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

$$\text{Lactose} + H_2O \xrightarrow{\beta\text{-galactosidase}} \text{Glucose} + \text{Galactose}$$

Question 1

Which of the following best describes the difference between inducible and constitutive enzymes in bacteria?

Accepted Answer

Inducible enzymes are produced only in the presence of specific substrates, while constitutive enzymes are produced continuously regardless of environmental conditions.

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 operator region (O^C) that prevents repressor binding.

Question 4

Which of the following best explains the role of the catabolite-activating protein (CAP) in the regulation of the lac operon?

Accepted Answer

CAP exerts positive control by binding to the promoter region only when complexed with cAMP, facilitating RNA polymerase binding and transcription.

Question 5

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

Accepted Answer

The structural genes will be expressed constitutively, regardless of lactose presence.

Question 6

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

Accepted Answer

$$	ext{Lactose} + H_2O \xrightarrow{\beta	ext{-galactosidase}} 	ext{Glucose} + 	ext{Galactose}$$

Question 7

What is the effect of glucose on the activity of adenylyl cyclase and the levels of cAMP in E. coli?

Accepted Answer

Glucose inhibits adenylyl cyclase, resulting in decreased cAMP levels.

Question 8

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

Accepted Answer

It is a repressible operon that is turned off in the presence of tryptophan.

Question 9

In the attenuation mechanism of the trp operon, what structure forms in the leader mRNA when tryptophan is abundant?

Accepted Answer

A terminator hairpin forms, causing premature termination of transcription.

Question 10

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

Accepted Answer

They can act as negative regulators by binding to mRNAs and preventing translation, or as positive regulators by enabling translation.

Question 11

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

Accepted Answer

Reduced transcription of the lac operon even in the absence of glucose.

Question 12

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

Accepted Answer

Using inducible promoters in biotechnology to control gene expression in recombinant bacteria.

Question 13

Which of the following statements about cis-acting and trans-acting elements is correct?

Accepted Answer

Cis-acting elements are DNA sequences that regulate genes on the same DNA molecule, while trans-acting elements are usually proteins that can diffuse and act on multiple DNA molecules.

Question 14

Which of the following best explains why the lac operon is not expressed at high levels when both glucose and lactose are present?

Accepted Answer

Glucose inhibits adenylyl cyclase, lowering cAMP levels and preventing CAP from binding to the promoter, thus reducing transcription.

Question 15

Which of the following is the correct sequence of events for positive control of the lac operon in the absence of glucose?

Accepted Answer

cAMP levels increase $$\rightarrow$$ cAMP binds to CAP $$\rightarrow$$ CAP-cAMP complex binds to promoter $$\rightarrow$$ RNA polymerase binds $$\rightarrow$$ transcription occurs

Question 16

A bacterium with a super-repressor mutation (I^s) in the lacI gene will exhibit which phenotype?

Accepted Answer

The lac operon will remain repressed even in the presence of lactose.

Question 17

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

Accepted Answer

They are mRNA domains that bind small ligands, causing conformational changes that regulate transcription or translation.

Question 18

Which of the following is a key feature of polycistronic mRNA in prokaryotes?

Accepted Answer

It contains coding sequences for multiple proteins, allowing coordinated expression of functionally related genes.

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

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