Question 1

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

Accepted Answer

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 ribosomal RNAs
E)  genes involved in the transport of the sugar maltose D

Question 2

The trp repressor blocks transcription of the trp operon when the repressor _____.

Accepted Answer

A)  binds to inducer
B)  is not bound to inducer
C)  binds to tryptophan
D)  is not bound to tryptophan
E)  is not bound to the operator C

Question 3

How does the inducer change the repressor's affinity for DNA?

Accepted Answer

A)  Inducer binding changes the repressor's shape so that the promoter is bent from a rectangular to a loop-like shape.
B)  Inducer binding changes the repressor's shape to alter the positioning of the repressor's DNA-binding domains so they no longer interact with DNA.
C)  Inducer binding makes the repressor's DNA-binding domains more positively charged,so they can no longer bind the operator.
D)  Inducer binding makes the repressor's DNA-binding domains more negatively charged,so they can no longer bind the operator. B

Question 4

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

Accepted Answer

A)  The lac operon would be transcribed constitutively.
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.

Question 5

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?

Accepted Answer

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.

Question 6

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 _____.

Accepted Answer

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

Question 7

In the first step of their experiments,Jacob and Monod treated E.coli cells with UV light or X-rays in order to _____.

Accepted Answer

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

Question 8

In negative control of transcription,a gene is activated when _____.

Accepted Answer

A)  a kinase adds a phosphate to DNA
B)  lactose is transported into the cell
C)  a regulatory protein binds to DNA
D)  a regulatory protein is removed from DNA

Question 9

Use the following information when answering the corresponding question(s).
The lac operon is used in countless ways for genetics research and biotechnology.One important use of the lac operon is to regulate the expression of cloned genes.In this application,some regulatory elements of the operon are fused to the cloned gene that is to be regulated.One application of such technology was reported by Cronin et al. ,The lac operator-repressor system is functional in the mouse,Genes and Development 15 (2001):1506-17.These investigators fused a portion of the lac operon to the mouse tyrosinase gene,a gene required for pigment production.This recombinant DNA,composed of E.coli lac operon sequences and the mouse tyrosinase gene,was introduced into albino mice using techniques that will be described in Chapter 19.Using the lac operon regulatory sequences,the investigators were able to regulate tyrosinase gene expression.In fact,they were able to convert albino (white)mice into brown mice.The following questions ask you to use your knowledge of the lac operon to deduce how Cronin et al.were able to regulate tyrosinase gene expression in mice.
-Refer to the paragraph about the lac operon experiment by Cronin et al.Controlling the production of a pigment is interesting,but in itself seems more a stunt than a valuable research endeavor.What is the broader significance of the results of Cronin et al.?

Accepted Answer

A)  The same genetic system will potentially cure most cancers.
B)  This same genetic system potentially can be used to treat hair loss in mice and humans.
C)  This same genetic system potentially can be used to control the expression of genes involved in weight control.
D)  This same genetic system potentially can be used to control the expression of almost any gene.

Question 10

Imagine that you discover a bacterial operon involved in the synthesis of vitamin B₁₂.This operon is regulated by a repressor protein that binds to an operator sequence.Vitamin B₁₂ is the allosteric effector of the repressor-the molecule that binds to the repressor to affect its activity.Predict how vitamin B₁₂ will influence repressor activity.

Accepted Answer

A)  When vitamin B₁₂ binds to the repressor,the repressor binds to the operator.
B)  When vitamin B₁₂ binds to the repressor,the repressor cannot bind to the operator.
C)  When vitamin B₁₂ binds to the repressor,the repressor can bind to RNA polymerase.
D)  When vitamin B₁₂ binds to the repressor,the repressor cannot bind to RNA polymerase.

Question 11

Extracellular glucose inhibits transcription of the lac operon by _____.

Accepted Answer

A)  strengthening the binding of repressor to the operator
B)  weakening the binding of repressor to the operator
C)  inhibiting RNA polymerase from opening the strands of DNA to initiate transcription
D)  reducing the levels of intracellular cAMP
E)  activating adenylyl cyclase

Question 12

The product of the lacI gene is most like a car's _____.

Accepted Answer

A)  parking brake
B)  accelerator pedal
C)  steering wheel
D)  engine
E)  gear shift

Question 13

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

Accepted Answer

A)  an intron
B)  an operon
C)  a repressor
D)  an activator
E)  an inducer

Question 14

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.What is X-Gal likely to be?

Accepted Answer

A)  a compound chemically similar to glucose
B)  a compound chemically similar to galactose
C)  a compound chemically similar to lactose
D)  a compound that cannot be transported into lacZ mutants
E)  a compound that can only be transported into lacA mutants

Question 15

Imagine that you've set up a genetic screen to identify E.coli mutants that cannot metabolize the amino acid tryptophan.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 _____.

Accepted Answer

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

Question 16

Use the following information when answering the corresponding question(s).
The lac operon is used in countless ways for genetics research and biotechnology.One important use of the lac operon is to regulate the expression of cloned genes.In this application,some regulatory elements of the operon are fused to the cloned gene that is to be regulated.One application of such technology was reported by Cronin et al. ,The lac operator-repressor system is functional in the mouse,Genes and Development 15 (2001):1506-17.These investigators fused a portion of the lac operon to the mouse tyrosinase gene,a gene required for pigment production.This recombinant DNA,composed of E.coli lac operon sequences and the mouse tyrosinase gene,was introduced into albino mice using techniques that will be described in Chapter 19.Using the lac operon regulatory sequences,the investigators were able to regulate tyrosinase gene expression.In fact,they were able to convert albino (white)mice into brown mice.The following questions ask you to use your knowledge of the lac operon to deduce how Cronin et al.were able to regulate tyrosinase gene expression in mice.
-Refer to the paragraph about the lac operon experiment by Cronin et al.How did Cronin et al.activate expression of tyrosinase to change the color of a white (albino)mouse to brown?

Accepted Answer

A)  They add sucrose to the drinking water.
B)  They fed the mice a diet rich in glucose.
C)  They added IPTG,a lactose analogue,to the drinking water.
D)  They added cAMP to the drinking water.

Question 17

Which of the following is associated with negative control of the lac operon?

Accepted Answer

A)  glucose
B)  repressor
C)  CAP
D)  adenylyl cyclase

Question 18

If a scientist wanted to prevent a regulatory protein from changing gene expression,he must prevent physical contact between the protein and _____.

Accepted Answer

A)  ribosomes
B)  DNA
C)  ribozymes
D)  mRNA

Question 19

E.coli adenylyl cyclase is most active when _____.

Accepted Answer

A)  extracellular glucose levels are low
B)  intracellular levels of lactose are low
C)  intracellular levels of lactose are high
D)  intracellular levels of cAMP are high

Question 20

The product of the lacI gene is _____.

Accepted Answer

A)  an enzyme
B)  a transport protein embedded within the membrane
C)  β-galactosidase
D)  the repressor

Exam 17: Control of Gene Expression in Bacteria

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