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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3:45 minutes

Problem 25a

Textbook Question

What is the likely effect of each of the following mutations of the trpL region on attenuation control of trp operon gene transcription? Explain your reasoning.
Region 3 is deleted.

Verified step by step guidance

Understand the role of the trpL region in attenuation control: The trpL region is part of the leader sequence of the trp operon and contains sequences that form stem-loop structures (regions 1-4). These structures are critical for regulating transcription termination based on tryptophan levels.

Identify the function of Region 3: Region 3 is involved in forming the antiterminator stem-loop (with Region 2) and the terminator stem-loop (with Region 4). The antiterminator prevents transcription termination, while the terminator promotes it.

Analyze the effect of deleting Region 3: Without Region 3, the antiterminator stem-loop cannot form, and the terminator stem-loop (Region 3-4 pairing) is also disrupted. This likely prevents attenuation control, as the operon cannot respond to tryptophan levels effectively.

Consider the transcriptional outcome: If attenuation control is lost, transcription of the trp operon genes may proceed unchecked, regardless of tryptophan levels. This could lead to overproduction of tryptophan biosynthesis enzymes even when tryptophan is abundant.

Summarize the reasoning: The deletion of Region 3 disrupts the structural elements required for attenuation control, impairing the operon's ability to regulate transcription termination in response to tryptophan availability.

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Key Concepts

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

Attenuation in Gene Regulation

Attenuation is a regulatory mechanism in prokaryotes, particularly in the trp operon, that controls gene expression based on the availability of tryptophan. It involves the formation of specific RNA structures during transcription that can either promote or inhibit the continuation of transcription, depending on the concentration of tryptophan in the cell.

trp Operon Structure

The trp operon consists of a set of genes responsible for tryptophan biosynthesis in bacteria. It includes regulatory regions such as the leader sequence (trpL), which contains sequences that can form alternative secondary structures, influencing whether transcription is terminated early or continues to produce the enzymes needed for tryptophan synthesis.

Role of Region 3 in Attenuation

Region 3 of the trpL sequence plays a critical role in the formation of the RNA secondary structures that determine whether transcription will be terminated. If Region 3 is deleted, it disrupts the ability to form the necessary structures for attenuation, likely leading to increased transcription of the trp operon genes, regardless of tryptophan levels.

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Textbook Question

A repressible operon system, like the trp operon, contains three genes, G, Z, and W. Operon genes are synthesized when the end product of the operon synthesis pathway is absent, but there is no synthesis when the end product is present. One of these genes is an operator, one is a regulatory protein, and the other is a structural enzyme involved in synthesis of the end product. In the table below, '+' indicates that the enzyme is synthesized by the operon and '−' means that no enzyme synthesis occurs. Use this information to determine which gene corresponds to each operon function.

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Textbook Question

The following figure depicts numerous critical regions of the leader sequence of mRNA that play important roles during the process of attenuation in the trp operon. A closer view of the leader sequence, which begins at about position 30 downstream from the 5' end, is shown below, running along both columns. Within this molecule are the sequences that cause the formation of the alternative hairpins. It also contains the successive triplets that encode tryptophan, where stalling during translation occurs.

Take a large piece of paper (such as manila wrapping paper) and, along with several other students from your genetics class, work through the base sequence to identify the trp codons and the parts of the molecule representing the base-pairing regions that form the terminator and antiterminator hairpins shown in the following figure.

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Textbook Question