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⁺ Oᶜ 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: I (lacI, the repressor gene), P (promoter), O (operator), Z (lacZ, encodes β-galactosidase), and Y (lacY, encodes permease). Mutations in these components can affect the operon's ability to transcribe genes in response to lactose presence.
2. Analyze the given genotype: The genotype provided is I⁺ P⁺ Oᶜ Z⁺ Y⁻. Here, I⁺ indicates a functional repressor protein, P⁺ indicates a functional promoter, Oᶜ indicates a constitutive operator mutation, Z⁺ indicates a functional β-galactosidase gene, and Y⁻ indicates a non-functional permease gene.
3. Determine the effect of the Oᶜ mutation: The Oᶜ (constitutive operator) mutation prevents the repressor protein (produced by I⁺) from binding to the operator. This means that transcription of the structural genes (Z and Y) will occur regardless of whether lactose is present, leading to constitutive expression of the operon.
4. Assess the strain's ability to grow on lactose-only medium: Since Z⁺ is functional, β-galactosidase will be produced, allowing lactose to be broken down into glucose and galactose. However, Y⁻ means permease is non-functional, so lactose cannot be efficiently transported into the cell. As a result, the strain is lac⁻ (cannot grow on lactose-only medium).
5. Summarize the findings: The operon transcribes genes constitutively due to the Oᶜ mutation, but the strain is lac⁻ because the Y⁻ mutation prevents lactose uptake, even though β-galactosidase is produced.

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), a promoter (P), and an operator (O). The operon is regulated by the presence or absence of lactose, which influences whether the genes are transcribed. Understanding the 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. Identifying whether a genotype leads to inducible or constitutive expression is essential for predicting the growth capabilities of the strain on lactose.

Genotype and Phenotype Relationship

The genotype refers to the genetic makeup of an organism, while the phenotype is the observable characteristics resulting from the genotype and its interaction with the environment. In the context of the lac operon, understanding how specific mutations (like Oᶜ or Y⁻) affect the ability to utilize lactose helps in determining whether a strain is lac⁺ or lac⁻, which is critical for answering the question.