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.
int

Verified step by step guidance

1. Understand the context: The λ phage can enter either the lytic or lysogenic life cycle. The 'int' gene encodes the integrase enzyme, which is essential for integrating the phage DNA into the host genome during the lysogenic cycle. A mutation in this gene would disrupt this process.
2. Step 1: Recall the role of the integrase enzyme. The integrase enzyme, encoded by the 'int' gene, facilitates site-specific recombination between the phage DNA and the host bacterial chromosome, allowing the phage to establish lysogeny.
3. Step 2: Analyze the effect of an 'int' gene inactivation. If the 'int' gene is inactivated by mutation, the integrase enzyme will not be produced. Without integrase, the phage DNA cannot integrate into the host genome, making the lysogenic cycle impossible.
4. Step 3: Predict the outcome for the life cycle. Since the lysogenic cycle cannot occur, the phage will default to the lytic cycle, where it replicates and lyses the host cell to release new phage particles.
5. Step 4: Conclude the impact of the mutation. A mutation in the 'int' gene forces the λ phage to exclusively follow the lytic life cycle, as the lysogenic pathway is entirely dependent on the function of the integrase enzyme.

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 host bacterium. 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.

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.

Mutations and Their Effects

Mutations are changes in the DNA sequence that can affect gene function. In the context of λ phage, mutations that inactivate genes essential for the lytic or lysogenic cycles can lead to altered phage behavior. For example, a mutation that inactivates a gene responsible for lysogeny may force the phage to enter the lytic cycle, resulting in increased viral replication and host cell lysis.