Identify which of the following lac operon haploid genotypes transcribe operon genes inducibly and which transcribe genes constitutively. Indicate whether the strain is lac⁺ (able to grow on lactose-only medium) or lac⁻ (cannot grow on lactose medium).
I⁻ P⁺ O⁺ Z⁺ Y⁺

Verified step by step guidance

1. Understand the components of the lac operon: The lac operon consists of regulatory and structural genes. The key components are: (1) I (lacI) - the repressor gene, (2) P - the promoter, (3) O - the operator, and (4) Z and Y - the structural genes encoding β-galactosidase and permease, respectively. Mutations in these components can affect transcription and lactose metabolism.
2. Analyze the given genotype: The genotype is I⁻ P⁺ O⁺ Z⁺ Y⁺. Here, I⁻ indicates a mutation in the lacI gene, which means the repressor protein is nonfunctional. P⁺ indicates a functional promoter, O⁺ indicates a functional operator, and Z⁺ and Y⁺ indicate functional structural genes for β-galactosidase and permease.
3. Determine the effect of the I⁻ mutation: Since the repressor protein (lacI) is nonfunctional, it cannot bind to the operator (O⁺). This means the operon is not regulated by the presence or absence of lactose, leading to constitutive transcription of the Z and Y genes.
4. Assess the strain's ability to grow on lactose-only medium: Because the Z⁺ and Y⁺ genes are transcribed constitutively, the strain produces β-galactosidase and permease regardless of lactose presence. These enzymes allow the strain to metabolize lactose, making it lac⁺ (able to grow on lactose-only medium).
5. Summarize the findings: The genotype I⁻ P⁺ O⁺ Z⁺ Y⁺ results in constitutive transcription of the operon genes (Z and Y) and the strain is lac⁺, meaning it can grow on lactose-only medium.

Key Concepts

Lac Operon Structure

The lac operon is a set of genes in E. coli that are involved in the metabolism of lactose. It consists of structural genes (Z, Y, A) that code for proteins necessary for lactose utilization, a promoter (P) where RNA polymerase binds to initiate transcription, and an operator (O) that regulates access to the promoter. Understanding the operon's structure is crucial for determining how mutations affect gene expression.

Inducible vs. Constitutive Expression

Inducible expression refers to genes that are turned on in response to specific signals, such as the presence of lactose in the case of the lac operon. In contrast, constitutive expression occurs when genes are continuously expressed regardless of environmental conditions. Recognizing these patterns is essential for analyzing how different genotypes affect the operon's function.

Genotype and Phenotype Relationship

The genotype refers to the genetic constitution of an organism, while the phenotype is the observable characteristics resulting from the genotype and environmental interactions. In the context of the lac operon, understanding how specific mutations (like I⁻, P⁺, O⁺, Z⁺, Y⁺) influence the ability to utilize lactose is key to determining whether a strain is lac⁺ or lac⁻.