Q: Which of the following equations correctly represents the breakdown of lactose by β-galactosidase in E. coli?

$$\text{Lactose} \xrightarrow{\beta\text{-galactosidase}} \text{Glucose} + \text{Galactose}$$

Question 1

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

Accepted Answer

It encodes the repressor protein, which binds to the operator to inhibit transcription.

Question 2

What happens to the lac operon when both glucose and lactose are present in the environment?

Accepted Answer

The operon is transcribed at a low rate due to catabolite repression by glucose.

Question 3

Which molecule acts as the inducer in the negative control of the lac operon?

Accepted Answer

Allolactose

Question 4

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

Accepted Answer

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

Question 5

Which of the following equations correctly represents the breakdown of lactose by β-galactosidase in E. coli?

Accepted Answer

$$	ext{Lactose} \xrightarrow{\beta	ext{-galactosidase}} 	ext{Glucose} + 	ext{Galactose}$$

Question 6

Which statement best explains why the lac operon is considered both positively and negatively regulated?

Accepted Answer

It is repressed by the lacI repressor and activated by CAP-cAMP complex.

Question 7

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

Accepted Answer

The operon is OFF because the repressor binds the operator and blocks RNA polymerase.

Question 8

Which of the following best describes the function of the CAP-cAMP complex in the lac operon?

Accepted Answer

It activates transcription by facilitating RNA polymerase binding to the promoter.

Question 9

A bacterial cell has a mutation in the lacI gene that prevents the repressor from binding to the operator. What is the phenotype of this cell regarding lac operon expression?

Accepted Answer

The lac operon is always ON, even in the absence of lactose.

Question 10

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

Accepted Answer

The operator (lacO)

Question 11

If a mutation prevents the production of permease in E. coli, what is the most likely effect on lactose metabolism?

Accepted Answer

Lactose will not be transported into the cell, so β-galactosidase cannot metabolize it.

Question 12

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

Accepted Answer

Low glucose, high lactose

Question 13

What is the role of allolactose in the regulation of the lac operon?

Accepted Answer

It binds to the repressor protein, causing it to release from the operator.

Question 14

Which equation best represents the relationship between cAMP and CAP in the activation of the lac operon?

Accepted Answer

$$	ext{CAP} + 	ext{cAMP} ightarrow 	ext{CAP-cAMP complex}$$

Question 15

Which of the following is NOT a structural gene of the lac operon?

Accepted Answer

lacI

Question 16

A mutation in the CAP binding site of the lac operon would most likely result in which phenotype?

Accepted Answer

Reduced transcription of the lac operon even when glucose is low and lactose is present.

Question 17

Which of the following best describes a trans-acting element in gene regulation?

Accepted Answer

A protein or RNA molecule that can regulate genes located on different DNA molecules.

Question 18

Which of the following is true regarding the effect of glucose on the lac operon?

Accepted Answer

High glucose decreases cAMP levels, reducing CAP activation and lac operon transcription.

Question 19

Which of the following is the correct order of regulatory regions in the lac operon from upstream to downstream?

Accepted Answer

CAP binding site, lac promoter, operator

Question 20

A patient’s E. coli strain is found to have a mutation in the lacZ gene. Which enzyme activity would be most affected?

Accepted Answer

β-galactosidase activity

Regulation of Gene Expression in Bacteria: The Lac Operon CH 12

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