Question 1

Which of the following best describes the regulatory mechanism of the lac operon in *E. coli* when both lactose and glucose are absent?

Accepted Answer

The lac repressor is bound to the operator, and CAP is bound to the promoter, so the operon is not transcribed.

Question 2

A mutation in the lacI gene results in a repressor protein that cannot bind to the operator. What is the expected outcome for the expression of the lac operon structural genes?

Accepted Answer

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

Question 3

Which of the following equations correctly represents the relationship between the rate of transcription ($$R) of $$the lac operon and the concentrations of lactose ($$[L]) $$and glucose ($$[G]$$)?

Accepted Answer

$$R \propto \frac{[L]}{[G]}$$

Question 4

In the trp operon, what is the role of tryptophan in regulating gene expression?

Accepted Answer

Tryptophan acts as a corepressor, activating the repressor to inhibit transcription.

Question 5

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

Accepted Answer

A mutation in the operator (O) region that prevents repressor binding

Question 6

Which of the following best explains why attenuation is necessary in the regulation of the trp operon?

Accepted Answer

The repressor alone is not 100% efficient at blocking transcription, so attenuation provides an additional layer of control.

Question 7

A bacterium has a mutation in the trpR gene such that the repressor protein cannot bind tryptophan. What is the expected effect on the trp operon?

Accepted Answer

The trp operon will be constitutively expressed, regardless of tryptophan levels.

Question 8

Which of the following hairpin structures in the leader region of the trp operon mRNA causes premature termination of transcription when tryptophan levels are high?

Accepted Answer

The 3-4 hairpin

Question 9

Which of the following statements about the coupling of transcription and translation in prokaryotes is correct?

Accepted Answer

Translation can begin before transcription is complete, allowing for attenuation mechanisms.

Question 10

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

Accepted Answer

Decreased transcription of the lac operon even when glucose is absent

Question 11

Which of the following best describes the function of the lac repressor in the absence of lactose?

Accepted Answer

It binds to the operator and blocks transcription of the lac operon.

Question 12

Which of the following is true regarding the regulation of the trp operon by attenuation?

Accepted Answer

When tryptophan is low, the ribosome stalls, allowing the 2-3 hairpin to form and transcription to continue.

Question 13

Which of the following best describes the difference between inducible and repressible operons?

Accepted Answer

Inducible operons are usually off and need to be turned on; repressible operons are usually on and need to be turned off.

Question 14

Which of the following is the correct sequence of events for the trp repressor to inhibit transcription?

Accepted Answer

Tryptophan binds to the repressor, activating it, which then binds to the operator.

Question 15

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

Accepted Answer

CAP is not bound to the promoter due to low cAMP levels, so transcription is minimal.

Question 16

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

Accepted Answer

Designing bacterial strains for industrial production of enzymes that are only expressed in the presence of specific sugars.

Question 17

If a mutation causes the trp operator to be deleted, what is the most likely outcome for the cell?

Accepted Answer

The trp operon will be constitutively expressed, regardless of tryptophan levels.

Question 18

Which of the following best describes the function of the CAP protein in the lac operon?

Accepted Answer

It acts as an activator in the absence of glucose by binding to cAMP and the promoter.

Question 19

Which of the following is the correct order of the structural genes in the trp operon?

Accepted Answer

trpE, trpD, trpC, trpB, trpA

Regulation of Gene Expression in Prokaryotes: Lac and Trp Operons

Study Guide - Practice Questions

Study Guide - Flashcards