Textbook Question
How do we know that the trp operon is a repressible control system, in contrast to the lac operon, which is an inducible control system?
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Ch. 16 - Regulation of Gene Expression in Bacteria
Klug12th EditionConcepts of Genetics ISBN: 9780135564776
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Table of contents
- Ch. 1 - Introduction to Genetics17
- Ch. 2 - Mitosis and Meiosis36
- Ch. 3 - Mendelian Genetics51
- Ch. 4 - Extensions of Mendelian Genetics74
- Ch. 5 - Chromosome Mapping in Eukaryotes51
- Ch. 6 - Genetic Analysis and Mapping in Bacteria and Bacteriophages46
- Ch. 7 - Sex Determination and Sex Chromosomes33
- Ch. 8 - Chromosome Mutations: Variation in Number and Arrangement40
- Ch. 9 - Extranuclear Inheritance31
- Ch. 10 - DNA Structure and Analysis43
- Ch. 11 - DNA Replication and Recombination44
- Ch. 12 - DNA Organization in Chromosomes30
- Ch. 13 - The Genetic Code and Transcription54
- Ch. 14 - Translation and Proteins47
- Ch. 15 - Gene Mutation, DNA Repair, and Transposition41
- Ch. 16 - Regulation of Gene Expression in Bacteria29
- Ch. 17 - Transcriptional Regulation in Eukaryotes41
- Ch. 18 - Post-transcriptional Regulation in Eukaryotes33
- Ch. 19 - Epigenetics33
- Ch. 20 - Recombinant DNA Technology44
- Ch. 21 - Genomic Analysis36
- Ch. 22 - Applications of Genetic Engineering and Biotechnology36
- Ch. 23 - Developmental Genetics37
- Ch. 24 - Cancer Genetics34
- Ch. 25 - Quantitative Genetics and Multifactorial Traits54
- Ch. 26 - Population and Evolutionary Genetics45
Chapter 16, Problem 4
Contrast the role of the repressor in an inducible system and in a repressible system.
Verified step by step guidance1
Begin by defining what a repressor protein is: a molecule that can bind to an operator region of DNA to regulate gene expression by preventing RNA polymerase from transcribing the gene.
Explain the role of the repressor in an inducible system: the repressor is normally bound to the operator, blocking transcription. When an inducer molecule is present, it binds to the repressor, causing it to change shape and release from the operator, allowing transcription to proceed.
Describe the role of the repressor in a repressible system: the repressor is normally inactive and does not bind to the operator, so transcription occurs. When a corepressor molecule is present, it binds to the repressor, activating it. The active repressor then binds to the operator and blocks transcription.
Highlight the key difference: in inducible systems, the repressor is active by default and is inactivated by the inducer, whereas in repressible systems, the repressor is inactive by default and is activated by the corepressor.
Summarize by noting that inducible systems typically regulate genes involved in catabolism (breaking down molecules), while repressible systems regulate genes involved in anabolism (building molecules), reflecting their different modes of repressor action.
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Inducible Operon System
An inducible operon is typically off and requires an inducer molecule to deactivate the repressor, allowing gene transcription. The repressor binds to the operator to block transcription until the inducer binds, causing the repressor to release and enabling gene expression, as seen in the lac operon.
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Induced Mutations
Repressible Operon System
A repressible operon is usually on and actively transcribed until a corepressor binds to the repressor, activating it. The active repressor then binds to the operator to block transcription, as in the trp operon, which regulates genes involved in amino acid synthesis.
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Role of the Repressor Protein
The repressor protein controls gene expression by binding to the operator region of an operon. Its activity is modulated by effectors: in inducible systems, the repressor is inactivated by an inducer, while in repressible systems, it is activated by a corepressor, thus either permitting or blocking transcription.
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Related Practice
Textbook Question
Write a brief essay that discusses why you think regulatory systems evolved in bacteria (i.e., what advantages do regulatory systems provide to these organisms?), and, in the context of regulation, discuss why genes related to common functions are found together in operons.
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Textbook Question
Contrast positive versus negative control of gene expression.
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Textbook Question
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⁺
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Textbook Question
For the genotypes and conditions (lactose present or absent) shown in the following table, predict whether functional enzymes, nonfunctional enzymes, or no enzymes are made.
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Textbook Question
The locations of numerous lacI⁻ and lacIˢ mutations have been determined within the DNA sequence of the lacI gene. Among these, lacI⁻ mutations were found to occur in the 5′-upstream region of the gene, while lacIˢ mutations were found to occur farther downstream in the gene. Are the locations of the two types of mutations within the gene consistent with what is known about the function of the repressor that is the product of the lacI gene?
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