9. Why lysogenic cycle is more beneficial to a virus than lytic cycle under certain circumstances?
(1) The Lysogenic cycle prevent local extinction of host while still retaining infectious potential
(2) By integrating with the bacterial chromosomes, the genetic instructions for the virus become refreshed after one or more replication events during binary fission
(3) Lysogenic infection cycles donot harm their host cells, so they can produce virus
C. Clear plaques are formed in Cll mutants
D. Turbid plaques are formed in integrase mutants
E. Clear plaques are formed in P mutants
F. No plaques are formed in cl mutants
Which of the following combination of statement is correct?
(1) A, B and F only (2) C, D and E only
(3) B and C only (4) D and F only
Viruses, particularly bacteriophages like lambda phage, have evolved two primary life cycles to reproduce within bacterial hosts: the lytic cycle and the lysogenic cycle. While the lytic cycle involves rapid viral replication and destruction of the host cell, the lysogenic cycle allows the viral genome to integrate into the host chromosome and replicate silently alongside it. Under certain conditions, the lysogenic cycle provides significant advantages for viral survival and propagation.
Why Is the Lysogenic Cycle More Beneficial Under Certain Circumstances?
The lysogenic cycle is often more advantageous to a virus than the lytic cycle because it allows the virus to persist in the host population without killing the host cells immediately. The key reasons include:
-
Prevention of Local Host Extinction
The lysogenic cycle prevents the virus from wiping out its host population. By integrating its genome into the host DNA and remaining dormant, the virus ensures that the bacterial cells survive and continue to reproduce. This maintains a reservoir of infected hosts, allowing the virus to persist in the environment and retain its infectious potential over time. -
Replication Alongside Host DNA
When the viral genome integrates into the bacterial chromosome as a prophage, it is replicated every time the host bacterium divides. This means the viral genetic material is “refreshed” and passed on to daughter cells without needing to produce new virions immediately, ensuring long-term maintenance of the viral genome. -
Minimal Harm to Host Cells During Lysogeny
Lysogenic infection does not harm the host cells, allowing them to grow and divide normally. This symbiotic relationship benefits the virus by providing a stable environment for its genome and a continuous supply of host cells for future viral production when conditions favor induction of the lytic cycle.
Summary of Benefits
| Benefit | Explanation |
|---|---|
| Prevents local extinction of host | Maintains host population for viral persistence |
| Viral genome replicates with host DNA | Ensures viral DNA is copied during bacterial binary fission |
| Lysogenic infection is non-lethal initially | Host cells survive and continue to propagate, preserving viral genetic material |
Understanding Plaque Formation in Lambda Phage Mutants
Plaque morphology—clear or turbid plaques—provides insights into the phage’s genetic and functional state during infection.
-
Clear plaques indicate that all infected bacteria are lysed, typical of mutants unable to establish lysogeny.
-
Turbid plaques indicate the presence of lysogens within the plaque, where some bacteria survive infection due to prophage integration.
Evaluating the Given Statements on Plaque Formation
-
C. Clear plaques are formed in cI mutants
The cI gene encodes the lambda repressor, which maintains lysogeny. Mutations in cI prevent lysogeny, causing immediate lytic infection and clear plaques. -
D. Turbid plaques are formed in integrase (int) mutants
Integrase is essential for prophage integration. Mutants lacking integrase cannot form lysogens, so plaques are typically clear, not turbid. Hence, this statement is incorrect. -
E. Clear plaques are formed in P mutants
The P gene is involved in phage DNA replication. Mutations here often prevent replication, resulting in no plaque formation or very small plaques, not necessarily clear plaques. This statement is generally incorrect. -
F. No plaques are formed in cI mutants
This is incorrect because cI mutants produce clear plaques due to failure to establish lysogeny and immediate lysis.
Correct Combination of Statements
Based on the above:
-
Statements A, B, and F relate to lysogenic advantages and plaque formation.
-
Statement F is incorrect as cI mutants form clear plaques, not no plaques.
-
Statements C, D, and E: Only C is correct; D and E are incorrect.
Therefore, the correct combination is:
(1) A, B and F only — but since F is incorrect, none of the options perfectly fit. The best supported by scientific understanding is that A and B are correct regarding lysogenic benefits, and C is correct about clear plaques in cI mutants.
Conclusion
The lysogenic cycle offers viruses a strategic advantage by allowing the viral genome to persist silently within the host, preventing host extinction and ensuring viral propagation through bacterial replication. The integration of the viral genome into the host chromosome allows the virus to replicate passively, maintaining its genetic material without harming the host immediately. This contrasts with the lytic cycle, where rapid viral replication leads to host cell destruction.
Regarding plaque formation, cI mutants produce clear plaques due to their inability to maintain lysogeny, leading to immediate lytic infection. Understanding these dynamics is crucial for comprehending viral life cycles, phage therapy applications, and bacterial genetics.
