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Ch. 12 - Regulation of Gene Expression in Bacteria and Bacteriophage
Sanders - Genetic Analysis: An Integrated Approach 3rd Edition
Sanders3rd EditionGenetic Analysis: An Integrated ApproachISBN: 9780135564172Not the one you use?Change textbook
All textbooksSanders 3rd EditionCh. 12 - Regulation of Gene Expression in Bacteria and BacteriophageProblem 17e
Chapter 12, Problem 17e

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 regulatory genes include I (lacI, the repressor), P (promoter), and O (operator). The structural genes include Z (lacZ, encoding β-galactosidase) and Y (lacY, encoding permease).
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 nonfunctional β-galactosidase gene, and Y⁺ indicates a functional permease gene.
Determine the transcriptional behavior: Since I⁺ produces a functional repressor, the operon will be inducible. This means that in the absence of lactose, the repressor binds to the operator and prevents transcription. In the presence of lactose, the repressor is inactivated, allowing transcription of the operon.
Assess the strain's ability to grow on lactose-only medium: The strain has a functional permease (Y⁺), which allows lactose to enter the cell. However, the strain lacks a functional β-galactosidase (Z⁻), which is required to break down lactose into glucose and galactose. Therefore, the strain is lac⁻ (cannot grow on lactose-only medium).
Summarize the findings: The operon is inducible, meaning transcription occurs only in the presence of lactose. However, the strain is lac⁻ because it cannot metabolize lactose due to the nonfunctional β-galactosidase gene (Z⁻).

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

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), a promoter (P), and an operator (O). The operon is regulated by the presence or absence of lactose, which influences whether the genes are transcribed. Understanding the structure is crucial for determining how mutations affect gene expression.
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Lac Operon Overview

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.
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Induced Mutations

Haploid Genotypes and Mutations

Haploid genotypes contain a single set of chromosomes, meaning any mutation will directly affect the phenotype. In the context of the lac operon, mutations in the genes or regulatory elements can lead to different expressions of the operon. For example, a mutation in the Z gene (which encodes β-galactosidase) can prevent lactose metabolism, impacting whether the strain can grow on lactose-only medium.
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Related Practice
Textbook Question

Complete the accompanying table, indicating whether functionally active -galactosidase and permease are produced in the presence and absence of lactose. Use '+' to indicate the presence of a functional enzyme and '−' to indicate its absence. Indicate whether the partial diploid strain is lac⁺ (able to grow on lactose-only medium) or lac⁻ (cannot grow on lactose medium).

Textbook Question

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⁺

Textbook Question

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⁺

Textbook Question

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⁻

Textbook Question

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⁺

Textbook Question

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⁺