Question 1

Which of the following best describes the function of the operator region in a prokaryotic operon?

Accepted Answer

It is a DNA sequence to which a repressor protein binds to regulate transcription.

Question 2

In the lac operon, what is the effect of a mutation that inactivates the lacI gene?

Accepted Answer

The operon will be constitutively expressed, even in the absence of lactose.

Question 3

Which enzyme is responsible for cleaving lactose into glucose and galactose in E. coli?

Accepted Answer

β-galactosidase

Question 4

What is the role of allolactose in the regulation of the lac operon?

Accepted Answer

It acts as an inducer, binding to the repressor and preventing it from binding the operator.

Question 5

Which of the following conditions will result in the highest expression of the lac operon?

Accepted Answer

Lactose present, glucose absent

Question 6

What is the function of the catabolite activator protein (CAP) in the lac operon?

Accepted Answer

It enhances RNA polymerase binding to the promoter in the absence of glucose.

Question 7

Which of the following best describes a repressible operon?

Accepted Answer

It is always transcribed unless a corepressor is present.

Question 8

In the trp operon, what is the role of tryptophan?

Accepted Answer

It acts as a corepressor, enabling the repressor to bind the operator and block transcription.

Question 9

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

Accepted Answer

Attenuation allows fine-tuning of transcription based on tryptophan levels.

Question 10

A mutation in the operator region of the lac operon that prevents repressor binding will result in which phenotype?

Accepted Answer

The operon will be constitutively expressed.

Question 11

Which equation correctly represents the reaction catalyzed 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 is true regarding polycistronic mRNA in prokaryotes?

Accepted Answer

It contains the coding sequences for several functionally related genes transcribed as a single unit.

Question 13

What would be the effect of a mutation that inactivates the CAP-binding site in the lac operon?

Accepted Answer

The operon would not be efficiently expressed even in the absence of glucose.

Question 14

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

Accepted Answer

Transcription is turned off by the binding of the repressor-tryptophan complex to the operator.

Question 15

If a cell has high levels of glucose and lactose, what will be the state of the lac operon?

Accepted Answer

Low expression due to catabolite repression by glucose.

Question 16

Which of the following is a key difference between negative and positive control of gene expression in prokaryotes?

Accepted Answer

Negative control involves a repressor shutting off transcription, while positive control involves an activator stimulating transcription.

Question 17

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

Accepted Answer

It involves the formation of alternative stem-loop structures in the leader mRNA that influence transcription termination.

Question 18

A nonsense mutation in the lacY gene would result in which phenotype?

Accepted Answer

The cell would be unable to import lactose efficiently.

Question 19

Which of the following equations correctly describes the conversion of ATP to cAMP by adenylyl cyclase?

Accepted Answer

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

Regulation of Gene Expression in Prokaryotes: The Lac and Trp Operons

Study Guide - Practice Questions