Viral Replication and Infection Cycles

Viral Entry into Host Cell

  • Viruses must enter a host cell to replicate.
  • Attachment to host cell:
    • Viruses attach using specific receptors on the host's plasma membrane.
    • Receptors vary among organisms, determining virus specificity and limiting cross-species transmission.
  • Entry of viral genetic material:
    • After attachment, the virus's genetic material (RNA or DNA) enters the host cell's cytoplasm.
    • In some cases, the entire virus enters, and the capsid breaks down to release the genetic material.

Lytic Cycle

  • Replication process:
    • The host cell is used to make copies of the viral RNA or DNA.
    • Viral genes direct the host cell to produce viral protein capsids and enzymes needed for replication.
    • New viruses are assembled with the protein coat forming around the nucleic acid.
  • Release of new viruses:
    • New viruses exit the cell via exocytosis or by causing the cell to lyse (burst).
    • Lysis releases new viruses that can infect other cells.
  • Active Infections:
    • Viruses replicating through the lytic cycle often cause active infections.
    • Symptoms appear quickly, typically one to four days after exposure.
    • Examples: common cold and influenza.

Lysogenic Cycle

  • Integration of viral DNA:
    • Viral DNA enters the host cell nucleus.
    • The viral DNA integrates into a chromosome in the host cell.
    • The infected cell permanently contains the viral genes.
  • Dormancy and Activation:
    • Viral genes can remain dormant for months or years.
    • Activation can be triggered by various factors, leading to the lytic cycle.
    • Once activated, the viral genes instruct the host cell to manufacture more viruses.
    • New viruses are released via exocytosis or cell lysis.
  • Latent Infections:
    • Many disease-causing viruses use the lysogenic cycle.
    • Example: Herpes simplex virus type 1 (HSV-1), which causes cold sores.
      • Transmitted orally.
      • Viral DNA remains inactive in the nucleus until reactivated.
      • Reactivation is thought to occur during times of physical, emotional, or environmental stress, leading to virus production.