Textbook Question
Describe the lytic and lysogenic life cycles of λ bacteriophage. What roles do λ repressor and Cro protein play in controlling transcription from PR and PRM, and how are these roles linked to lysis and lysogeny?
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12. Gene Regulation in Prokaryotes
Lambda Bacteriophage and Life Cycle Regulation
2:30 minutesProblem 28f
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.
N
Verified step by step guidance1
Understand the role of the N gene in λ phage: The N gene encodes an antiterminator protein that allows RNA polymerase to bypass transcriptional terminators. This is essential for the expression of downstream genes required for both the lytic and lysogenic cycles.
Recognize the impact of an inactivated N gene: If the N gene is mutated and its protein is nonfunctional, transcription of downstream genes necessary for both the lytic and lysogenic pathways will be prematurely terminated. This would disrupt the phage's ability to proceed with either life cycle.
Analyze the effect on the lytic cycle: In the lytic cycle, the N protein is required to transcribe genes involved in DNA replication, structural proteins, and lysis. Without N, these genes cannot be expressed, and the lytic cycle cannot proceed.
Analyze the effect on the lysogenic cycle: In the lysogenic cycle, the N protein is also required to transcribe genes like cII, which is critical for the activation of the cI gene (encoding the λ repressor). Without N, the cI gene cannot be expressed, and the phage cannot establish lysogeny.
Conclude the overall effect: A mutation in the N gene would prevent the λ phage from successfully entering either the lytic or lysogenic life cycle, effectively rendering the phage nonfunctional in terms of its ability to propagate or integrate into the host genome.
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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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Decision Between Lytic and Lysogenic Cycles
Gene Function in Phage Life Cycles
Specific genes in λ phage are crucial for determining whether the phage enters the lytic or lysogenic cycle. For instance, genes that regulate the expression of proteins involved in the decision-making process, such as the cI repressor, play a significant role. Mutations that inactivate these genes can disrupt the balance between the two cycles, potentially favoring one pathway over the other.
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Mutations and Their Effects
Mutations are changes in the DNA sequence that can alter gene function. In the context of λ phage, mutations that inactivate genes essential for the lytic or lysogenic cycles can lead to a failure in the phage's ability to switch between these pathways. Understanding the specific roles of these genes helps predict how mutations will affect the phage's life cycle and its interaction with host bacteria.
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