Q: Which of the following equations correctly represents the reaction catalyzed by adenyl cyclase?

$$\mathrm{ATP} \xrightarrow{\text{adenyl cyclase}} \mathrm{cAMP} + 2\mathrm{P_i}$$

Question 1

A frameshift mutation occurs in the DNA-binding domain of the lacI gene. What is the most likely effect on lac operon expression in the presence and absence of lactose?

Accepted Answer

The lac operon will be constitutively expressed regardless of lactose presence.

Question 2

A mutation in the operator sequence of the lac operon prevents the lacI repressor from binding. What is the expected outcome for lac operon gene expression?

Accepted Answer

Constitutive expression of the lac operon, regardless of lactose presence.

Question 3

If a mutation occurs in the lactose-binding site of the lacI repressor protein, what will happen to lac operon expression when lactose is present?

Accepted Answer

The lac operon will remain repressed because the repressor cannot be inactivated by lactose.

Question 4

Which of the following best describes the role of the lacI gene product in the lac operon system?

Accepted Answer

It is a trans-acting repressor that can diffuse and regulate multiple operons.

Question 5

In E. coli, which molecule is preferred for energy metabolism when both glucose and lactose are available?

Accepted Answer

Glucose

Question 6

What is the relationship between glucose and cAMP levels in E. coli?

Accepted Answer

As glucose increases, cAMP decreases.

Question 7

Which of the following equations correctly represents the reaction catalyzed by adenyl cyclase?

Accepted Answer

$$\mathrm{ATP} \xrightarrow{	ext{adenyl cyclase}} \mathrm{cAMP} + 2\mathrm{P_i}$$

Question 8

What is the role of the CAP-cAMP complex in the regulation of the lac operon?

Accepted Answer

It binds to the promoter and facilitates RNA polymerase binding, increasing transcription.

Question 9

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

Accepted Answer

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

Question 10

Which of the following scenarios will result in maximal expression of the lac operon?

Accepted Answer

Lactose present, glucose absent

Question 11

The trp operon in E. coli is best described as:

Accepted Answer

A repressible gene system that is turned off in the presence of tryptophan.

Question 12

What would happen if the trp repressor protein was mutated so that it could no longer bind to DNA?

Accepted Answer

Transcription of the trp operon would occur all the time, regardless of tryptophan presence.

Question 13

Which of the following best explains why the lac operon is considered under negative control?

Accepted Answer

Transcription occurs only when the repressor fails to bind the operator region.

Question 14

Which of the following is a function of small noncoding RNAs (sRNAs) in bacteria?

Accepted Answer

They can bind to mRNAs and regulate translation either positively or negatively.

Question 15

A mutation in the CAP-binding site of the lac operon promoter prevents CAP-cAMP complex binding. What is the expected effect on lac operon expression when glucose is absent and lactose is present?

Accepted Answer

Lac operon expression will be significantly reduced, even though lactose is present.

Question 16

Which of the following best describes the mechanism by which tryptophan regulates the trp operon?

Accepted Answer

Tryptophan acts as a corepressor, binding to the repressor protein and enabling it to bind the operator.

Question 17

Which of the following is true regarding the polycistronic mRNA produced by the trp operon?

Accepted Answer

It encodes multiple enzymes involved in tryptophan biosynthesis.

Question 18

A bacterial cell is placed in a medium lacking tryptophan. What will happen to the trp operon?

Accepted Answer

The trp operon will be transcribed to synthesize tryptophan.

Question 19

Which of the following best explains why the lac operon is not expressed when both glucose and lactose are present?

Accepted Answer

Glucose inhibits adenyl cyclase, lowering cAMP and preventing CAP from activating transcription.

Regulation of Gene Expression in Bacteria: Lac and Trp Operons

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