For the lac genotypes shown in the following table, predict whether the structural genes (Z) are constitutive, permanently repressed, or inducible in the presence of lactose. Genotype Constitutive Repressed Inducible I⁺O⁺Z⁺ x I⁻O⁺Z⁺ I⁻OᶜZ⁺ I⁻OᶜZ⁺/F'O⁺ I⁺OᶜZ⁺/F'O⁺ IˢO⁺Z⁺ IˢO⁺Z⁺/F'I⁺

Verified step by step guidance

1. Identify the components of the lac operon: I (repressor gene), O (operator), and Z (structural gene for β-galactosidase).
2. Understand the role of each component: I⁺ produces a functional repressor, I⁻ produces a non-functional repressor, Iˢ produces a super-repressor, O⁺ is a normal operator, and Oᶜ is a constitutive operator that cannot bind the repressor.
3. For each genotype, determine the functionality of the repressor and operator: I⁺O⁺Z⁺ has a functional repressor and operator, I⁻O⁺Z⁺ has a non-functional repressor, I⁻OᶜZ⁺ has a non-functional repressor and constitutive operator, etc.
4. Predict the expression of the Z gene based on the functionality of the repressor and operator: If the repressor cannot bind to the operator (due to I⁻ or Oᶜ), the Z gene is constitutively expressed. If the repressor can bind and is not inhibited by lactose, the Z gene is repressed.
5. Consider the presence of lactose: In the presence of lactose, a functional repressor (I⁺) will be inhibited, allowing the Z gene to be inducible. A super-repressor (Iˢ) will not be inhibited by lactose, keeping the Z gene repressed.

Key Concepts

Lac Operon Structure

The lac operon is a genetic regulatory system found in E. coli that controls the metabolism of lactose. It consists of three structural genes (Z, Y, A) and regulatory elements, including the promoter (P) and operator (O). Understanding the operon's structure is crucial for predicting gene expression based on different genotypes, as it determines how the genes respond to the presence or absence of lactose.

Regulatory Genes and Their Functions

Regulatory genes, such as I (the repressor gene), play a vital role in controlling the expression of the lac operon. The I⁺ allele produces a functional repressor that can bind to the operator, while I⁻ produces a non-functional repressor, allowing for gene expression. The presence of mutations in these regulatory genes affects whether the structural genes are constitutive, repressed, or inducible.

Inducibility and Repression Mechanisms

Inducibility refers to the ability of a gene to be expressed in response to an inducer, such as lactose, which binds to the repressor and prevents it from inhibiting transcription. Conversely, repression occurs when the repressor is active and binds to the operator, blocking transcription. Understanding these mechanisms is essential for predicting the behavior of the lac operon under different genetic configurations.