Question

Which genotype will allow the bacteria to make ß-galactosidase (lacZ) in the absence of lactose? (superscripts don't work in this panopto quiz, so check the lecture slide itself)

1) O+Z+/F’O+Z+

2) O+Z-/F’OCZ+

3) O+Z+/F’OCZ-

B) 1Which genotype will allow the bacteria to make ß-galactosidase (lacZ) in the absence of lactose?

1) I+O+Z+/F’I+O+Z+

2)I-O+Z-/F’I+O+Z+

3) I+O+Z-/F’I-O+Z+

4) I-O+Z+/F’I-O+Z+

Answer 1

Operon can be defined as a group or cluster of genes which are functionally related that have a common operator. Thus, the lac operon is a group of three genes - lacZ, lacY, and lacA that are involved in the metabolism of lactose and are controlled by a common operator.

The lac operon is an inducible operon - which means that generally, the operon is in a repressed state or inactive, but in the presence of the inducer allolactose (lactose is rearranged inside the cell to its isomer form - allolactose), the operon is activated.

The lac operon consists of a promoter site where the RNA polymerase binds. It consists of an operator site where the lac repressor is generally bound. Generally, when the lac repressor is bound to the operator region, the RNA polymerase cannot bind to the promoter region and start transcription because of the overlapping of some portion of the operator and promoter regions.

The lac Z gene produces the enzyme beta-galactosidase which breaks down the disaccharide lactose into glucose and galactose.

The lac Y gene produces beta-galactoside permease. Beta-galactoside permease, as the name suggests increases the permeability of the cell to the beta-galactosides like lactose.

The lac Z gene encodes beta-galactoside transacetylase.

The genes in the lac operon are either the cis-acting (coupling) genes or the trans-acting (repulsion) genes. The cis-acting genes are the operator and the structural genes while the trans-acting genes are the repressor genes.

The genotype is written as: repressor (I), promoter (P), the operator (O), Lac Z (Z), and Lac Y (Y).

Bacteria are generally monoploid - i.e., they have one copy of each gene. But placing another copy of the gene on a plasmid (F') can make the bacteria partially diploid.

Question a:

Oc is the mutated form of the operator gene. Due to this mutation, the repressor cannot bind to the operator sequence and hence, the RNA polymerase binds to the promoter sequence. This causes the RNA polymerase to induce the transcription of the lac Z, lac Y, and lac A genes irrespective of whether lactose is present or absent in the medium.

The mutant form of the Z- gene means that no functional protein (i.e., the enzyme beta-galactosidase) will be produced. Thus, for the enzyme to be produced a good copy of the gene (i.e., Z+) should be present.

Thus, the correct option is Option 2 - O+Z-/F'Oc Z+. Here, the first chromosome has a good operator which can bind the lac suppressor, so the beta-galactosidase will not be produced in the absence of lactose. However, the second chromosome has a mutated operator sequence were the lac repressor will not bind. It also has a good Z gene (i.e., Z+) which is capable of producing the beta-galactosidase enzyme. Thus, the enzyme beta-galactosidase will be produced even in the absence of lactose.

Question b:

I+ repressor is fully functional and can bind to the operator region. This causes the inhibition of the transcription of the lac Z, lac Y, and lac A genes in the absence of lactose.

I- mean that the repressor is nonfunctional and thus cannot bind to the operator region. This means that the RNA polymerase can bind freely to the promoter region (since some portion of the promoter and operator regions overlap, both cannot bind to their respective regions together) and induce the transcription of the lac Z, lac Y, and lac A genes. This is a constitutive kind of expression where the gens are constitutively expressed regardless of whether lactose is present in the environment or not).

Since the regulator genes are transacting, the genes in both the chromosomes have to be I- for the bacteria to make beta-galactosidase in the absence of lactose.

Thus, the correct option is an option (4) - I-O+Z+/F'I-O+Z+. Here, since the repressor gene in both the chromosomes is mutated, the repressor cannot bind to the operator. this causes constitutive expression of the lac Z, lac Y, and lac A genes.

Even though the operator genes are not mutated, the repressor cannot bind to the operator region.

The Z genes are not mutated are fully functional (i.e., Z+) so they can produce the enzyme beta-galactosidase.