ASU BIO 340 Exam 3 WITH QUESTIONS AND
ANSWERS
What will be the effect on transcription when an inhibitor is in play?
A. Allow transcription B. Prevent transcription - ANS-B Explanation: An inhibitor
prevents activator from binding to DNA; activators are used in positive regulation
(increasing transcription) - thus, the inhibitor prevents the activator from positive
regulating (increasing) transcription.
What will be the effect on transcription when an inducer is in play?
A. Allow transcription B. Prevent transcription - ANS-A Explanation: When an inducer is
in play, it will allow transcription.
For a gene under positive control, which of the following would prevent transcription?
A. Repressor B. Corepressor C. Inducer D. Effector E. Inhibitor - ANS-E Explanation:
Positive regulation includes activators (affected by effectors and inhibitors); inhibitors
prevent activator from binding to DNA, thus preventing transcription.
For a gene under negative control, which of the following would you expect to find
bound to the repressor if there is active transcription? A. Repressor B. Corepressor C.
Inducer D. Effector E. Inhibitor - ANS-C Explanation: In negative regulation, there are
repressors (affected by correpressors and inducers). If there were a corepressor, the
repressor would successfully bind to the operator and prevent transcription. Inducers,
however, prevent the binding of the repressor - thus causing a prevention of it doing its
job, and allowing transcription.
Which molecule serves as the inducer for the lac operon?
A. cAMP B. Glucose C. Lactose D. Allolactose - ANS-D Explanation: Allolactose is the
molecule that binds to repressor protein, making it unable to bind to the operator.
Which of the following conditions will result in the greatest levels of transcription of the
lac operon?
A. Lactose absent, glucose absent B. Lactose absent, glucose present C. Lactose
present, glucose present D. Lactose present, glucose absen - ANS-D Explanation:
From the perspective of the cell, it is most efficient to repress expression of the genes
allowing for metabolism of lactose when it's absent, to allow it when present, but only to
prioritize it when glucose (the preferred food source) is absent. In C, allolactose will also
be present, allowing transcription, but positive regulation (i.e. increase in transcription)
only happens as in D, when glucose is absent.
Which of the following mutations will result in the lowest level of transcription of the lac
operon when lactose is present?
A. An operator mutation that prevents binding of the lac repressor B. A mutation in the
lac repressor that prevents it from binding to the operator C. A mutation in CAP that
, prevents it from binding to the CAP binding site D. A mutation in the lac repressor that
prevents it from binding allolactose - ANS-D Explanation: A - if repressor can't bind, the
gene will be permanently turned on, there will be transcription. B - exactly the same
effect as A. C - "tempting but wrong"; CAP can't bind, will cause lower but not total
prevention of transcription. D - correct answer; if the inducer is totally ineffective, then
the repressor will permanently bind to operator and totally prevent transcription.
The regulatory elements in the lac operon act to:
A. Repress transcription when there is no lactose present B. Repress transcription when
there is lactose present C. Activate transcription when there is no glucose present D.
Activate transcription when there is glucose present E. A and C - ANS-E Explanation:
The cell wants to both repress transcription when there is no lactose present and
activate it when glucose is absent (C "slightly less good" answer than A, but still true).
Similar to Clicker Question 6
A bacterium needs to make a certain lipid that it cannot obtain from its environment.
That same lipid is involved in regulating the expression of the enzymes that make it.
Which of the following is the most likely regulatory function of the lipid?
A. Effector B. Activator C. Inducer D. Co-repressor E. Operator - ANS-D Explanation: If
you have that lipid, you would want it to not make any more. So, if you have the lipid in
the cell, you would want to turn off the transcription, so it would likely be involved in
negative regulation. Effectors and activators are both involved in positive regulation. An
inducer is under negative regulation but its job is to turn on transcription. A co-repressor
will only turn the gene off if there is a signal the cell can sense. Operator is a piece of
DNA.
A bacterium is unable to transport lactose into the cell to be broken down. Which gene
is likely mutated in this bacterium?
A. lacZ B. lacY C. lacI D. lacP E. lacO - ANS-B Explanation: If you can't transport
lactose, then it means permease isn't working. So lacY is likely mutated since that is the
gene that encodes permease.
Under which conditions will E. coli make the least amount of beta-galactosidase?
A. I+ O+ Z+, glucose absent, lactose present B. Is Oc Z+, glucose present, lactose
absent C. I- O+ Z+, glucose absent, lactose absent D. Is O+ Z+, glucose absent,
lactose present - ANS-D Explanation: A is wild type, so the expression level would be
high, B has a super repressor, meaning is not able to bind allolactose, but since we
have constitutive operators, the gene is going to turn "on". For C, the repressor can't
bind the operator, so the gene will not undergo negative regulation. For D, the super
repressor bound to the wild-type operator will keep the gene off no matter what, even
though lactose is present.
In terms of lac operon regulation, what happens when an I+, O+, P+, Z- mutant E. coli is
grown in medium containing very low glucose and high lactose?
A. CAP is bound to the DNA but the lac repressor is not, E. coli grows B. CAP is bound
to the DNA but the lac repressor is not, E. coli does not grow C. Lac repressor is bound
ANSWERS
What will be the effect on transcription when an inhibitor is in play?
A. Allow transcription B. Prevent transcription - ANS-B Explanation: An inhibitor
prevents activator from binding to DNA; activators are used in positive regulation
(increasing transcription) - thus, the inhibitor prevents the activator from positive
regulating (increasing) transcription.
What will be the effect on transcription when an inducer is in play?
A. Allow transcription B. Prevent transcription - ANS-A Explanation: When an inducer is
in play, it will allow transcription.
For a gene under positive control, which of the following would prevent transcription?
A. Repressor B. Corepressor C. Inducer D. Effector E. Inhibitor - ANS-E Explanation:
Positive regulation includes activators (affected by effectors and inhibitors); inhibitors
prevent activator from binding to DNA, thus preventing transcription.
For a gene under negative control, which of the following would you expect to find
bound to the repressor if there is active transcription? A. Repressor B. Corepressor C.
Inducer D. Effector E. Inhibitor - ANS-C Explanation: In negative regulation, there are
repressors (affected by correpressors and inducers). If there were a corepressor, the
repressor would successfully bind to the operator and prevent transcription. Inducers,
however, prevent the binding of the repressor - thus causing a prevention of it doing its
job, and allowing transcription.
Which molecule serves as the inducer for the lac operon?
A. cAMP B. Glucose C. Lactose D. Allolactose - ANS-D Explanation: Allolactose is the
molecule that binds to repressor protein, making it unable to bind to the operator.
Which of the following conditions will result in the greatest levels of transcription of the
lac operon?
A. Lactose absent, glucose absent B. Lactose absent, glucose present C. Lactose
present, glucose present D. Lactose present, glucose absen - ANS-D Explanation:
From the perspective of the cell, it is most efficient to repress expression of the genes
allowing for metabolism of lactose when it's absent, to allow it when present, but only to
prioritize it when glucose (the preferred food source) is absent. In C, allolactose will also
be present, allowing transcription, but positive regulation (i.e. increase in transcription)
only happens as in D, when glucose is absent.
Which of the following mutations will result in the lowest level of transcription of the lac
operon when lactose is present?
A. An operator mutation that prevents binding of the lac repressor B. A mutation in the
lac repressor that prevents it from binding to the operator C. A mutation in CAP that
, prevents it from binding to the CAP binding site D. A mutation in the lac repressor that
prevents it from binding allolactose - ANS-D Explanation: A - if repressor can't bind, the
gene will be permanently turned on, there will be transcription. B - exactly the same
effect as A. C - "tempting but wrong"; CAP can't bind, will cause lower but not total
prevention of transcription. D - correct answer; if the inducer is totally ineffective, then
the repressor will permanently bind to operator and totally prevent transcription.
The regulatory elements in the lac operon act to:
A. Repress transcription when there is no lactose present B. Repress transcription when
there is lactose present C. Activate transcription when there is no glucose present D.
Activate transcription when there is glucose present E. A and C - ANS-E Explanation:
The cell wants to both repress transcription when there is no lactose present and
activate it when glucose is absent (C "slightly less good" answer than A, but still true).
Similar to Clicker Question 6
A bacterium needs to make a certain lipid that it cannot obtain from its environment.
That same lipid is involved in regulating the expression of the enzymes that make it.
Which of the following is the most likely regulatory function of the lipid?
A. Effector B. Activator C. Inducer D. Co-repressor E. Operator - ANS-D Explanation: If
you have that lipid, you would want it to not make any more. So, if you have the lipid in
the cell, you would want to turn off the transcription, so it would likely be involved in
negative regulation. Effectors and activators are both involved in positive regulation. An
inducer is under negative regulation but its job is to turn on transcription. A co-repressor
will only turn the gene off if there is a signal the cell can sense. Operator is a piece of
DNA.
A bacterium is unable to transport lactose into the cell to be broken down. Which gene
is likely mutated in this bacterium?
A. lacZ B. lacY C. lacI D. lacP E. lacO - ANS-B Explanation: If you can't transport
lactose, then it means permease isn't working. So lacY is likely mutated since that is the
gene that encodes permease.
Under which conditions will E. coli make the least amount of beta-galactosidase?
A. I+ O+ Z+, glucose absent, lactose present B. Is Oc Z+, glucose present, lactose
absent C. I- O+ Z+, glucose absent, lactose absent D. Is O+ Z+, glucose absent,
lactose present - ANS-D Explanation: A is wild type, so the expression level would be
high, B has a super repressor, meaning is not able to bind allolactose, but since we
have constitutive operators, the gene is going to turn "on". For C, the repressor can't
bind the operator, so the gene will not undergo negative regulation. For D, the super
repressor bound to the wild-type operator will keep the gene off no matter what, even
though lactose is present.
In terms of lac operon regulation, what happens when an I+, O+, P+, Z- mutant E. coli is
grown in medium containing very low glucose and high lactose?
A. CAP is bound to the DNA but the lac repressor is not, E. coli grows B. CAP is bound
to the DNA but the lac repressor is not, E. coli does not grow C. Lac repressor is bound
