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. Identify the components of the lac operon: I (repressor), P (promoter), O (operator), Z (β-galactosidase), and Y (permease).
2. Determine the functionality of each component in the given genotype: I⁺ (functional repressor), P⁺ (functional promoter), O⁺ (functional operator), Z⁻ (non-functional β-galactosidase), Y⁺ (functional permease).
3. Assess the ability of the operon to be inducibly transcribed: Since I⁺ is functional, it can bind to the operator O⁺ and block transcription in the absence of an inducer. In the presence of an inducer, the repressor will not bind, allowing transcription.
4. Evaluate the ability of the operon to transcribe constitutively: The presence of a functional repressor (I⁺) and operator (O⁺) means the operon is not constitutively active; it requires an inducer for transcription.
5. Determine the lac phenotype: Since Z⁻ is non-functional, the strain cannot produce β-galactosidase, which is necessary for lactose metabolism, making the strain lac⁻ (cannot 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) and regulatory elements (promoter P and operator O). The operon is controlled by the presence or absence of lactose, which influences whether the genes are transcribed. Understanding the structure is essential 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 crucial for predicting the growth capabilities of the strain on lactose.

Haploid Genotypes and Mutations

Haploid genotypes contain a single set of chromosomes, making them particularly useful for studying gene function and mutations. In the context of the lac operon, mutations in genes such as Z (β-galactosidase) or Y (lactose permease) can affect the operon's ability to metabolize lactose. Understanding how these mutations influence the operon's function is key to determining whether a strain is lac⁺ or lac⁻.