Question 1

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

Accepted Answer

An inducible system that is usually OFF and must be turned ON by the presence of lactose.

Question 2

What is the role of the lacY gene in the lac operon?

Accepted Answer

It encodes permease, which allows lactose to enter the cell.

Question 3

A mutation in the operator region of the lac operon (Oc) results in which of the following?

Accepted Answer

The operon is always ON, regardless of lactose presence.

Question 4

Which equation correctly represents the reaction catalyzed by $eta$-galactosidase encoded by lacZ?

Accepted Answer

$\mathrm{Lactose} + \mathrm{H}_2\mathrm{O} \rightarrow \mathrm{Galactose} + \mathrm{Glucose}$

Question 5

In the absence of both glucose and lactose, what is the state of the lac operon?

Accepted Answer

The operon is OFF because the repressor is bound to the operator and CAP-cAMP is present.

Question 6

Which of the following best explains the effect of glucose on the lac operon?

Accepted Answer

Glucose inhibits adenyl cyclase, decreasing cAMP levels and reducing CAP binding.

Question 7

A mutation in the lacI gene that prevents the repressor from binding lactose (Is) will result in which phenotype?

Accepted Answer

The operon is always OFF, even when lactose is present.

Question 8

Which of the following is a cis-acting element in the lac operon?

Accepted Answer

The operator sequence.

Question 9

What is the default state of the trp operon in E. coli?

Accepted Answer

ON, because the repressor cannot bind operator without tryptophan.

Question 10

Which statement best describes attenuation in the trp operon?

Accepted Answer

It is a mechanism that decreases transcription when tryptophan is abundant.

Question 11

In the trp operon, what happens when tryptophan is present in high concentrations?

Accepted Answer

Tryptophan binds the repressor, which then binds the operator and blocks transcription.

Question 12

Which of the following best describes the difference between inducible and repressible operons?

Accepted Answer

Inducible operons are usually OFF and require an inducer to turn ON; repressible operons are usually ON and require a corepressor to turn OFF.

Question 13

A bacterial cell has a mutation in the lacI gene such that no repressor protein is produced. What is the expected outcome for lac operon expression?

Accepted Answer

The operon will be constitutively ON, regardless of lactose presence.

Question 14

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

Accepted Answer

High lactose, low glucose

Question 15

Which formula correctly represents the relationship between cAMP and CAP binding in the lac operon?

Accepted Answer

$\mathrm{CAP} + \mathrm{cAMP} \rightarrow \mathrm{CAP{-}cAMP}$ (binds promoter)

Question 16

In a partially diploid E. coli cell (merodiploid) with genotype $I^s O^+ Z^+ / I^+ O^+ Z^+$, what is the expected phenotype for $eta$-galactosidase production in the presence of lactose?

Accepted Answer

No $eta$-galactosidase produced because $I^s$ is dominant and prevents induction.

Question 17

Which of the following is true regarding the leader sequence and attenuation in the trp operon?

Accepted Answer

The leader sequence contains codons for tryptophan and regulates transcription via formation of hairpin structures.

Question 18

If a ribosome stalls at the leader sequence of the trp operon due to low tryptophan, what is the result?

Accepted Answer

Formation of an antiterminator hairpin and continued transcription.

Question 19

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

Accepted Answer

It is usually OFF and is turned ON in response to the presence of lactose.

Regulation of Gene Expression in Bacteria: The Lac and Trp Operons

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