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.
cII and cro

Verified step by step guidance

1. Understand the context: The λ phage can enter either the lytic or lysogenic life cycle. The decision is regulated by a genetic switch involving key genes such as cII and cro. The cII gene promotes lysogeny, while the cro gene promotes the lytic cycle. Mutations in these genes can disrupt this balance.
2. Analyze the role of cII: The cII protein activates the transcription of genes required for lysogeny, such as the cI gene, which encodes the λ repressor. If cII is inactivated by mutation, the transcription of cI will be impaired, reducing the likelihood of lysogeny and favoring the lytic cycle.
3. Analyze the role of cro: The cro protein represses the transcription of the cI gene, thereby inhibiting lysogeny and promoting the lytic cycle. If cro is inactivated by mutation, the repression of cI will be lifted, allowing the λ repressor to accumulate and favor lysogeny over the lytic cycle.
4. Consider the combined effects: A mutation in cII would make it difficult for the phage to establish lysogeny, as the cI gene would not be activated. Conversely, a mutation in cro would make it difficult for the phage to enter the lytic cycle, as cI repression would be lost. The specific life cycle outcome depends on which gene is mutated and the environmental conditions.
5. Summarize the outcomes: Inactivation of cII skews the balance toward the lytic cycle, while inactivation of cro skews the balance toward lysogeny. These effects highlight the importance of the genetic switch in determining the phage's life 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 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.

cII Protein

The cII protein is a key regulatory protein in λ phage that promotes the lysogenic cycle. It activates the transcription of genes necessary for lysogeny, including the repressor protein cI, which maintains the phage in a dormant state within the host. If cII is inactivated by mutations, the phage is more likely to enter the lytic cycle due to the lack of lysogenic promotion.

cro Protein

The cro protein is another regulatory protein in λ phage that plays a crucial role in determining the lytic cycle. It represses the expression of the cI gene, thereby favoring the lytic pathway. Mutations that inactivate cro can lead to increased levels of cI, promoting lysogeny and preventing the lytic cycle, as the phage would be unable to effectively suppress the lysogenic state.