycle

  Chemical trigger or lack of food for host
• Retrovirus

  A type of virus that uses RNA as its genetic material
• Retrovirus life cycle

  1. Infects cell

  2. Employs reverse transcriptase to create DNA copy of RNA

  3. Integrates DNA into host cell's genome (becomes provirus)

  4. Actively transcribes DNA when host DNA is transcribed

  5. Host cell produces RNA (mRNA) required for assembling new virus particles

  6. Buds off, using host's membrane to envelope itself
• Retrovirus

  • Reverse Transcriptase: reads viral RNA and reverse transcribes it into complementary DNA (cDNA)
  • Integrase: clips off 3' ends of cDNA, forming sticky ends, integrates retroviral DNA into host's DNA
  • Protease: formed as RNA enters cytoplasm, cleaves other proteins to ensure they're fully functional
• HIV (human immunodeficiency virus)


  A well-known retrovirus
• HIV life cycle

  1. Enters cells through direct fusion

  2. Undergoes uncoating

  3. Integrates its DNA into the host's genome