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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2:09 minutes

Problem 8

Textbook Question

Describe the experimental rationale that allowed the lac repressor to be isolated.

Verified step by step guidance

Understand the lac operon system: The lac operon in *E. coli* is a set of genes involved in lactose metabolism. The lac repressor is a protein that binds to the operator region of the operon to inhibit transcription in the absence of lactose. Familiarize yourself with this regulatory mechanism to understand the context of the experiment.

Identify the need for isolation: Scientists needed to isolate the lac repressor to study its structure, function, and interaction with DNA and inducers (like allolactose). This required a method to separate the repressor protein from other cellular components.

Use of mutant strains: Researchers used mutant strains of *E. coli* that overproduced the lac repressor protein. These mutants were engineered to have multiple copies of the lacI gene (the gene encoding the lac repressor), leading to higher levels of the repressor protein in the cell.

Affinity for DNA and inducers: The lac repressor binds specifically to the operator DNA sequence and can also bind to inducers like IPTG (isopropyl β-D-1-thiogalactopyranoside). This property was exploited to isolate the repressor by using DNA or IPTG as bait in affinity chromatography techniques.

Purification process: The lac repressor was purified using biochemical techniques such as affinity chromatography, where IPTG or operator DNA was immobilized on a column to selectively bind the repressor. The bound repressor was then eluted using a high concentration of IPTG or by altering the ionic conditions, allowing its isolation for further study.

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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 responsible for the metabolism of lactose. It consists of structural genes (lacZ, lacY, and lacA) and regulatory elements, including the lac repressor. Understanding the lac operon is crucial for grasping how gene expression is controlled in response to environmental changes, particularly the presence or absence of lactose.

Repressor Proteins

Repressor proteins are molecules that bind to specific DNA sequences to inhibit the transcription of genes. In the case of the lac operon, the lac repressor binds to the operator region, preventing RNA polymerase from transcribing the downstream genes when lactose is absent. This mechanism is a key example of negative regulation in gene expression.

Experimental Isolation Techniques

Experimental isolation techniques, such as affinity chromatography and gel electrophoresis, are methods used to purify specific proteins from complex mixtures. In the context of isolating the lac repressor, researchers utilized these techniques to separate the repressor from other cellular components, allowing for detailed study of its function and interaction with the lac operon.

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Related Practice

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 InducibleI⁺O⁺Z⁺ xI⁻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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Textbook Question

The CAP binding site in the lac promoter is the location of positive regulation of gene expression for the operon. Identify what binds at this site to produce positive regulation, under what circumstances binding occurs, and how binding exerts a positive effect.

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Textbook Question

What role does cAMP play in transcription of lac operon genes? What role does CAP play in transcription of lac operon genes?

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Textbook Question

What properties demonstrate that the lac repressor is a protein? Describe the evidence that it indeed serves as a repressor within the operon system.

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Textbook Question

How would a cap⁻ mutation that produces an inactive CAP protein affect transcriptional control of the lac operon?