Question 1

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

Accepted Answer

Inducible systems are OFF unless an inducer is present, while repressible systems are ON unless a corepressor is present.

Question 2

In the lac operon, what is the direct effect of allolactose binding to the lac repressor?

Accepted Answer

It causes the repressor to release from the operator, allowing transcription to proceed.

Question 3

Which of the following is true regarding the regulation of the trp operon in E. coli?

Accepted Answer

The trp operon is repressed when tryptophan binds to the repressor, activating it.

Question 4

A mutation in the lacI gene (I−) results in which of the following phenotypes?

Accepted Answer

The lac operon structural genes are constitutively transcribed, regardless of lactose presence.

Question 5

Which of the following best explains why glucose inhibits lac operon expression even when lactose is present?

Accepted Answer

Glucose decreases cAMP levels, preventing CAP-cAMP complex formation and reducing RNA polymerase recruitment.

Question 6

Which of the following is a correct description of a cis-acting element in gene regulation?

Accepted Answer

A DNA sequence located on the same molecule as the gene it regulates, such as an operator.

Question 7

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

Accepted Answer

Lactose present, glucose absent

Question 8

A bacterial cell has a mutation in the operator region of the lac operon (Oc). What is the expected outcome?

Accepted Answer

The repressor cannot bind, so the operon is always transcribed.

Question 9

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

Accepted Answer

A mechanism where transcription is prematurely terminated in the leader region depending on tryptophan availability.

Question 10

Which of the following statements about polycistronic mRNA is correct?

Accepted Answer

It is a single mRNA molecule that encodes multiple proteins, typical of prokaryotic operons.

Question 11

Which of the following is the correct equation for the hydrolysis of lactose by β-galactosidase?

Accepted Answer

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

Question 12

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

Accepted Answer

It facilitates RNA polymerase binding to the promoter, increasing transcription.

Question 13

A bacterium is grown in a medium containing both glucose and lactose. Which of the following best describes the expected expression of the lac operon?

Accepted Answer

Low expression because glucose reduces cAMP levels, limiting CAP activation.

Question 14

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

Accepted Answer

Using IPTG to induce expression of recombinant proteins in E. coli.

Question 15

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

Accepted Answer

The presence of tryptophan activates the repressor, which inhibits further tryptophan synthesis.

Question 16

Which of the following statements about riboswitches is correct?

Accepted Answer

Riboswitches are mRNA domains that bind small ligands, causing conformational changes that affect gene expression.

Question 17

Which of the following best describes the effect of a lacIs (superrepressor) mutation?

Accepted Answer

The repressor cannot bind the inducer, so the operon is always OFF.

Question 18

Which of the following equations correctly represents the conversion of ATP to cAMP by adenylate cyclase?

Accepted Answer

$$\mathrm{ATP} \xrightarrow{	ext{adenylate cyclase}} \mathrm{cAMP} + 2\mathrm{P}_i$$

Question 19

Which of the following is a function of small noncoding RNAs (sRNAs) in bacterial gene regulation?

Accepted Answer

They bind to mRNAs to either inhibit or promote translation by affecting ribosome binding sites.

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

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