Table of contents

1. Introduction to Genetics51m
2. Mendel's Laws of Inheritance3h 37m
3. Extensions to Mendelian Inheritance2h 41m
4. Genetic Mapping and Linkage2h 28m
5. Genetics of Bacteria and Viruses1h 21m
6. Chromosomal Variation1h 48m
7. DNA and Chromosome Structure56m
8. DNA Replication1h 10m
9. Mitosis and Meiosis1h 34m
10. Transcription1h 0m
11. Translation58m
12. Gene Regulation in Prokaryotes1h 19m
13. Gene Regulation in Eukaryotes44m
14. Genetic Control of Development44m
15. Genomes and Genomics1h 50m
16. Transposable Elements47m
17. Mutation, Repair, and Recombination1h 6m
18. Molecular Genetic Tools19m
- Genetic Cloning
  9m
- Methods for Analyzing DNA
  9m
19. Cancer Genetics29m
- Overview of Cancer
  21m
- Cancer Mutations
  7m
20. Quantitative Genetics1h 26m
21. Population Genetics50m
- Hardy Weinberg
  19m
- Allelic Frequency Changes
  31m
22. Evolutionary Genetics29m

12. Gene Regulation in Prokaryotes

Tryptophan Operon and Attenuation

Genetics

12. Gene Regulation in Prokaryotes

Tryptophan Operon and Attenuation

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3:11 minutes

Problem 4c

Textbook Question

Identify similarities and differences between an inducible operon and a repressible operon in terms of
The organization of structural genes of the operon.

Verified step by step guidance

Understand the concept of an operon: An operon is a cluster of genes under the control of a single promoter and regulatory elements. It is a key feature of prokaryotic gene regulation.

Define an inducible operon: An inducible operon is typically off by default and requires the presence of an inducer molecule to activate transcription. A classic example is the lac operon in *E. coli*.

Define a repressible operon: A repressible operon is typically on by default and can be turned off when a specific molecule (corepressor) binds to the repressor protein. A classic example is the trp operon in *E. coli*.

Compare the organization of structural genes: Both inducible and repressible operons have structural genes organized in a contiguous sequence, which are transcribed together as a single mRNA (polycistronic mRNA). This allows coordinated expression of genes involved in a common pathway.

Highlight the regulatory differences: In an inducible operon, the repressor protein is active by default and binds to the operator to block transcription until the inducer molecule inactivates the repressor. In a repressible operon, the repressor protein is inactive by default and requires a corepressor molecule to bind to the operator and inhibit transcription.

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Key Concepts

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

Operon Structure

An operon is a cluster of genes under the control of a single promoter, allowing for coordinated expression. In prokaryotes, operons can be classified as inducible or repressible based on their regulatory mechanisms. The structural genes within an operon are transcribed together, producing a single mRNA that encodes multiple proteins, which can be functionally related.

Inducible Operon

An inducible operon is typically off and can be turned on in response to an inducer molecule. This type of operon, such as the lac operon in E. coli, allows for the expression of genes involved in metabolizing specific substrates when they are present. The presence of the inducer leads to the inactivation of a repressor protein, enabling transcription of the structural genes.

Repressible Operon

A repressible operon is usually active and can be turned off by a corepressor molecule. An example is the trp operon, which regulates the synthesis of tryptophan. When tryptophan levels are high, it binds to the repressor, activating it and blocking transcription of the operon, thus preventing the unnecessary production of tryptophan.

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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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Textbook Question

Transcriptional regulation of operon gene expression involves the interaction of molecules with one another and of regulatory molecules with segments of DNA. In this context, define and give an example of each of the following:

Attenuation

Corepressor

Identify similarities and differences between an inducible operon and a repressible operon in terms of

The presence and action of allosteric regulatory molecules.

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Textbook Question

Is attenuation the product of an allosteric effect? Is attenuation the result of a transcriptional or a translational activity? Explain your answers.

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Textbook Question

The trpL region contains four repeated DNA sequences that lead to the formation of stem-loop structures in mRNA. What are these stem-loop structures, and how do they affect transcription of the structural genes of the trp operon?

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Textbook Question

Explain the circumstances under which attenuation of operon gene expression is advantageous to a bacterial organism. Would you expect attenuation to be found in a single-celled eukaryote? In a multicelled eukaryote?

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Textbook Question

Describe the role of attenuation in the regulation of tryptophan biosynthesis.

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