Replication in Viruses (HL) (HL IB Biology)

Viral replication

• Being non-living, viruses do not undergo cell division
• All viruses are parasitic, meaning they can only reproduce by infecting living cells, referred to as host cells
• To replicate all viruses must:
  • Attach to a specific attachment site on the host cell
  • Inject their nucleic acid into the cytoplasm of the host cell
  • Use the protein synthesis machinery of their host cell to produce viral proteins
  • Assemble new viral particles
  • Release the new viral particles from  the host cell
• Viral replication occurs via a lytic pathway, but some viruses undergo a series of events known as the lysogenic pathway in between reproductive cycles

The lytic pathway

• This is named as such because the new virus particles are released during lysis (bursting) of the host cell
  • This is caused by an enzyme called lysozyme which is coded for by the virus genetic material
• Lysis occurs after the production of fully functional virus particles called virions
• The steps of the lytic pathway are as follows:
  1. The virus attaches to the cell membrane of the host cell using attachment proteins
  2. The virus infects the host cell by injecting its DNA into the cytoplasm
  3. Next, the virus uses proteins and enzymes within the host cell to produce new virus particles in a process called biosynthesis
  4. Virus particles are assembled and matured into virions
  5. Finally, the host cell undergoes lysis, releasing the virions into the host organism to infect more cells

Lytic cycle diagram

Viruses use the protein synthesis machinery of host cells to replicate themselves in the lytic pathway

The lysogenic pathway

• A key difference between the lytic pathway and the lysogenic pathway is that here new virus particles are not immediately released and will not immediately cause disease once they infect a host cell
• During the lysogenic pathway viral nucleic acid combines with the host DNA
• A viral gene coding for a repressor protein prevents the viral nucleic acid from being transcribed and translated
  • This is called latency and the time during which it occurs is known as a period of lysogeny
• The host cell will continue to function as normal, including reproduction and cell division which means that subsequent cells will contain the virus nucleic acid within the host's genome
  • This can result in continuous production of host cells containing the virus nucleic acid within its genome
  • This stage of the lysogenic pathway can continue until a lytic event is triggered
• The viral DNA is inactive, or dormant, until a change in the cell's environment triggers the virus DNA to enter the lytic pathway
  • Changes include exposure to UV rays and certain chemicals

Lysogenic cycle diagram

Viruses lay dormant in the lysogenic pathway until the lytic pathway is triggered