Question 1

Which of the following best defines an operon in bacterial gene regulation?

Accepted Answer

A cluster of genes under the control of a single promoter and operator, transcribed together as a single mRNA.

Question 2

What is the primary difference between an activator protein and a repressor protein in bacterial gene regulation?

Accepted Answer

An activator increases transcription by binding to DNA, while a repressor decreases transcription by binding to DNA.

Question 3

Which of the following statements correctly distinguishes between an inducer and a corepressor?

Accepted Answer

An inducer binds to a repressor and inactivates it, while a corepressor binds to a repressor and activates it.

Question 4

In the lac operon, what is the effect of high glucose and high lactose concentrations on operon expression?

Accepted Answer

The operon is off because cAMP levels are low and CAP/cAMP is not bound.

Question 5

Which of the following best describes the role of allolactose in the lac operon?

Accepted Answer

Allolactose acts as an inducer by binding to the lac repressor and causing it to release from the operator.

Question 6

The tryptophan (trp) operon is different from the lac operon in that:

Accepted Answer

The trp operon uses a corepressor (tryptophan) to activate the repressor, while the lac operon uses an inducer (allolactose) to inactivate the repressor.

Question 7

Why is the lac operon considered an inducible operon, while the trp operon is considered a repressible operon?

Accepted Answer

The lac operon is usually off and is turned on in the presence of an inducer (lactose), while the trp operon is usually on and is turned off in the presence of a corepressor (tryptophan).

Question 8

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

Accepted Answer

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

Question 9

If a bacterium has a lacOC mutation (operator constitutive), what is the expected effect on lac operon expression?

Accepted Answer

The operon will be expressed constitutively, regardless of the presence or absence of lactose.

Question 10

A bacterium with a lacIs (superrepressor) mutation will show which phenotype?

Accepted Answer

The operon will always be off, even in the presence of lactose.

Question 11

Which of the following best describes the effect of a lacZ- mutation?

Accepted Answer

The cell cannot metabolize lactose because it cannot produce functional β-galactosidase.

Question 12

In a scenario where glucose is absent and lactose is present, what are the expected levels of cAMP and the binding status of CAP/cAMP to the lac operon?

Accepted Answer

High cAMP; CAP/cAMP is bound to the promoter, enhancing transcription.

Question 13

A bacterium with a lacY- mutation will exhibit which phenotype?

Accepted Answer

The cell cannot transport lactose into the cell due to lack of permease.

Question 14

If a cell has the genotype I-P+O+Z+Y+/I+P+O+Z-Y-, what will be the phenotype regarding lactose metabolism?

Accepted Answer

The cell will be able to metabolize lactose because the functional Z+ and Y+ genes are present on at least one copy.

Question 15

Which of the following equations correctly represents the relationship between glucose, cAMP, and CAP binding in the lac operon?

Accepted Answer

$$[	ext{Glucose}] \downarrow \implies [	ext{cAMP}] \uparrow \implies 	ext{CAP/cAMP binds to promoter}$$

Question 16

A patient is infected with a strain of E. coli that has a lacP- mutation. What is the expected effect on the lac operon?

Accepted Answer

The operon will not be transcribed because the promoter is nonfunctional.

Question 17

Which of the following best describes the effect of complementing a lacI- mutation with a plasmid carrying a functional lacI+ gene?

Accepted Answer

The repressor will be produced in trans and restore normal regulation of the operon.

Question 18

In the trp operon, when tryptophan levels are high, what happens to the repressor and transcription?

Accepted Answer

Tryptophan acts as a corepressor, binds to the repressor, and the repressor binds to the operator to block transcription.

Question 19

A multi-step scenario: An E. coli cell has the genotype I+P+OCZ-Y+/ISP+O+Z+Y+. Which of the following statements is correct regarding β-galactosidase and permease activity?

Accepted Answer

β-galactosidase will be produced constitutively from the OCZ- allele, and permease will be produced from the Y+ allele.

Genetics Study Guide: Bacterial Gene Regulation and the Lac Operon

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