$$[CAP_{active}] = [CAP] \times [cAMP]$$

Question 1

Which of the following best describes the role of the lacI gene in the regulation of the lac operon in E. coli?

Accepted Answer

It encodes the Lac repressor protein, which binds to the operator and blocks transcription in the absence of lactose.

Question 2

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

Accepted Answer

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

Question 3

Which condition leads to maximal transcription of the lac operon?

Accepted Answer

Low glucose, high lactose

Question 4

A mutation in the lacO (operator) region that prevents repressor binding will result in which phenotype?

Accepted Answer

Constitutive expression of the lac operon, regardless of lactose presence

Question 5

Which equation correctly represents the relationship between cAMP and CAP activation in the lac operon?

Accepted Answer

$$[CAP_{active}] = [CAP] 	imes [cAMP]$$

Question 6

Jacob and Monod’s study of the lac operon was significant because it:

Accepted Answer

Demonstrated that gene expression is regulated and identified key principles of gene regulation.

Question 7

Which of the following is true regarding mutations in structural genes versus regulatory regions of an operon?

Accepted Answer

Mutations in regulatory regions typically affect all structural genes in the operon.

Question 8

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

Accepted Answer

The trp operon is transcribed only when tryptophan levels are low.

Question 9

What is the role of attenuation in the regulation of the trp operon?

Accepted Answer

It allows early termination of transcription depending on tryptophan levels.

Question 10

Which secondary structure in the trp operon leader mRNA leads to transcription termination?

Accepted Answer

Stem-loop formed by regions 3 and 4

Question 11

If a cell has a lacI- mutation (loss of function), what phenotype would you expect?

Accepted Answer

Constitutive expression of lac operon genes, regardless of lactose presence

Question 12

Which of the following best explains why a LacI^s (super-repressor) mutation is dominant to LacI^-?

Accepted Answer

LacI^s protein cannot bind allolactose and always represses the operon.

Question 13

Which of the following is a cis-regulatory element in the lac operon?

Accepted Answer

The lacO operator sequence

Question 14

What is the effect of a loss-of-function mutation in the gene encoding the trp repressor?

Accepted Answer

The trp operon is not fully repressed, leading to increased transcription even when tryptophan is present.

Question 15

Which of the following best describes the relationship between glucose, cAMP, and CAP in the regulation of the lac operon?

Accepted Answer

Low glucose leads to high cAMP, which activates CAP and increases transcription.

Question 16

Which of the following is true about the attenuation mechanism in the trp operon?

Accepted Answer

It is controlled by the position of the ribosome on the leader peptide sequence.

Question 17

In a partial diploid E. coli cell with a lacI- mutation on the chromosome and a wild-type lacI+ gene on a plasmid, what is the expected phenotype?

Accepted Answer

Normal regulation of the lac operon

Question 18

Which of the following is a feature shared by both the lac and trp operons?

Accepted Answer

Regulation by a repressor protein that binds to the operator sequence

Question 19

Which of the following best explains why transcription and translation can occur simultaneously in E. coli?

Accepted Answer

E. coli is a prokaryote, so both processes occur in the cytoplasm without a nuclear membrane.

Question 20

Which of the following equations best represents the probability $$P of $$transcription of the lac operon when both glucose and lactose are present?

Accepted Answer

$$P \approx 0$$

Regulation of Prokaryotic Transcription: The lac and trp Operons

Study Guide - Practice Questions