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

$$\text{Lactose} + \text{H}_2\text{O} \xrightarrow{\beta\text{-galactosidase}} \text{Glucose} + \text{Galactose}$$

Question 1

Which of the following best describes the difference between inducible and repressible gene systems in bacteria?

Accepted Answer

Inducible systems are turned on by the presence of a substrate, while repressible systems are turned off by the abundance of an end product.

Question 2

In the lac operon, what is the function of the lacY gene product?

Accepted Answer

It encodes permease, which facilitates the entry of lactose into the bacterial cell.

Question 3

Which of the following scenarios would result in constitutive expression of the lac operon structural genes?

Accepted Answer

A mutation in the lacI gene that prevents the repressor from binding to the operator.

Question 4

What is the role of the catabolite-activating protein (CAP) in the regulation of the lac operon?

Accepted Answer

CAP binds to the promoter region and stimulates transcription when bound to cAMP.

Question 5

Which of the following best explains why the presence of glucose inhibits lac operon expression?

Accepted Answer

Glucose inhibits adenylyl cyclase, lowering cAMP levels and preventing CAP-cAMP complex formation.

Question 6

A mutation in the operator region (O^C) of the lac operon prevents the repressor from binding. What is the expected outcome?

Accepted Answer

The operon will be constitutively expressed, even in the absence of lactose.

Question 7

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

Accepted Answer

$$	ext{Lactose} + 	ext{H}_2	ext{O} \xrightarrow{\beta	ext{-galactosidase}} 	ext{Glucose} + 	ext{Galactose}$$

Question 8

In the trp operon, what happens when tryptophan is abundant in the environment?

Accepted Answer

Tryptophan binds to the repressor, activating it and blocking transcription of the operon.

Question 9

Which of the following best describes attenuation in the trp operon?

Accepted Answer

It is a regulatory mechanism where the leader mRNA forms a terminator hairpin in the presence of tryptophan, causing premature termination of transcription.

Question 10

Which of the following statements about riboswitches is correct?

Accepted Answer

Riboswitches are mRNA domains that bind small ligands, causing conformational changes that affect transcription or translation.

Question 11

A bacterial cell has a mutation in the lacI gene that produces a repressor unable to bind lactose. What is the expected phenotype regarding lac operon expression?

Accepted Answer

The operon will be permanently repressed, even in the presence of lactose.

Question 12

Which of the following is a correct description of positive control in gene regulation?

Accepted Answer

A regulator molecule must bind to DNA to stimulate transcription.

Question 13

Which of the following best explains the paradox of permease expression in the lac operon?

Accepted Answer

The repressor occasionally dissociates from the operator, allowing low-level expression of permease even in the absence of lactose.

Question 14

Which of the following is true regarding the structure of the lac operon regulatory region?

Accepted Answer

It contains three operator sites, and all must be bound for maximum repression.

Question 15

Which of the following best describes the function of small noncoding RNAs in bacterial gene regulation?

Accepted Answer

They can act as both negative and positive regulators by binding to mRNAs and affecting translation.

Question 16

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

Accepted Answer

Low, because glucose inhibits cAMP production, preventing CAP-cAMP complex formation.

Question 17

Which of the following is a correct statement about polycistronic mRNA in prokaryotes?

Accepted Answer

It is an mRNA that encodes multiple proteins from a single transcript.

Question 18

Which of the following best describes the effect of a mutation in the CAP-binding site of the lac operon?

Accepted Answer

The operon will have reduced transcription even in the absence of glucose.

Regulation of Gene Expression in Bacteria: The lac and trp Operons

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