Model Answer
0 min readIntroduction
Viruses, obligate intracellular parasites, exhibit diverse replication strategies. These strategies are broadly categorized into two main life cycles: the lytic cycle and the lysogenic cycle. While the lytic cycle results in immediate viral replication and host cell destruction, the lysogenic cycle involves the integration of the viral genome into the host cell’s DNA, establishing a latent infection. This integration allows the virus to replicate along with the host cell without causing immediate lysis, representing a sophisticated survival mechanism for the virus. Understanding the lysogenic cycle is crucial for comprehending viral pathogenesis and developing antiviral strategies.
The Lysogenic Cycle: A Detailed Explanation
The lysogenic cycle is a viral replication process characterized by a period of latency where the viral genome integrates into the host cell’s genome. This integrated viral DNA is referred to as a prophage in bacteria and a provirus in eukaryotic cells. The cycle unfolds in several key steps:
1. Attachment and Penetration
Similar to the lytic cycle, the lysogenic cycle begins with the virus attaching to the host cell surface via specific receptor interactions. Following attachment, the viral genetic material penetrates the host cell.
2. Integration
This is the defining step of the lysogenic cycle. The viral DNA integrates into the host cell’s chromosomal DNA. This integration is often site-specific, mediated by a viral enzyme called integrase. The integrated viral DNA becomes a prophage (in bacteria) or provirus (in eukaryotes).
3. Replication with Host DNA
Once integrated, the prophage/provirus is passively replicated along with the host cell’s DNA during cell division. This means that every daughter cell receives a copy of the viral genome. The virus remains dormant during this phase, not actively producing viral particles.
4. Induction (Triggering the Lytic Cycle)
Under certain stress conditions, such as UV radiation, nutrient deprivation, or exposure to chemicals, the prophage/provirus can be induced to excise itself from the host genome and enter the lytic cycle. This induction process is often triggered by the host cell’s DNA repair mechanisms.
5. Lytic Cycle Resumption
Once induced, the virus enters the lytic cycle, replicating its genome, synthesizing viral proteins, assembling new viral particles, and ultimately lysing the host cell to release progeny viruses.
Lysogenic vs. Lytic Cycle: A Comparative Overview
| Feature | Lysogenic Cycle | Lytic Cycle |
|---|---|---|
| Viral Genome Fate | Integrates into host DNA (prophage/provirus) | Replicates independently within the host cell |
| Host Cell Destruction | No immediate destruction | Host cell lyses, releasing new viruses |
| Viral Replication | Replicates along with host DNA | Rapid replication of viral components |
| Latency | Yes, a period of dormancy | No latency |
| Induction | Can be induced to enter the lytic cycle | Does not require induction |
Examples: Bacteriophage lambda (λ) in E. coli is a classic example of a virus utilizing the lysogenic cycle. HIV, a retrovirus, also employs a lysogenic-like cycle, integrating its RNA genome (converted to DNA via reverse transcriptase) into the host cell’s DNA as a provirus.
Conclusion
The lysogenic cycle represents a sophisticated viral strategy for long-term survival and propagation. By integrating into the host genome, viruses can evade the host’s immune system and replicate passively alongside the host cell. While seemingly dormant, the potential for induction into the lytic cycle allows for periodic viral production and spread. Understanding the intricacies of the lysogenic cycle is vital for developing effective antiviral therapies and managing viral infections, particularly those exhibiting latency.
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.