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 the regulatory gene (I), the promoter (P), the operator (O), and the structural genes (Z and Y). The I gene encodes the repressor protein, the P region is where RNA polymerase binds, the O region is where the repressor binds, and Z and Y encode β-galactosidase and permease, respectively.
2. Analyze the given genotype: I⁺ P⁺ Oᶜ Z⁺ Y⁺. Here, I⁺ indicates a functional repressor protein, P⁺ indicates a functional promoter, Oᶜ indicates a constitutive operator mutation (the repressor cannot bind to the operator), and Z⁺ and Y⁺ indicate functional β-galactosidase and permease genes.
3. Determine the effect of the Oᶜ mutation: The Oᶜ mutation prevents the repressor protein from binding to the operator. As a result, RNA polymerase can transcribe the structural genes (Z and Y) regardless of whether lactose is present. This means the operon is transcribed constitutively.
4. Assess the strain's ability to grow on lactose-only medium: Since Z⁺ and Y⁺ are functional, the strain can produce β-galactosidase (to break down lactose) and permease (to transport lactose into the cell). Therefore, the strain is lac⁺ and can grow on lactose-only medium.
5. Summarize the findings: The genotype I⁺ P⁺ Oᶜ Z⁺ Y⁺ results in constitutive transcription of the operon genes, and the strain is lac⁺ because it can metabolize lactose.

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, and A) that code for proteins necessary for lactose utilization, a promoter (P) where RNA polymerase binds, and an operator (O) that regulates transcription. 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. Identifying whether a genotype leads to inducible or constitutive expression is essential for predicting the growth capabilities of the strain on lactose.

Genetic Mutations and Their Effects

Mutations in the lac operon can lead to different phenotypes, such as lac⁺ (able to utilize lactose) or lac⁻ (unable to utilize lactose). For example, a mutation in the operator (Oᶜ) can cause constitutive expression of the operon, leading to continuous transcription of the genes regardless of lactose presence. Understanding these mutations helps in analyzing the genotype's impact on lactose metabolism.