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Chapter 21: Problem 25

Researchers have isolated bacterial cells with mutations in various segments of the lac operon. What is the effect on gene expression if a mutation in the operator occurs so that the repressor cannot bind? What happens when lactose is added to the growth medium of these mutants?

Short Answer

Expert verified
The mutation causes continuous gene expression; lactose presence has no additional effect.

Step by step solution

01

Understand the Lac Operon Components

The lac operon consists of the promoter, operator, and the genes coding for enzymes needed for lactose metabolism. The operator acts as a switch controlled by a repressor protein, which regulates the expression of these genes by binding to the operator.
02

Effect of Operator Mutation

A mutation in the operator that prevents the repressor from binding leads to the inability of the repressor to turn off the operon. This means the genes in the lac operon will be expressed continuously (constitutively), regardless of lactose presence.
03

Adding Lactose to Mutant Cells

Adding lactose to the growth medium of these mutant cells has no additional effect on gene expression because the operon is already being continuously expressed due to the mutation preventing repressor binding.
04

Summarize the Outcome

Without the ability of the repressor to bind, the lac operon remains active, and the genes for lactose metabolism are constantly transcribed, leading to continuous production of the enzymes, regardless of lactose availability.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Operator Mutation
The lac operon is a crucial part of gene regulation in bacteria. It involves a segment called the operator, which plays a key role in controlling gene expression. When an operator mutation occurs, it causes changes in the operator's DNA sequence.
This mutation can have significant effects. Specifically, if the repressor protein cannot bind to the mutated operator, it results in a condition known as constitutive expression of the operon.
  • This means that the genes downstream of the operon are expressed continuously, irrespective of environmental cues like the presence or absence of lactose.
  • In normal conditions, the operator acts like an on-off switch, but this switch is broken in the case of a mutation that prevents repressor binding.
Constitutive expression can have metabolic consequences for the cell as it may lead to unnecessary production of enzymes.
Gene Expression
Gene expression refers to the process by which information from a gene is used to synthesize a functional gene product, typically proteins like enzymes. In the context of the lac operon, gene expression is regulated by environmental signals.
Without regulation, such as in the case of an operator mutation, genes are expressed without control
  • This could mean that cells produce enzymes even when they are not needed.
  • In efficient cells, genes are turned on only when necessary, helping the cell conserve resources.
Understanding how gene expression is controlled is key in biotechnology and medicine, as it opens doors to modifying organisms for various purposes.
Repressor Protein
The repressor protein is fundamental in the regulation of the lac operon. It functions by binding to the operator to inhibit gene expression. This binding prevents RNA polymerase from transcribing the operon.
The absence of functional repressor binding due to a mutation means that this inhibitory mechanism fails.
  • Normally, the presence of a repressor protein ensures that the operon's genes are not expressed when lactose is absent.
  • Thus, the failure of the repressor to bind, as happens in operator mutations, leads to unscheduled expression of these genes.
The repressor protein is an example of a negative control mechanism in bacterial gene regulation.
Lactose Metabolism
Lactose metabolism involves the breakdown of lactose into simpler sugars for use by the cell. The lac operon plays an instrumental role in this process.
When lactose is present, it induces a change in the repressor protein, relieving inhibition and allowing gene expression. However:
  • With an operator mutation preventing repressor binding, lactose presence doesn't alter gene expression because the operon is already unregulated.
  • This leads to a constant production of the enzymes needed for lactose breakdown, regardless of whether lactose is available or not.
In normal conditions, efficient lactose metabolism helps bacteria extract energy only when lactose is present, helping them survive and thrive in changing environments.

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Most popular questions from this chapter

In bacterial cells, the genes that code for the enzymes of the tryptophan biosynthetic pathway are organized in an operon as shown below. Another gene encodes a repressor protein that binds tryptophan. How does the repressor protein control the expression of the genes in the trp operon?

Proteins can interact with DNA through relatively weak forces, such as hydrogen bonds and van der Waals interactions, as well as through stronger electrostatic interactions such as ion pairs. Which types of interactions predominate for sequence-specific DNA-binding proteins and for sequence- independent binding proteins?

Why is it effective for a bacterial cell to organize genes for related functions as an operon? How do eukaryotes achieve the same benefits?

DNA methylation requires the methyl group donor \(S\)-adenosylmethionine, which is produced by the condensation of methionine with ATP. The sulfonium ion's methyl group is used in methyl-group transfer reactions. a. The demethylated \(S\)-adenosylmethionine is then hydrolyzed to produce adenosine and a nonstandard amino acid. Draw the structure of this amino acid. How does the cell convert this compound back to methionine to regenerate \(S\)-adenosylmethionine? b. The proper regulation of gene expression requires methylation as well as demethylation of cytosine residues in DNA. If a demethylase carries out a hydrolytic reaction to restore cytosine residues, what is the other reaction product?

A bacterial strain expresses a mutant lac repressor protein that retains its ability to bind to the operator but cannot bind lactose. What is the effect on gene expression in these mutants? What happens when lactose is added to the growth medium?
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