Q: Which equation correctly represents the conversion catalyzed by adenylyl cyclase in the formation of cAMP?

$\mathrm{ATP} \xrightarrow{\text{adenylyl cyclase}} \mathrm{cAMP} + \mathrm{PP_i}$

Question 1

Which of the following best describes the role of the lacI gene in the lac operon of E. coli?

Accepted Answer

It produces a diffusible repressor protein that regulates transcription by binding to the operator.

Question 2

A mutation in the operator site (O^c) of the lac operon leads to which of the following outcomes?

Accepted Answer

The repressor protein cannot bind, resulting in constitutive expression of the lac operon.

Question 3

In the lacI^S (super-repressor) mutation, what is the effect on lac operon transcription?

Accepted Answer

The operon is permanently repressed and cannot be induced by lactose.

Question 4

Which experimental technique provided evidence that the lac repressor is a protein and not an RNA molecule?

Accepted Answer

Equilibrium dialysis with radioactive IPTG

Question 5

What is the function of the Catabolite-Activating Protein (CAP) in the regulation of the lac operon?

Accepted Answer

CAP facilitates RNA polymerase binding to the promoter, promoting transcription in the absence of glucose.

Question 6

Which molecule must bind to CAP for it to activate transcription of the lac operon?

Accepted Answer

Cyclic Adenosine Monophosphate (cAMP)

Question 7

How does glucose affect the activity of adenylyl cyclase and the levels of cAMP in E. coli?

Accepted Answer

Glucose inhibits adenylyl cyclase, decreasing cAMP levels.

Question 8

Which equation correctly represents the conversion catalyzed by adenylyl cyclase in the formation of cAMP?

Accepted Answer

$\mathrm{ATP} \xrightarrow{	ext{adenylyl cyclase}} \mathrm{cAMP} + \mathrm{PP_i}$

Question 9

A partially diploid (merozygote) E. coli cell contains both a wild-type lacI+ gene and a lacI- mutant gene. What is the expected effect on lac operon regulation?

Accepted Answer

The wild-type lacI+ gene will produce a functional repressor, restoring normal regulation.

Question 10

Which of the following statements about the trp operon is correct?

Accepted Answer

It is a repressible operon regulated by the presence of tryptophan.

Question 11

What happens to transcription of the trp operon when tryptophan is absent?

Accepted Answer

The repressor cannot bind the operator, and transcription occurs.

Question 12

Which sequence in the trp operon is responsible for attenuation of transcription?

Accepted Answer

The leader sequence containing the attenuator

Question 13

If tryptophan levels are high in E. coli, which hairpin structure forms in the trp operon mRNA and what is its effect?

Accepted Answer

Terminator hairpin (regions 3-4); transcription terminates prematurely.

Question 14

Which of the following is a common feature of attenuation in amino acid biosynthetic operons such as trp, his, leu, and phe?

Accepted Answer

Attenuation occurs in the leader sequence upstream of the structural genes.

Question 15

Which of the following best explains the difference between negative and positive control of transcription in bacteria?

Accepted Answer

Negative control involves repressor proteins blocking transcription, while positive control involves activator proteins facilitating transcription.

Question 16

A bacterial cell is grown in a medium containing both glucose and lactose. What is the expected level of lac operon transcription?

Accepted Answer

Low, because glucose inhibits cAMP production and CAP cannot activate transcription.

Question 17

Which of the following is NOT a prediction made by the operon model for the lac operon?

Accepted Answer

The lacZ gene produces a diffusible repressor protein.

Question 18

Which of the following best describes the effect of an inducer molecule in negative control of transcription?

Accepted Answer

It binds to the repressor protein, preventing it from binding the operator and allowing transcription.

Regulation of Gene Expression in Bacteria: The lac and trp Operons (CH 15)

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