Overview
This lecture explains how viruses replicate by hijacking host cells, detailing the two main viral replication cycles: the lytic and lysogenic cycles.
Viral Entry Into Host Cells
- Viruses lack organelles and must enter host cells to replicate.
- Bacteriophages inject genetic material directly into the cell.
- Non-enveloped viruses use cell surface receptors to enter cells.
- Enveloped viruses enter by either fusing with the membrane or using surface receptors.
The Lytic Cycle ("Impatient" Virus)
- Virus quickly takes over host machinery to replicate its genetic material and proteins.
- Viral components self-assemble into new virus particles.
- Host cell lyses (breaks open), releasing many new viruses to infect other cells.
- Rapid method suited for environments with many potential new hosts.
The Lysogenic Cycle ("Hitch a Ride" Virus)
- Viral genome integrates into the host’s DNA and becomes inactive (provirus/prophage).
- Virus remains dormant (latent phase) and is replicated with the host genome.
- Repressor genes keep the viral genes inactive.
- Environmental triggers (e.g., UV light) or random chance can reactivate the virus.
- Activated virus enters the lytic cycle: replication, assembly, cell lysis.
Key Terms & Definitions
- Lytic Cycle — Viral replication cycle involving rapid production of new viruses and lysis of the host cell.
- Lysogenic Cycle — Viral replication cycle where viral DNA integrates into the host genome and remains dormant.
- Provirus/Prophage — Dormant viral genome integrated into host DNA.
- Latent Phase — Period when the viral genome is inactive inside the host.
- Lysis — Breaking open of the host cell to release new viruses.
- Repressor Genes — Genes that prevent expression of viral DNA during the lysogenic cycle.
Action Items / Next Steps
- Review diagrams of lytic and lysogenic cycles for visual understanding.
- Prepare to compare and contrast the two cycles in upcoming assignments.