Q: Which of the following equations correctly represents the relationship between glucose, cAMP, and CRP activity in the lac operon system?

$$[\text{cAMP}] \propto \frac{1}{[\text{glucose}]}$$

Question 1

Which of the following best describes the default state of gene expression in prokaryotes and the predominant type of transcriptional regulator?

Accepted Answer

Default state is activated; regulators are mostly repressors.

Question 2

A mutation abolishes the binding of the lac repressor to the operator. What is the most likely outcome for lac operon expression?

Accepted Answer

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

Question 3

Which molecule acts as the inducer for the lac operon, and what is its effect on the repressor?

Accepted Answer

Allolactose; it inactivates the repressor by changing its conformation.

Question 4

Which of the following best explains why the lac operon is not expressed at high levels even when the repressor is not bound, if glucose is present?

Accepted Answer

The lac promoter is a weak promoter and requires an activator for efficient RNA polymerase recruitment.

Question 5

Which of the following equations correctly represents the relationship between glucose, cAMP, and CRP activity in the lac operon system?

Accepted Answer

$$[	ext{cAMP}] \propto \frac{1}{[	ext{glucose}]}$$

Question 6

A bacterium is growing in an environment with high glucose and high lactose. What is the expected level of lac operon transcription?

Accepted Answer

Very low, because glucose inhibits cAMP production and CRP activation.

Question 7

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

Accepted Answer

It is repressed by tryptophan, which acts as a corepressor.

Question 8

What is the molecular mechanism underlying attenuation in the trp operon?

Accepted Answer

Formation of a stem-loop structure in the mRNA leader sequence that causes premature transcription termination.

Question 9

Which of the following best describes a cis-acting element?

Accepted Answer

A DNA sequence that regulates the expression of a gene located on the same DNA molecule.

Question 10

A mutation in the core domain of the lac repressor prevents inducer binding. What is the expected phenotype?

Accepted Answer

The lac operon will be permanently repressed, regardless of lactose presence.

Question 11

Which of the following is true regarding polycistronic mRNA in bacteria?

Accepted Answer

It is transcribed from a single promoter and encodes multiple proteins.

Question 12

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

Accepted Answer

Low glucose, high lactose

Question 13

Which of the following best explains why attenuation is an effective regulatory mechanism for the trp operon?

Accepted Answer

It allows the cell to rapidly adjust trp operon expression in response to changes in charged tRNA-Trp levels.

Question 14

Which of the following is a correct statement about the interaction between cis-acting elements and trans-acting factors?

Accepted Answer

Cis-acting elements are DNA sequences; trans-acting factors are proteins that bind to these sequences.

Question 15

Which of the following best describes the effect of a mutation that abolishes the ability of the lac repressor to dimerize?

Accepted Answer

The repressor cannot bind the operator, leading to constitutive expression of the lac operon.

Question 16

Which of the following is the correct sequence of events for activation of the lac operon in the absence of glucose and presence of lactose?

Accepted Answer

cAMP levels rise $$\rightarrow$$ CRP binds cAMP $$\rightarrow$$ CRP-cAMP complex binds DNA $$\rightarrow$$ RNA polymerase recruited $$\rightarrow$$ repressor inactivated by allolactose

Question 17

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

Accepted Answer

The operon is turned off when the repressor is activated by binding to tryptophan.

Question 18

Which of the following is a real-world application of understanding the lac operon system?

Accepted Answer

Using IPTG as an artificial inducer to control gene expression in recombinant protein production.

Question 19

Which of the following best describes the effect of attenuation on trp operon expression?

Accepted Answer

It reduces expression by causing premature transcription termination when tRNA-Trp is abundant.

Prokaryotic Transcription Regulation: Operons and Gene Control in Bacteria

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