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

Lac Operon

Genetics

12. Gene Regulation in Prokaryotes

Lac Operon

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2:18 minutes

Problem 16c

Textbook Question

In the lac operon, what are the likely effects on operon gene transcription of the mutations described in a–e?
Mutation of the lacI gene affecting the allosteric site of the protein

Verified step by step guidance

Understand the role of the lacI gene: The lacI gene encodes the lac repressor protein, which binds to the operator region of the lac operon to inhibit transcription. The allosteric site of the lac repressor is where the inducer (allolactose) binds, causing a conformational change that prevents the repressor from binding to the operator.

Analyze the mutation: A mutation in the allosteric site of the lacI gene would likely affect the ability of the repressor protein to bind the inducer (allolactose). This could result in the repressor being unable to undergo the conformational change necessary to release from the operator.

Predict the effect on transcription: If the repressor cannot bind the inducer, it will remain bound to the operator, continuously blocking RNA polymerase from transcribing the operon genes. This would lead to a lack of transcription, even in the presence of lactose.

Consider alternative scenarios: If the mutation causes the allosteric site to mimic the induced state permanently, the repressor may never bind to the operator, leading to constitutive (continuous) transcription of the operon genes, regardless of lactose presence.

Summarize the outcomes: Depending on the specific nature of the mutation, the lac operon could either be permanently repressed (no transcription) or constitutively active (continuous transcription). The exact effect depends on whether the mutation prevents inducer binding or mimics the induced state.

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Key Concepts

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

Lac Operon

The lac operon is a set of genes in E. coli that are involved in the metabolism of lactose. It consists of structural genes (lacZ, lacY, and lacA) and regulatory elements that control their expression. The operon is typically off when lactose is absent and is activated in its presence, allowing the bacteria to utilize lactose as an energy source.

LacI Gene and Repressor Protein

The lacI gene encodes the lac repressor protein, which binds to the operator region of the lac operon to inhibit transcription when lactose is not present. The repressor can undergo conformational changes when bound to allolactose, a lactose metabolite, allowing RNA polymerase to access the promoter and initiate transcription of the operon genes.

Allosteric Regulation

Allosteric regulation refers to the modulation of a protein's function through the binding of an effector molecule at a site other than the active site. In the context of the lac operon, mutations affecting the allosteric site of the lac repressor can disrupt its ability to bind allolactose, potentially leading to continuous repression of the operon, even in the presence of lactose.

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Textbook Question

Neelaredoxin is a 15-kDa protein that is a gene product common in anaerobic bacteria. It has superoxide-scavenging activity, and it is constitutively expressed. In addition, its expression is not further induced during its exposure to O₂ or H₂O₂ [Silva, G. et al. (2001). J. Bacteriol. 183:4413 4420]. What do the terms constitutively expressed and induced mean in terms of neelaredoxin synthesis?

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Textbook Question

The creation of milk products such as cheeses and yogurts is dependent on the conversion by various anaerobic bacteria, including several Lactobacillus species, of lactose to glucose and galactose, ultimately producing lactic acid. These conversions are dependent on both permease and β-galactosidase as part of the lac operon. After selection for rapid fermentation for the production of yogurt, one Lactobacillus subspecies lost its ability to regulate lac operon expression [Lapierre, L., et al. (2002). J. Bacteriol. 184:928–935]. Would you consider it likely that in this subspecies the lac operon is on or off? What genetic events would likely contribute to the loss of regulation as described above?

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Textbook Question

In the lac operon, what are the likely effects on operon gene transcription of the mutations described in a–e?

Mutation of the CAP binding site of the lac promoter

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Textbook Question

In the lac operon, what are the likely effects on operon gene transcription of the mutations described in a–e?

Mutation of the lacI gene affecting the DNA-binding site of the protein

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Textbook Question

In the lac operon, what are the likely effects on operon gene transcription of the mutations described in a–e?

Mutation of the repressor binding site on the operator sequence

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Textbook Question

In the lac operon, what are the likely effects on operon gene transcription of the mutations described in a–e?

Mutation of consensus sequence in the lac promoter

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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⁺

I⁺ P⁺ Oᶜ Z⁺ Y⁻