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.
cro

Verified step by step guidance

1. Understand the role of the cro gene: The cro gene in λ phage encodes the Cro protein, which is a key regulator in determining whether the phage enters the lytic or lysogenic cycle. Cro protein represses the expression of the cI gene, which encodes the λ repressor protein. The λ repressor promotes the lysogenic cycle by repressing lytic genes.
2. Analyze the effect of inactivating cro: If the cro gene is inactivated, the Cro protein will not be produced. Without Cro, the cI gene will not be repressed, leading to increased production of the λ repressor protein.
3. Predict the impact on the life cycle: With higher levels of the λ repressor protein, the phage is more likely to enter or remain in the lysogenic cycle, as the λ repressor actively suppresses the expression of genes required for the lytic cycle.
4. Consider the balance of regulatory proteins: Normally, the decision between the lytic and lysogenic cycles depends on the balance between Cro and the λ repressor. In the absence of Cro, this balance is disrupted, favoring the lysogenic cycle.
5. Summarize the outcome: Mutations that inactivate the cro gene would likely result in a bias toward the lysogenic life cycle, as the repression of the cI gene by Cro is a critical step in promoting the lytic cycle.

Key Concepts

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.

Role of the cro Gene

The cro gene in λ phage is crucial for regulating the switch between the lytic and lysogenic cycles. It encodes a protein that promotes the lytic cycle by inhibiting the expression of genes necessary for lysogeny, such as cI, which encodes the repressor protein. When cro is active, it favors the production of new phage particles, while its inactivation can lead to a preference for the lysogenic pathway.

Mutations and Gene Function

Mutations are changes in the DNA sequence that can affect gene function. In the context of the cro gene, mutations that inactivate it would prevent the production of the cro protein, disrupting the balance between the lytic and lysogenic cycles. This could lead to an increased likelihood of lysogeny, as the repression of lysogenic genes would be lifted, allowing the phage to integrate its DNA into the host genome instead of immediately replicating.