Q: Which of the following equations correctly represents the conversion of ATP to cAMP by adenyl cyclase?

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

Question 1

Which of the following best describes the function of an inducer in bacterial gene regulation?

Accepted Answer

An inducer binds to a repressor protein and prevents it from binding to DNA, increasing transcription.

Question 2

In the lac operon, what is the effect of high glucose and low lactose on transcription?

Accepted Answer

Transcription is low because there is no allolactose to inactivate the repressor and no cAMP-CAP complex to activate transcription.

Question 3

Which mutation in the lac operon would result in constitutive expression of the structural genes, regardless of lactose presence?

Accepted Answer

A mutation in the operator (lacO) that prevents the repressor from binding.

Question 4

Which of the following equations correctly represents the conversion of ATP to cAMP by adenyl cyclase?

Accepted Answer

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

Question 5

A bacterium has a mutation in the lacI gene that prevents the repressor from binding to the operator. What is the expected outcome for lac operon transcription?

Accepted Answer

Transcription will be constitutive, occurring regardless of lactose presence.

Question 6

Which of the following best explains why the trp operon is considered repressible?

Accepted Answer

It is always on until a corepressor (tryptophan) binds to the repressor, turning it off.

Question 7

What is the role of the 2-3 stem-loop structure in the trp operon leader sequence?

Accepted Answer

It allows transcription to proceed when tryptophan levels are low.

Question 8

Which of the following scenarios would result in the highest level of lac operon transcription?

Accepted Answer

Glucose absent, lactose present

Question 9

A mutation in the trpR gene results in a nonfunctional repressor protein. What is the expected effect on the trp operon?

Accepted Answer

The operon will be constitutively expressed, regardless of tryptophan levels.

Question 10

Which of the following best describes catabolite repression in the context of the lac operon?

Accepted Answer

The presence of glucose decreases cAMP levels, preventing CAP from activating the lac operon.

Question 11

Which of the following is true regarding the difference between cis-acting and trans-acting elements in the lac operon?

Accepted Answer

Cis-acting elements are DNA sequences (like the operator), while trans-acting elements are proteins (like the repressor).

Question 12

Which of the following best explains the molecular mechanism of attenuation in the trp operon?

Accepted Answer

Attenuation is the process by which the leader peptide sequence forms alternative stem-loop structures in mRNA, influencing transcription termination.

Question 13

A bacterium is grown in medium containing both glucose and lactose. Which of the following best describes the expected transcriptional activity of the lac operon?

Accepted Answer

Basal (very low) transcription because the repressor is inactivated by allolactose, but CAP cannot bind efficiently due to low cAMP.

Question 14

Which of the following is a correct description of a repressible gene?

Accepted Answer

A gene that is always on until turned off by a corepressor.

Question 15

Which of the following best describes the effect of an inhibitor on an activator protein in bacterial gene regulation?

Accepted Answer

The inhibitor binds to the activator, preventing it from binding to DNA and decreasing transcription.

Question 16

Which of the following is a direct result of the binding of the cAMP-CAP complex to the lac operon?

Accepted Answer

Increased affinity of RNA polymerase for the promoter, enhancing transcription.

Question 17

Which of the following best explains why the lac operon is considered an inducible operon?

Accepted Answer

It is always off until an inducer (allolactose) inactivates the repressor, allowing transcription.

Question 18

Which of the following would most likely occur if a mutation prevented the formation of the 3-4 stem-loop in the trp operon leader sequence?

Accepted Answer

Transcription would proceed into the structural genes even when tryptophan is abundant.

Gene Regulation in Bacteria: lac and trp Operons

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