Model Answer
0 min readIntroduction
Viruses are obligate intracellular parasites, meaning they can only replicate inside a host cell. Their replication strategies are diverse, but broadly categorized into two main cycles: the lytic cycle and the lysogenic cycle. These cycles represent different approaches viruses take to utilize the host cell machinery for their own propagation. Understanding these cycles is crucial for comprehending viral pathogenesis, developing antiviral therapies, and controlling viral infections. The choice between these cycles often depends on the virus type and the host cell's condition.
Lytic Cycle
The lytic cycle is a rapid and destructive process resulting in the immediate lysis (rupture) of the host cell. It’s characterized by active viral replication and the production of numerous viral particles.
- Attachment (Adsorption): The virus attaches to specific receptors on the host cell surface.
- Penetration (Entry): The viral genome enters the host cell. This can occur through various mechanisms, including direct injection, endocytosis, or membrane fusion.
- Replication: The viral genome takes control of the host cell's machinery, directing it to replicate viral nucleic acids and synthesize viral proteins.
- Assembly (Maturation): Newly synthesized viral components (nucleic acids and proteins) are assembled into new viral particles (virions).
- Release (Lysis): The host cell lyses, releasing the newly formed virions, which can then infect other cells. This lysis is often facilitated by viral enzymes like lysozyme.
Examples of viruses that primarily utilize the lytic cycle include bacteriophage T4 and influenza virus.
Lysogenic Cycle
The lysogenic cycle is a more subtle and prolonged process where the viral genome integrates into the host cell's genome, becoming a prophage (in bacteria) or provirus (in eukaryotes). The host cell is not immediately destroyed.
- Attachment & Penetration: Similar to the lytic cycle, the virus attaches to and enters the host cell.
- Integration: The viral genome integrates into the host cell's DNA. In bacteria, this is often site-specific integration.
- Prophage/Provirus Replication: The integrated viral genome (prophage/provirus) is replicated along with the host cell's DNA during cell division. Each daughter cell receives a copy of the viral genome.
- Induction: Under certain stress conditions (e.g., UV radiation, nutrient deprivation), the prophage/provirus can excise itself from the host genome and enter the lytic cycle.
Examples of viruses that can undergo lysogeny include bacteriophage lambda (λ) and HIV.
Comparison of Lytic and Lysogenic Cycles
| Feature | Lytic Cycle | Lysogenic Cycle |
|---|---|---|
| Host Cell Survival | Host cell is destroyed (lysed) | Host cell survives (initially) |
| Viral Replication | Rapid and immediate | Delayed; viral genome replicates with host DNA |
| Viral Genome | Replicates independently | Integrates into host genome |
| Progeny Virus Production | Large number of virions released quickly | No immediate virion production; potential for later lytic cycle |
| Induction | Not applicable | Can be induced to enter lytic cycle |
Temperate Viruses
Viruses capable of undergoing both lytic and lysogenic cycles are called temperate viruses. The decision between the two cycles is often influenced by environmental factors and the physiological state of the host cell.
Conclusion
In conclusion, the lytic and lysogenic cycles represent distinct strategies employed by viruses to replicate and propagate. The lytic cycle is characterized by rapid replication and host cell destruction, while the lysogenic cycle involves integration into the host genome and delayed replication. Understanding these cycles is fundamental to comprehending viral infections and developing effective antiviral strategies. The ability of some viruses to switch between these cycles adds complexity to their pathogenesis and control.
Answer Length
This is a comprehensive model answer for learning purposes and may exceed the word limit. In the exam, always adhere to the prescribed word count.