Textbook Question
How would mutations that inactivate each of the following genes affect the determination of the lytic or lysogenic life cycle in mutated λ phage strains? Explain your answers.
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12. Gene Regulation in Prokaryotes
Lambda Bacteriophage and Life Cycle Regulation
2:42 minutesProblem 28a
Textbook Question
How would mutations that inactivate each of the following genes affect the determination of the lytic or lysogenic life cycle in mutated λ phage strains? Explain your answers.
cI
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Understand the role of the cI gene: The cI gene in λ phage encodes the λ repressor protein, which is crucial for maintaining the lysogenic state. This protein binds to specific operator sites on the phage DNA, repressing the expression of lytic genes and promoting lysogeny.
Analyze the effect of a mutation that inactivates cI: If the cI gene is inactivated, the λ repressor protein will not be produced. Without this repressor, the lytic genes will not be suppressed, and the phage will be unable to establish or maintain the lysogenic state.
Determine the life cycle outcome: In the absence of the λ repressor, the phage will default to the lytic cycle. This is because the lytic genes will be expressed unchecked, leading to the production of new phage particles and eventual lysis of the host cell.
Consider the broader implications: The inability to enter or maintain lysogeny means that the phage cannot integrate its genome into the host's genome, which is a hallmark of the lysogenic cycle. This could affect the phage's ability to persist in environments where host cells are scarce.
Summarize the mutation's impact: A mutation that inactivates the cI gene will prevent the phage from entering or maintaining the lysogenic cycle, forcing it to exclusively follow the lytic pathway.
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Lytic and Lysogenic Cycles
The lytic and lysogenic cycles are two distinct pathways that bacteriophages, like λ phage, can follow after infecting a bacterial host. In the lytic cycle, the phage replicates rapidly, leading to the destruction of the host cell and the release of new phage particles. In contrast, the lysogenic cycle involves the integration of the phage DNA into the host genome, allowing it to replicate along with the host cell without causing immediate harm.
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cI Gene Function
The cI gene in λ phage encodes a repressor protein that is crucial for maintaining the lysogenic state. This protein binds to the operator region of the phage genome, inhibiting the expression of genes required for the lytic cycle. When cI is functional, it promotes lysogeny by preventing the activation of lytic genes, thus allowing the phage to coexist with the host.
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Mutations and Gene Inactivation
Mutations that inactivate the cI gene can disrupt the balance between the lytic and lysogenic cycles. If cI is nonfunctional, the repressor cannot inhibit lytic gene expression, leading to the activation of the lytic cycle. This results in the phage entering a lytic phase, causing cell lysis and the release of new phage particles, rather than integrating into the host genome and remaining dormant.
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