Question 1

If a researcher moves the promoter for the lac operon to the region between the beta galactosidase (lacZ) gene and the permease (lacY) gene, which of the following results would be most likely?
A) The three genes of the lac operon will be expressed normally.
B) RNA polymerase will no longer transcribe permease.
C) The operon will still transcribe the lacZ and lacY genes, but the mRNA will not be translated.
D) Beta galactosidase will not be produced.

Accepted Answer

Moving the promoter to the region between the lacZ and lacY genes means that RNA polymerase will not initiate transcription at the normal site, leading to the lack of beta galactosidase production since the lacZ gene is upstream of the new promoter location.

Question 2

Under what conditions does the trp repressor block transcription of the trp operon?
A) when the repressor binds to the inducer
B) when the repressor binds to tryptophan
C) when the repressor is not bound to RNA polymerase
D) when the repressor is not bound to the operator

Accepted Answer

The trp repressor blocks transcription of the trp operon when it binds to tryptophan. This complex then binds to the operator region of the operon, preventing RNA polymerase from transcribing the genes involved in tryptophan synthesis.

Question 3

A rise in extracellular glucose results in which of the following changes at the lac operon?
A) RNA polymerase binds the lac promoter with greater affinity
B) cAMP receptor protein (CRP) detaches from the lac promoter
C) lac repressor is allosterically inactivated
D) lac operator binds inducer with greater affinity

Accepted Answer

When extracellular glucose levels are high, the intracellular levels of cyclic AMP (cAMP) decrease. The cAMP receptor protein (CRP), also known as the catabolite activator protein (CAP), requires cAMP to bind to the DNA. Therefore, with lower cAMP levels due to high glucose, CRP detaches from the lac promoter, reducing the transcription of the lac operon.

Question 4

How does breakdown of glucose inhibit transcription of the lac operon?
A) by strengthening the binding of the repressor to the operator
B) by weakening the binding of the repressor to the operator
C) by inhibiting RNA polymerase from opening the strands of DNA to initiate transcription
D) by reducing the levels of intracellular cAMP

Accepted Answer

The breakdown of glucose reduces the levels of intracellular cAMP, which is necessary for the activation of CAP (catabolite activator protein). CAP, when bound to cAMP, facilitates the binding of RNA polymerase to the lac operon promoter, enhancing transcription. Low cAMP levels due to glucose presence result in reduced transcription of the lac operon.

Question 5

If a researcher moves the repressor gene (lacI) and its promoter, to a position at some several thousand base pairs away from its normal position, which of the following describes the most likely effect on the lac operon?
A) The repressor will no longer bind to the operator.
B) The repressor will no longer bind to the inducer.
C) The lac operon will be expressed continuously.
D) The lac operon will function normally.

Accepted Answer

The lac operon's functionality is primarily determined by the interaction between the repressor protein and the operator region. Moving the lacI gene and its promoter does not inherently alter the ability of the repressor protein to be synthesized or its ability to bind to the operator. Thus, as long as the repressor can still be produced and can reach the operator, the operon can function normally. The physical distance in the DNA between the lacI gene and the operon does not directly affect the interaction between the repressor protein and the operator, thanks to the nature of DNA looping and protein diffusion within the cell.

Question 6

Which of the following conditions is most likely to result in transcription of the lac operon?
A) There is more glucose in the cell than lactose.
B) There is glucose but no lactose in the cell.
C) The cAMP and lactose levels are both high within the cell.
D) The cAMP level is high and the lactose level is low.

Accepted Answer

The lac operon is transcribed when lactose is present to act as an inducer, removing the repressor from the operon, and when glucose levels are low, leading to high cAMP levels which facilitate the binding of CAP (catabolite activator protein) to the DNA, enhancing transcription.

Question 7

The cAMP receptor protein (CRP) is said to be responsible for positive regulation of the lac operon because ________.
A) CRP levels rise when lactose is present in the environment
B) CRP promotes the production of allolactose
C) CRP prevents binding of the repressor to the operator
D) CRP bound to the CRP-binding site stimulates the transcription of the lac operon

Accepted Answer

CRP (cAMP receptor protein), when bound to cAMP, binds to a specific site near the lac operon, enhancing the binding of RNA polymerase to the promoter, thereby stimulating the transcription of the lac operon genes. This action does not directly involve lactose, allolactose production, or prevention of repressor binding to the operator.

Question 8

When taken up by a cell, which of the following molecules binds to a repressor so that the repressor no longer binds to an operator?
A) inducer
B) promoter
C) cAMP
D) corepressor

Accepted Answer

Inducers bind to repressors and cause them to change shape, which prevents the repressor from binding to the operator. This allows transcription to proceed by freeing up the DNA for RNA polymerase action.

Question 9

According to the lac operon model proposed by Jacob and Monod, what is predicted to occur if the operator is removed from the operon?
A) The lac operon would be transcribed continuously.
B) Only lacZ would be transcribed.
C) Only lacY would be transcribed.
D) Genes involved in glucose metabolism would not be transcribed.

Accepted Answer

Removing the operator from the lac operon removes the binding site for the repressor protein, leading to continuous transcription of the operon genes, as there is no longer a mechanism to block RNA polymerase.

Question 10

Which of the following molecules is a protein produced by a regulatory gene?
A) operon
B) operator
C) promoter
D) repressor

Accepted Answer

Repressor proteins are produced by regulatory genes and are involved in the process of gene regulation, specifically in prokaryotes, by binding to operator regions and inhibiting gene transcription.

Question 11

If a mutation deactivated a regulatory gene of a repressible operon in an E. coli cell which of the following describes the most likely outcome?
A) continuous transcription of the structural gene controlled by that regulator
B) complete inhibition of transcription of the structural gene controlled by that regulator
C) irreversible binding of the repressor to the operator
D) continuous translation of the mRNA because of alteration of its structure

Accepted Answer

In a repressible operon, the regulatory gene typically codes for a repressor protein that can bind to the operator region and inhibit transcription of the structural genes when a specific corepressor is present. If a mutation deactivates this regulatory gene, the repressor protein would not be produced, leading to continuous transcription of the structural genes because there is nothing to inhibit their transcription.

Question 12

Which of the following predictions about the survival of a lactose intolerant E. coli cell is most likely correct?
A) The cell will survive if the environment has sufficient glucose.
B) The cell will survive if the cell has high levels of lacI expression.
C) The cell will not survive because E. coli require lactose as a nutrient source for life.
D) The cell will not survive because inducible operons must be stimulated for the cell to survive.

Accepted Answer

E. coli cells can utilize glucose as a primary energy source, so a lactose intolerant E. coli cell can survive in an environment with sufficient glucose, even if it cannot metabolize lactose.

Question 13

Under which condition do high levels of transcription of structural genes occur in an inducible operon?
A) It occurs continuously in the cell.
B) It starts when the pathway's substrate is present.
C) It starts when the pathway's product is present.
D) It stops when the pathway's product is present.

Accepted Answer

In an inducible operon, high levels of transcription of structural genes occur when the substrate for the pathway controlled by the operon is present. This substrate acts as an inducer, binding to a repressor protein and preventing it from repressing the operon, thus allowing transcription to proceed.

Question 14

Which of the following changes in conditions results in an increase in cAMP receptor protein (CRP) mediated initiation of transcription?
A) an increase in glucose and an increase in the repressor
B) a decrease in glucose and an increase in cAMP
C) an increase in glucose and a decrease in cAMP
D) a decrease in glucose and a decrease in the repressor

Accepted Answer

CRP-mediated initiation of transcription is enhanced when glucose levels are low, leading to an increase in cAMP. CRP binds cAMP, and the complex then promotes transcription of genes, including those for the utilization of alternative sugars.

Question 15

If a researcher moves the operator to the far end of the operon, which of the following processes would likely occur when the cell is exposed to lactose?
A) The inducer will no longer bind to the repressor.
B) The repressor will no longer bind to the operator.
C) The operon will never be transcribed.
D) The genes of the lac operon will be transcribed continuously.

Accepted Answer

Moving the operator to the far end of the operon would likely prevent the repressor from effectively blocking RNA polymerase, leading to continuous transcription of the lac operon genes, regardless of lactose presence.

Question 16

Which of the following is a molecule that helps to "turn off" the expression of genes in a cell?
A) operator
B) inducer
C) promoter
D) corepressor

Accepted Answer

Corepressors are molecules that can bind to repressor proteins and increase their affinity for operator sequences, thereby helping to "turn off" the expression of genes by preventing transcription.

Question 17

For a repressible operon to be transcribed, which of the following conditions must occur?
A) RNA polymerase must bind to the promoter and a corepressor must be present.
B) RNA polymerase and active repressor must occupy the promoter.
C) RNA polymerase must bind to the promoter, and the repressor must be inactive.
D) RNA polymerase must bind the inducer, and the repressor must be inactive.

Accepted Answer

For transcription of a repressible operon to occur, RNA polymerase must be able to bind to the promoter, and the repressor protein must be inactive or not bound to the operator. This allows transcription to proceed.

Question 18

Prokaryote's ability to regulate patterns of gene expression most likely promotes the organism's survival by ________.
A) organizing gene expression, so that genes are expressed in a given order
B) allowing each gene to be expressed an equal number of times
C) allowing an organism to adjust to changes in environmental conditions
D) allowing environmental changes to alter a prokaryote's genome

Accepted Answer

Prokaryotes can rapidly adjust their gene expression in response to environmental changes, enabling them to survive in varying conditions by efficiently utilizing resources and responding to stressors.

Question 19

Which of the following molecules binds with a repressor to alter its conformation and therefore affect its function?
A) inducer
B) promoter
C) transcription factor
D) cAMP

Accepted Answer

Inducers bind to repressors or activators to alter their conformation, affecting their ability to bind DNA, thereby regulating gene expression.

Question 20

A mutation in E. coli results in a molecule known as a "super-repressor" because the operon is permanently repressed. Which of the following describes the most likely effect of the mutation on the repressor protein?
A) It cannot bind to the operator.
B) It cannot make a functional repressor.
C) It cannot bind to the inducer.
D) It makes a repressor that binds CAP.

Accepted Answer

The mutation likely alters the repressor protein in a way that it cannot bind to the inducer. Normally, the inducer would bind to the repressor, causing it to change shape and release from the operator, allowing transcription. If the repressor cannot bind the inducer, it remains bound to the operator, continuously repressing the operon.

Quiz 18: Control of Gene Expression

If a researcher moves the promoter for the lac operon to the region between the beta galactosidase (lacZ) gene and the permease (lacY) gene, which of the following results would be most likely?

Under what conditions does the trp repressor block transcription of the trp operon?

A rise in extracellular glucose results in which of the following changes at the lac operon?

How does breakdown of glucose inhibit transcription of the lac operon?

If a researcher moves the repressor gene (lacI) and its promoter, to a position at some several thousand base pairs away from its normal position, which of the following describes the most likely effect on the lac operon?

Which of the following conditions is most likely to result in transcription of the lac operon?

The cAMP receptor protein (CRP) is said to be responsible for positive regulation of the lac operon because ________.

When taken up by a cell, which of the following molecules binds to a repressor so that the repressor no longer binds to an operator?

According to the lac operon model proposed by Jacob and Monod, what is predicted to occur if the operator is removed from the operon?

Which of the following molecules is a protein produced by a regulatory gene?

If a mutation deactivated a regulatory gene of a repressible operon in an E. coli cell which of the following describes the most likely outcome?

Which of the following predictions about the survival of a lactose intolerant E. coli cell is most likely correct?

Under which condition do high levels of transcription of structural genes occur in an inducible operon?

Which of the following changes in conditions results in an increase in cAMP receptor protein (CRP) mediated initiation of transcription?

If a researcher moves the operator to the far end of the operon, which of the following processes would likely occur when the cell is exposed to lactose?

Which of the following is a molecule that helps to "turn off" the expression of genes in a cell?

For a repressible operon to be transcribed, which of the following conditions must occur?

Prokaryote's ability to regulate patterns of gene expression most likely promotes the organism's survival by ________.

Which of the following molecules binds with a repressor to alter its conformation and therefore affect its function?

A mutation in E. coli results in a molecule known as a "super-repressor" because the operon is permanently repressed. Which of the following describes the most likely effect of the mutation on the repressor protein?