Question 1

Which of the following levels of gene expression allows the most rapid response to environmental change?
A) transcriptional control
B) translational control
C) post-translational control

Accepted Answer

Post-translational control allows for the most rapid response to environmental change because it involves quick modifications to existing proteins, rather than waiting for transcription and translation to create new ones.

Question 2

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

Accepted Answer

A repressor is a protein produced by a regulatory gene that can bind to the operator region of an operon and prevent transcription of the structural genes. The other choices listed are not proteins produced by regulatory genes.

Question 3

The greatest expression of the lac operon occurs when lactose levels are ________.
A) low and glucose levels are low
B) low and glucose levels are high
C) high and glucose levels are low
D) high and glucose levels are high

Accepted Answer

The lac operon is turned on when lactose is present and glucose is absent. This is because lactose serves as an inducer that binds to the repressor protein and prevents it from binding to the operator region of the operon, allowing for RNA polymerase to transcribe the genes involved in lactose metabolism. High levels of lactose indicate that it is a good energy source for the cell, so it makes sense for the lac operon to be turned on under these conditions. However, if glucose is also present, the cell will preferentially use glucose as the primary energy source, even if lactose is available. This is known as catabolite repression and is mediated by the cAMP-CAP complex, which binds to the promoter region of the lac operon and enhances RNA polymerase binding, leading to increased transcription. Therefore, the greatest expression of the lac operon occurs when lactose levels are high and glucose levels are low.

Question 4

A compound known as X-gal is widely used in molecular genetic research. When wild-type (normal) E. coli is grown on medium containing X-gal, the bacterial colonies turn blue. In contrast, when lacZ mutants are grown on medium containing X-gal, the bacterial colonies remain their normal white color. X-gal is likely to be a compound ________.
A) chemically similar to glucose
B) chemically similar to galactose
C) chemically similar to lactose
D) that cannot be transported into lacZ mutants
E) that can be transported only into lacA mutants

Accepted Answer

X-gal is a synthetic compound that is chemically similar to lactose, a sugar that is the natural substrate for the enzyme β-galactosidase. When wild-type E. coli with functional β-galactosidase is grown on medium containing X-gal, the enzyme cleaves X-gal and produces a blue color compound. Mutant strains of E. coli that do not have a functional β-galactosidase enzyme cannot cleave X-gal, and therefore the colonies remain white.

Question 5

Allolactose, an isomer of lactose, is formed in small amounts from lactose. An E. coli cell is presented for the first time with the sugar lactose (containing allolactose) as a potential food source. Which of the following occurs when lactose enters the cell?
A) The repressor protein attaches to the regulator.
B) Allolactose binds to the repressor protein.
C) Allolactose binds to the regulator gene.
D) The repressor protein and allolactose bind to RNA polymerase.
E) RNA polymerase attaches to the regulator.

Accepted Answer

When lactose enters the E. coli cell, some of it will be converted into allolactose. Allolactose acts as an inducer for the lac operon, which regulates the expression of genes involved in lactose metabolism. The lac operon contains a gene for a repressor protein that normally binds to the operator region of the operon, preventing transcription of the genes. However, when allolactose is present, it binds to the repressor, causing a conformational change that prevents it from binding to the operator. This allows RNA polymerase to transcribe the genes, leading to the production of enzymes that can break down lactose into usable energy for the cell.

Question 6

Suppose several genes are grouped together in a bacterial genome and the group results in a single enzyme. If one of these genes is mutated and the enzyme is no longer active, this gene must be part of ________.
A) an intron
B) an operon
C) a repressor
D) an activator
E) an inducer

Accepted Answer

The group of genes that result in a single enzyme is called an operon. In an operon, all the genes are transcribed together as a single mRNA and the resulting protein products work together in a biochemical pathway. If one of the genes in the operon is mutated and the enzyme is no longer active, it will affect the entire pathway, indicating that the gene must be part of the operon. An intron is a non-coding region of DNA within a gene that is transcribed but later removed during RNA splicing. Repressors and activators are regulatory proteins that control gene expression, and an inducer is a small molecule that activates gene expression by binding to a regulatory protein. These terms are not directly relevant to the given scenario.

Question 7

If you wanted to prevent a regulatory protein from changing gene expression, you would have to prevent physical contact between the protein and ________.
A) ribosomes
B) DNA
C) ribozymes
D) mRNA

Accepted Answer

The regulatory protein acts by binding to specific regions of DNA, such as promoters, to either enhance or inhibit gene expression. Therefore, preventing physical contact between the protein and DNA would effectively prevent any change in gene expression.

Question 8

In negative control, a gene is switched off when ________.
A) a kinase adds a phosphate to DNA
B) lactose is transported into the cell
C) a regulatory protein binds to DNA and shuts down transcription
D) a regulatory protein is removed from DNA and shuts down transcription

Accepted Answer

Negative control refers to the regulation of gene expression by preventing the transcription of a gene. In this case, a regulatory protein binds to DNA and shuts down transcription, preventing the production of mRNA and protein.

Question 9

Imagine that you have set up a genetic screen to identify E. coli mutants that cannot metabolize the amino acid tryptophan for energy. Beginning with a master plate containing many colonies, you prepare replica plates on medium with glucose or tryptophan as the only energy source. You would look for colonies that ________.
A) cannot grow on either type of media
B) can grow on both types of media
C) can grow only on the plates with tryptophan
D) can grow only on the plates with glucose

Accepted Answer

The correct answer is D because the goal is to identify mutants that cannot metabolize tryptophan for energy. Therefore, these mutants should be able to grow on glucose (since their inability to metabolize tryptophan does not affect their ability to metabolize glucose) but not on tryptophan.

Question 10

In the first step of their experiments, Jacob and Monod treated E. coli cells with ultraviolet light or X-rays to ________.
A) decrease the rate of gene expression
B) induce DNA repair enzymes
C) increase the frequency of mutations in all genes
D) selectively mutate the lac operon, leaving all other genes unmutated

Accepted Answer

Jacob and Monod treated E. coli cells with ultraviolet light or X-rays to increase the frequency of mutations in all genes, not just the lac operon. This allowed them to isolate mutant strains with specific defects in the lac operon.

Question 11

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) Galactosidase permease would be produced but would be incapable of transporting lactose.

Accepted Answer

In the lac operon model, the operator serves as a regulatory element that blocks RNA polymerase from transcribing the structural genes (lacZ, lacY, lacA) in the absence of lactose. The removal of the operator would remove this block and allow RNA polymerase to bind and transcribe the genes continuously, regardless of the presence or absence of lactose.

Question 12

Bacterial and eukaryotic cells primarily control gene expression at the level of transcription. If instead cells exerted control of gene expression primarily at the post-translational level, what would be different?
A) The ability to rapidly respond to environmental change would be reduced.
B) Cells would expend significantly more energy.
C) Genes would no longer be transcribed.
D) Translation of mRNA into protein would not occur.

Accepted Answer

Controlling gene expression at the post-translational level means that the cell would first transcribe and translate proteins before regulating their activity or stability. This process is more energy-intensive because it involves synthesizing proteins that may not be needed immediately or in large quantities, leading to wasted resources.

Question 13

Most repressor proteins are allosteric. Which of the following binds with the repressor to alter its conformation?
A) inducer
B) promoter
C) RNA polymerase
D) transcription factor
E) cAMP

Accepted Answer

Inducer molecules bind to the repressor protein and cause a conformational change, leading to the release of the repressor from the DNA binding site and allowing gene expression.

Question 14

Which of the following, when taken up by a cell, binds to a repressor so that the repressor no longer binds to the operator?
A) ubiquitin
B) inducer
C) promoter
D) repressor
E) corepressor

Accepted Answer

An inducer is a small molecule that binds to a repressor protein, which prevents the repressor from binding to the operator. This allows transcription to occur, and the gene is expressed.

Question 15

The product of the lacI gene is ________.
A) an enzyme
B) a transport protein embedded within the membrane
C) β-galactosidase
D) the repressor

Accepted Answer

The lacI gene encodes a repressor protein that inhibits the transcription of the lac operon. Hence, the product of the lacI gene is the repressor. Choices A and B are incorrect because they do not pertain to the lacI gene; β-galactosidase is encoded by the lacZ gene.

Question 16

Jacob and Monod were intellectually primed to draw the conclusions they did concerning regulation of the lac operon. In part, this was due to their fascination with mechanisms of enzyme regulation. They knew that the activity of some enzymes is regulated when their reaction product binds to the enzyme, changing its shape and, therefore, its activity. This knowledge allowed them to easily make the intellectual leap to propose ________.
A) the existence of the CAP binding site
B) allosteric regulation of the repressor
C) positive regulation of the lac operon
D) the existence of the operator
E) co-transcribed and co-regulated genes of bacterial operons

Accepted Answer

Jacob and Monod's understanding of enzyme regulation through product binding and shape change led them to propose the concept of allosteric regulation, which is key to how the lac operon repressor functions.

Question 17

Altering patterns of gene expression in prokaryotes would most likely serve an 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 young organisms to respond differently than more mature organisms
E) allowing environmental changes to alter a prokaryote's genome

Accepted Answer

Altering patterns of gene expression in prokaryotes would allow for an organism to adjust to changes in environmental conditions, providing a survival advantage. This allows for the organism to respond to different situations as needed and increase their chances of survival.

Question 18

Although the expression of most genes is tightly regulated, some genes are expressed at roughly constant rates. Which of the following genes would you predict to be constitutively (constantly) expressed?
A) genes involved in the biosynthesis of the amino acid tryptophan
B) genes involved in the degradation of tryptophan
C) genes involved in the degradation of arabinose, a sugar
D) genes that code for regulatory proteins

Accepted Answer

Genes that code for regulatory proteins are often expressed constitutively because these proteins are required at all times to maintain basic cellular functions and respond to changes in the cell's environment. Regulatory proteins include transcription factors and enzymes involved in signaling pathways, which are essential for the constant regulation of gene expression and other cellular processes.

Question 19

The product of the lacI gene functions most like a car's ________.
A) parking brake
B) accelerator pedal
C) steering wheel
D) engine
E) gear shift

Accepted Answer

The lacI gene acts as a repressor for the lac operon, preventing the expression of genes involved in lactose metabolism. This repression can be thought of as a "brake" that stops the expression of these genes until lactose is present and needed as an energy source. Just like a parking brake prevents a car from moving until released.

Question 20

An E. coli cell without a functional lacI gene is expected to ________.
A) never produce β-galactosidase
B) always produce β-galactosidase
C) be unable to transport lactose into the cell
D) be unable to metabolize lactose within the cell

Accepted Answer

The lacI gene encodes for the lac repressor protein that binds to the operator region of the lac operon, inhibiting transcription of the lac genes. If the lacI gene is not functional, there will be no inhibition of the lac operon and β-galactosidase will always be produced in the presence of lactose. Therefore, choice B is the correct answer. Choices A, C, and D are incorrect because the absence of a functional lacI gene does not affect the production, transport, or metabolism of lactose in E. coli cells.

Quiz 18: Control of Gene Expression in Bacteria

Which of the following levels of gene expression allows the most rapid response to environmental change?

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

The greatest expression of the lac operon occurs when lactose levels are ________.

Allolactose, an isomer of lactose, is formed in small amounts from lactose. An E. coli cell is presented for the first time with the sugar lactose (containing allolactose) as a potential food source. Which of the following occurs when lactose enters the cell?

Suppose several genes are grouped together in a bacterial genome and the group results in a single enzyme. If one of these genes is mutated and the enzyme is no longer active, this gene must be part of ________.

If you wanted to prevent a regulatory protein from changing gene expression, you would have to prevent physical contact between the protein and ________.

In negative control, a gene is switched off when ________.

In the first step of their experiments, Jacob and Monod treated E. coli cells with ultraviolet light or X-rays to ________.

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

Bacterial and eukaryotic cells primarily control gene expression at the level of transcription. If instead cells exerted control of gene expression primarily at the post-translational level, what would be different?

Most repressor proteins are allosteric. Which of the following binds with the repressor to alter its conformation?

Which of the following, when taken up by a cell, binds to a repressor so that the repressor no longer binds to the operator?

The product of the lacI gene is ________.

Altering patterns of gene expression in prokaryotes would most likely serve an organism's survival by ________.

Although the expression of most genes is tightly regulated, some genes are expressed at roughly constant rates. Which of the following genes would you predict to be constitutively (constantly) expressed?

The product of the lacI gene functions most like a car's ________.

An E. coli cell without a functional lacI gene is expected to ________.