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1. The lac operon of E. coli controls the expression of genes that code for enzymes involved in lactose metabolism. The operon system is inducible in that it can be ‘turned on’ by the presence of lactose. Mutations to various regions of the lac operon have been discovered which affect both the control of gene expression and the function of the gene products.

These mutations include:

I - = produces a repressor protein that can not bind to the operator region

I s = produces a repressor protein that can not be removed from the operator region

O C = can not bind a repressor protein

Z - = produces a defective (non-functioning) β-galactosidase enzyme

( I- are recessive to I+; I+ recessive to IS )

Detail the functionality of the lac

operon under the following genetic and cellular conditions.

GENES

Lactose

present

Repressed

(yes/no)

Constitutive

(yes/no)

Inducible

(yes/no)

I + O + Z +

no

I + O C Z +

no

I S O + Z +

yes

I - O + Z +

yes

I S O C Z +

no

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Jarrod Robel
Jarrod RobelLv2
28 Sep 2019

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Related questions

1. The following shows the genotype of a partial diploid bacterial cell - where one chromosomal region containing the lac operon in E,coli is given, and the other fragment is from a plasmid carrying another lac operon from another source. The two are separated by a slash (/).

The possible answers indicate with a ʺ+ʺ or a ʺ-ʺ whether β-galactosidase would be expected to be produced at induced levels under two circumstances: 1) first in the absence of lactose and 2) second in the presence of lactose. (Assume that glucose is not present in the medium.)

Genotype C: Is Oc Z+/ Fʹ I + O+ Z+
KEY:

I+ = wild-type repressor
I- = mutant repressor (unable to bind to the operator)
Is = mutant repressor (insensitive to lactose)
O+ = wild-type operator
Oc = constitutive operator (insensitive to repressor)

a. +,+ d. -,+

b. -,- e. cannot tell

c. +,-

2. The following shows the genotype of a partial diploid bacterial cell - where one chromosomal region containing the lac operon in E,coli is given, and the other fragment is from a plasmid carrying another lac operon from another source. The two are separated by a slash (/).

The possible answers indicate with a ʺ+ʺ or a ʺ-ʺ whether β-galactosidase would be expected to be produced at induced levels under two circumstances: 1) first in the absence of lactose and 2) second in the presence of lactose. (Assume that glucose is not present in the medium.)

Genotype D: I- O+ Z+/ Fʹ I- O+ Z+
KEY:

I+ = wild-type repressor
I- = mutant repressor (unable to bind to the operator)
Is = mutant repressor (insensitive to lactose)
O+ = wild-type operator
Oc = constitutive operator (insensitive to repressor)

a. +,+ d. -,+

b. -,- e. cannot tell

c. +,-

3. The following shows the genotype of a partial diploid bacterial cell - where one chromosomal region containing the lac operon in E,coli is given, and the other fragment is from a plasmid carrying another lac operon from another source. The two are separated by a slash (/).

The possible answers indicate with a ʺ+ʺ or a ʺ-ʺ whether β-galactosidase would be expected to be produced at induced levels under two circumstances: 1) first in the absence of lactose and 2) second in the presence of lactose. (Assume that glucose is not present in the medium.)

Genotype E: I + O+ Z+/ Fʹ I + O+ Z+
KEY:

I+ = wild-type repressor
I- = mutant repressor (unable to bind to the operator)
Is = mutant repressor (insensitive to lactose)
O+ = wild-type operator
Oc = constitutive operator (insensitive to repressor)

a. +,+ d. -,+

b. -,- e. cannot tell

c. +,-

Below are possible alleles found in the lac operon.

Repressor gene:

Is (superrepressor; can't bind allolactose, but can bind operator)

I+ (inducible; wild-type)

I- (constitutive; can bind allolactose, but can't bind operator)

Operator:

Oc (constitutive; can't bind repressor)

O+ (inducible; wild-type)

A. Which of the following combinations below will not express β-galactosidase even when lactose is present? Circle and explain your answer.

I+O+

I-Oc

IsOc

IsO+

I-O+

B. Which of the following combinations below will express β-galactosidase only if lactose is present? Circle and explain your answer.

I+O+

I-Oc

IsOc

IsO+

I-O+

Note: This question is for an INTRODUCTORY genetics class, so please try to answer in a way that an INTRODUCTORY student would be able to understand.

1. The following shows the genotype of a partial diploid bacterial cell - where one chromosomal region containing the lac operon in E,coli is given, and the other fragment is from a plasmid carrying another lac operon from another source. The two are separated by a slash (/).

The possible answers indicate with a ʺ+ʺ or a ʺ-ʺ whether β-galactosidase would be expected to be produced at induced levels under two circumstances: 1) first in the absence of lactose and 2) second in the presence of lactose. (Assume that glucose is not present in the medium.)

Genotype A: I + O+ Z+/ Fʹ I - O+ Z+

KEY:

I+ = wild-type repressor
I- = mutant repressor (unable to bind to the operator)
Is = mutant repressor (insensitive to lactose)
O+ = wild-type operator
Oc = constitutive operator (insensitive to repressor)

a. +,+ d. -,+

b. -,- e. cannot tell for certain

c. +,-

2. The following shows the genotype of a partial diploid bacterial cell - where one chromosomal region containing the lac operon in E,coli is given, and the other fragment is from a plasmid carrying another lac operon from another source. The two are separated by a slash (/).

The possible answers indicate with a ʺ+ʺ or a ʺ-ʺ whether β-galactosidase would be expected to be produced at induced levels under two circumstances: 1) first in the absence of lactose and 2) second in the presence of lactose. (Assume that glucose is not present in the medium.)

Genotype B: I - Oc Z+/ Fʹ I - O+ Z-

KEY:

I+ = wild-type repressor

I- = mutant repressor (unable to bind to the operator)
Is = mutant repressor (insensitive to lactose)
O+ = wild-type operator
Oc = constitutive operator (insensitive to repressor)

a. +,+ d. -,+

b. -,- e. cannot tell for certain

c. +,-

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