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⁺

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 different 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 medium.

Haploid Genotypes and Mutations

Haploid organisms, like certain bacterial strains, have a single set of chromosomes, meaning any mutation will directly affect the phenotype. In the context of the lac operon, mutations such as I⁻ (non-functional repressor) or P⁺ (functional promoter) can significantly alter the operon's ability to transcribe genes. Understanding these mutations helps in determining whether a strain can grow on lactose-only medium (lac⁺) or not (lac⁻).