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: I (lacI, the repressor gene), P (promoter), O (operator), Z (lacZ, encodes β-galactosidase), and Y (lacY, encodes permease).
2. Analyze the given genotype: I⁺ P⁺ O⁺ Z⁺ Y⁻. Here, I⁺ indicates a functional repressor, P⁺ indicates a functional promoter, O⁺ indicates a functional operator, Z⁺ indicates a functional β-galactosidase gene, and Y⁻ indicates a non-functional permease gene.
3. Determine the transcriptional behavior: Since I⁺ produces a functional repressor, the operon will be inducible. In the absence of lactose, the repressor binds to the operator (O⁺), preventing transcription. In the presence of lactose, the repressor is inactivated, allowing transcription of the operon.
4. Assess the strain's ability to grow on lactose-only medium: The strain is lac⁻ because Y⁻ indicates a non-functional permease. Permease is required to transport lactose into the cell, so even though β-galactosidase (Z⁺) is functional, the cell cannot utilize lactose as a carbon source.
5. Summarize the findings: The operon is inducible (transcription occurs only in the presence of lactose), but the strain is lac⁻ (cannot grow on lactose-only medium) due to the non-functional permease (Y⁻).

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 (lacZ, lacY, and lacA), a promoter (P), an operator (O), and regulatory elements. Understanding the operon's structure is crucial for determining how genes are expressed in response to lactose presence or absence.

Inducible vs. Constitutive Expression

Inducible expression refers to genes that are turned on in the presence of a specific substrate, such as lactose for the lac operon. In contrast, constitutive expression occurs when genes are continuously expressed regardless of the substrate's presence. This distinction is essential for analyzing how different genotypes affect the operon's function.

Haploid Genotypes and Phenotypes

Haploid genotypes consist of a single set of chromosomes, which means that any mutation or allele present will directly affect the phenotype. In the context of the lac operon, understanding how specific mutations (like I⁺, P⁺, O⁺, Z⁺, and Y⁻) influence the ability to utilize lactose is key to determining whether a strain is lac⁺ or lac⁻.