Question 1

A lac operon of genotype lacI^- lacP⁺ lacO⁺ lacZ⁺ lacY⁺ will produce $\beta$-galactosidase and permease when:

Accepted Answer

The lacI- mutation results in a non-functional repressor, meaning the operon is constitutively expressed regardless of lactose presence.

Question 2

Where would the lac repressor be bound in a (nonmutant) E. coli cell that is growing in low glucose and high lactose? (I = lac repressor gene; Z, Y, A = lac operon structural genes; P = lac promoter; O = lac operator)

Accepted Answer

The answer of Where would the lac repressor be bound...

Question 3

A lac operon of genotype lacI⁺ lacP⁺ lacO⁺ lacZ⁺lacY^- will produce $\beta$-galactosidase but not permease when:

Accepted Answer

The lac operon is only induced in the presence of lactose and absence of glucose. In the given genotype, the lacY gene is mutated, so the permease will not be produced. Therefore, only β-galactosidase will be produced in the presence of lactose.

Question 4

Describe one similarity and one difference in how the trp and lac repressor proteins function.

Accepted Answer

The answer of Describe one similarity and one difference in...

Question 5

E. coli lac operon control by lacI is:

Accepted Answer

The lac operon is controlled by the lacI gene, which encodes a repressor protein. This repressor binds to the operator region of the lac operon and inhibits transcription. When an inducer (such as allolactose) is present, it binds to the repressor, causing it to release from the operator, allowing transcription to proceed. This is an example of negative inducible control.

Question 6

Suppose that you perform an experiment where you construct a plasmid that carries a copy of the lac operator region (lacO) but no other part of the lac operon. (The lac repressor can bind to single operator regions.) This plasmid is placed in an E. coli cell, which has a normal copy of the lac operon in its chromosome. When this strain is grown, the number of plasmids reaches about 50 copies per bacterial cell. What is the expected phenotype of such a strain in the absence of glucose?

Accepted Answer

The excess lac operator regions from the plasmids will bind the lac repressor proteins, preventing them from binding to the chromosomal lac operon, thus keeping it off even in the presence of lactose.

Question 7

A mutant E. coli strain is found that synthesizes $\beta$-galactosidase in the absence of glucose whether or not lactose is present. What two mutations might lead to this outcome?

Accepted Answer

The lacI^- mutation results in a non-functional repressor, allowing expression of the lac operon regardless of lactose presence. The lacO^c mutation alters the operator sequence, preventing the repressor from binding and allowing constitutive expression.

Question 8

Which parts of the DNA region shown in the diagram encode proteins? (I = lac repressor gene; Z, Y, A = lac operon structural genes; P = lac promoter; O = lac operator)

Accepted Answer

The answer of Which parts of the DNA region shown...

Question 9

It is possible for a repressor to negatively regulate the expression of an operon because:

Accepted Answer

The repressor can bind to the operator region, which often overlaps with the promoter, blocking RNA polymerase from binding and initiating transcription.

Question 10

A partial diploid E. coli cell of lacI⁺lacP⁺lacO^clacZ^-lacY^-/ lacI^- lacP⁺ lacO⁺lacZ^-lacY⁺genotype will synthesize:

Accepted Answer

The lacO^c mutation causes constitutive expression of downstream genes, but since lacZ^- is non-functional, only lacY will be expressed in the presence of lactose from the lacI^- lacP^+ lacO^+ lacZ^- lacY^+ operon.

Question 11

In the presence of both lactose and glucose, which of the following occurs with the E. coli lac operon?

Accepted Answer

In the presence of glucose, the cAMP levels are low, preventing the formation of the cAMP-CAP complex, which is necessary for the activation of the lac operon. Thus, the lac operon is not fully activated even if lactose is present.

Question 12

A promoter that affects only genes that are on the same piece of DNA is referred to as a(n) _____-acting promoter.

Accepted Answer

A cis-acting promoter affects only genes that are located on the same piece of DNA, as it is part of the same DNA molecule and regulates the expression of adjacent genes.

Question 13

Assume that a mutation occurs in the promoter for the lacI regulatory gene and this mutation results in a tenfold increase in the transcription of lacI. What would be the expected consequences of such a mutation?

Accepted Answer

A tenfold increase in lacI transcription would lead to more repressor proteins being produced, making it harder for lactose to induce the lac operon, thus preventing full induction even when lactose is present.

Question 14

A lac operon of genotype lacI⁺ lacP⁺ lacO⁺ lacZ^- lacY⁺ will not produce $\beta$-galactosidase but will produce permease when:

Accepted Answer

The answer of A lac operon of genotype lacI⁺ lacP⁺...

Question 15

Which of the following E. coli strains will synthesize permease in the presence of lactose and absence of glucose?

Accepted Answer

The answer of Which of the following E. coli strains...

Question 16

A mutant E. coli strain, grown under conditions that normally induce the lac operon, produces particularly high amounts of ß-galactosidase. What is a possible genotype of the cells?

Accepted Answer

The lacO^c mutation (constitutive operator) allows continuous expression of the lac operon, leading to high levels of ß-galactosidase production regardless of the presence of an inducer.

Question 17

What would happen to the lac operon in the absence of allolactose?

Accepted Answer

In the absence of allolactose, the repressor protein binds to the operator, preventing transcription of the lac operon.

Question 18

The following table shows several bacterial strain lac operon genotypes (some are partial diploids). a. Fill in the blanks in the "lactose absent" and "lactose present" columns in this table. (+) means significant levels of active ß-galactosidase enzyme can be detected. (-) means no significant levels of active ß-galactosidase enzyme are present. The first line is filled in for reference. $\begin{array}{|l|c|c|} \hline \text { Strain genotype }&\text { Lactose absent } & \text { Lactose present } \ \hline 1. l a c I^{+} l a c P^{+} l a c O^{+} l a c Z^{+} l a c Y^{+} &-&+\ \hline \text { 2. } \operatorname{lacI}^{+} \operatorname{lac}^{+} \operatorname{lac} O^{+} \operatorname{lacZ^{+}} \operatorname{lac} Y^{+}\ \hline 3. l a c I^{+} l a c P^{+} \operatorname{lac} O^{+} \operatorname{lacZ^{-}lacY^{+}} \ 4. l a c \mathrm{l}^{*} l a c P^{+} \operatorname{lac} O^{+} \operatorname{lacZ}^{+} l a c \mathrm{Y}^{+} \ \text { 5. } \operatorname{lacI}^{-} \operatorname{lacP^{+}} \operatorname{lacO^{+}\operatorname {lac}Z^{+}lacY^{+}/lacI^{+}}\ \text { 6. } \operatorname{lac} I^{\top} \operatorname{lacP^{+}} \operatorname{lac} O^{+} \operatorname{lacZ^{+}} \operatorname{lac} Y^{+} / \operatorname{lac} I^{+}\ \hline \end{array}$ b. Using your answers in the previous table, explain the dominance relationships among the three lacI alleles (lacI⁺, lacI⁻, lacI^s).

Accepted Answer

The answer of The following table shows several bacterial strain...

Question 19

If there are mutations that inactivate lacP and lacI, which of the following statements is true? (I = lac repressor gene; Z, Y, A = lac operon structural genes; P = lac promoter; O = lac operator)

Accepted Answer

The answer of If there are mutations that inactivate lacP...

Question 20

A mutant E. coli strain, grown under conditions that normally induce the lac operon, does not produce ß-galactosidase. What is a possible genotype of the cells?

Accepted Answer

The mutation in the promoter (lacP^-) prevents RNA polymerase from binding, thus inhibiting transcription of the lac operon, including the lacZ gene that encodes ß-galactosidase.

Quiz 16: Control of Gene Expression in Prokaryotes

A lac operon of genotype lacI^- lacP⁺ lacO⁺ lacZ⁺ lacY⁺ will produce $\beta$ -galactosidase and permease when:

Where would the lac repressor be bound in a (nonmutant) E. coli cell that is growing in low glucose and high lactose? (I = lac repressor gene; Z, Y, A = lac operon structural genes; P = lac promoter; O = lac operator)

A lac operon of genotype lacI⁺ lacP⁺ lacO⁺ lacZ⁺lacY^- will produce $\beta$ -galactosidase but not permease when:

Describe one similarity and one difference in how the trp and lac repressor proteins function.

E. coli lac operon control by lacI is:

A mutant E. coli strain is found that synthesizes $\beta$ -galactosidase in the absence of glucose whether or not lactose is present. What two mutations might lead to this outcome?

Which parts of the DNA region shown in the diagram encode proteins? (I = lac repressor gene; Z, Y, A = lac operon structural genes; P = lac promoter; O = lac operator)

It is possible for a repressor to negatively regulate the expression of an operon because:

A partial diploid E. coli cell of lacI⁺lacP⁺lacO^clacZ^-lacY^-/ lacI^- lacP⁺ lacO⁺lacZ^-lacY⁺genotype will synthesize:

In the presence of both lactose and glucose, which of the following occurs with the E. coli lac operon?

A promoter that affects only genes that are on the same piece of DNA is referred to as a(n) _____-acting promoter.

Assume that a mutation occurs in the promoter for the lacI regulatory gene and this mutation results in a tenfold increase in the transcription of lacI. What would be the expected consequences of such a mutation?

A lac operon of genotype lacI⁺ lacP⁺ lacO⁺ lacZ^- lacY⁺ will not produce $\beta$ -galactosidase but will produce permease when:

Which of the following E. coli strains will synthesize permease in the presence of lactose and absence of glucose?

A mutant E. coli strain, grown under conditions that normally induce the lac operon, produces particularly high amounts of ß-galactosidase. What is a possible genotype of the cells?

What would happen to the lac operon in the absence of allolactose?

If there are mutations that inactivate lacP and lacI, which of the following statements is true? (I = lac repressor gene; Z, Y, A = lac operon structural genes; P = lac promoter; O = lac operator)

A mutant E. coli strain, grown under conditions that normally induce the lac operon, does not produce ß-galactosidase. What is a possible genotype of the cells?

Quiz 16: Control of Gene Expression in Prokaryotes

A lac operon of genotype lacI- lacP+ lacO+ lacZ+ lacY+ will produce β\betaβ -galactosidase and permease when:

Where would the lac repressor be bound in a (nonmutant) E. coli cell that is growing in low glucose and high lactose? (I = lac repressor gene; Z, Y, A = lac operon structural genes; P = lac promoter; O = lac operator)

A lac operon of genotype lacI+ lacP+ lacO+ lacZ+ lacY- will produce β\betaβ -galactosidase but not permease when:

Describe one similarity and one difference in how the trp and lac repressor proteins function.

E. coli lac operon control by lacI is:

A mutant E. coli strain is found that synthesizes β\betaβ -galactosidase in the absence of glucose whether or not lactose is present. What two mutations might lead to this outcome?

Which parts of the DNA region shown in the diagram encode proteins? (I = lac repressor gene; Z, Y, A = lac operon structural genes; P = lac promoter; O = lac operator)

It is possible for a repressor to negatively regulate the expression of an operon because:

A partial diploid E. coli cell of lacI+lacP+lacOc lacZ-lacY-/ lacI- lacP+ lacO+lacZ-lacY+ genotype will synthesize:

In the presence of both lactose and glucose, which of the following occurs with the E. coli lac operon?

A promoter that affects only genes that are on the same piece of DNA is referred to as a(n) _____-acting promoter.

Assume that a mutation occurs in the promoter for the lacI regulatory gene and this mutation results in a tenfold increase in the transcription of lacI. What would be the expected consequences of such a mutation?

A lac operon of genotype lacI+ lacP+ lacO+ lacZ- lacY+ will not produce β\betaβ -galactosidase but will produce permease when:

Which of the following E. coli strains will synthesize permease in the presence of lactose and absence of glucose?

A mutant E. coli strain, grown under conditions that normally induce the lac operon, produces particularly high amounts of ß-galactosidase. What is a possible genotype of the cells?

What would happen to the lac operon in the absence of allolactose?

If there are mutations that inactivate lacP and lacI, which of the following statements is true? (I = lac repressor gene; Z, Y, A = lac operon structural genes; P = lac promoter; O = lac operator)

A mutant E. coli strain, grown under conditions that normally induce the lac operon, does not produce ß-galactosidase. What is a possible genotype of the cells?

A lac operon of genotype lacI^- lacP⁺ lacO⁺ lacZ⁺ lacY⁺ will produce $\beta$ -galactosidase and permease when:

A lac operon of genotype lacI⁺ lacP⁺ lacO⁺ lacZ⁺lacY^- will produce $\beta$ -galactosidase but not permease when:

A mutant E. coli strain is found that synthesizes $\beta$ -galactosidase in the absence of glucose whether or not lactose is present. What two mutations might lead to this outcome?

A partial diploid E. coli cell of lacI⁺lacP⁺lacO^clacZ^-lacY^-/ lacI^- lacP⁺ lacO⁺lacZ^-lacY⁺genotype will synthesize:

A lac operon of genotype lacI⁺ lacP⁺ lacO⁺ lacZ^- lacY⁺ will not produce $\beta$ -galactosidase but will produce permease when: