Q: Which equation correctly represents the synthesis of cAMP from ATP by adenylate cyclase?

$$\mathrm{ATP} \xrightarrow{\text{adenylate cyclase}} \mathrm{cAMP} + \mathrm{PP_i}$$

Question 1

Which of the following best describes the mechanism of negative control of transcription in bacteria?

Accepted Answer

A repressor protein binds to the operator region, preventing RNA polymerase from initiating transcription.

Question 2

In the lac operon, what is the role of allolactose?

Accepted Answer

It acts as an inducer by binding to the allosteric domain of the lac repressor, causing it to release from the operator.

Question 3

A mutation in the lacO operator (lacO^C) results in which phenotype?

Accepted Answer

Constitutive transcription of the lac operon, regardless of lactose presence.

Question 4

Which equation correctly represents the synthesis of cAMP from ATP by adenylate cyclase?

Accepted Answer

$$\mathrm{ATP} \xrightarrow{	ext{adenylate cyclase}} \mathrm{cAMP} + \mathrm{PP_i}$$

Question 5

Which of the following scenarios would result in the highest level of lac operon transcription?

Accepted Answer

Lactose present, glucose absent

Question 6

A partial diploid E. coli strain has the genotype $$F'\,I^+\,P^+\,O^+\,Z^-\,Y^+ / I^+\,P^+\,O^+\,Z^+\,Y^- . $$What is the expected phenotype when grown on lactose?

Accepted Answer

Wild-type: both β-galactosidase and permease are produced.

Question 7

Which structural motif is most commonly found in bacterial DNA-binding regulatory proteins?

Accepted Answer

Helix-turn-helix (HTH) motif

Question 8

What is the effect of a lacI^S (super-repressor) mutation on the lac operon?

Accepted Answer

The operon is never transcribed, even in the presence of lactose.

Question 9

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

Accepted Answer

Formation of a 3–4 stem-loop in the leader mRNA causes premature termination of transcription.

Question 10

In the trp operon, what happens when tryptophan is scarce in the cell?

Accepted Answer

The ribosome stalls at the tryptophan codons, allowing the 2–3 stem-loop to form and transcription to continue.

Question 11

Which of the following is a real-life application of riboswitches in bacterial gene regulation?

Accepted Answer

Regulation of thiamin pyrophosphate (TPP) biosynthesis by binding TPP to a riboswitch in the thi operon mRNA.

Question 12

Which of the following mutations would most likely result in no transcription of the lac operon, even when lactose is present?

Accepted Answer

A mutation in the promoter region (lacP−)

Question 13

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

Accepted Answer

It binds to the CAP binding site, enhancing RNA polymerase binding and increasing transcription.

Question 14

A bacterium has a mutation in the trpR gene, resulting in a nonfunctional repressor. What is the expected outcome for trp operon expression when tryptophan is present?

Accepted Answer

The operon will be transcribed at a higher rate than wild-type, but not 100%.

Question 15

Which of the following is true regarding the regulation of mRNA stability by riboswitches?

Accepted Answer

Binding of a metabolite to the riboswitch can induce mRNA cleavage, preventing translation.

Question 16

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

Accepted Answer

It is turned on only when an inducer compound (allolactose) is available.

Question 17

Which of the following is a key difference between repressible and inducible operons?

Accepted Answer

Repressible operons are typically involved in anabolic pathways, while inducible operons are involved in catabolic pathways.

Regulation of Gene Expression in Bacteria and Bacteriophage

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