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Lac Operon: State Of Transcription

The lac operon encodes a set of genes for the metabolism of lactose. There are three genes encode by the lac operon: lac Z gene, lac Y gene, and lac A gene. (1 )
The lac Z gene translate into B-galactosidase. (1)
The lac Y gene translate into lactose permease. (1)
The lac A gene translate into lactose transacetylase. (1)

These three genes are the structural genes of the lac operon. (1,2) The direction of the structural gene on the lac operon is Z-Y-A. (1, 2)

The lac I gene is close to the structural genes, and it encodes for the lac repressor. (1, 2) The function of the lac repressor is that it can bind to the sequence of DNA called the operator (lac O), and stop transcription of the structural genes Z-Y-A. (1, 2) The repressor has another function in that it can bind to the inducer, which is a metabolite of lactose called allolactose. (1, 2) When the repressor bind to the inducer allolactose or lactose, it becomes inactive and cannot bind to the operator. Therefore, transcription of the structural genes B-galactosidase to metabolize lactose is induced when there is no repressor. (2) This induction of transcription only occur in the present of lactose, so the inducer is lactose.

On the other hand, if the inducer allolactose or lactose is absent, then it cannot bind to repressor. The repressor will stay active and bind to the operator. Binding of repressor to the operator will prevent transcription of lac Z-Y-A. (1) As a result, there is no B-galactosidase production because lactose is absent. Thus, the lac operon is an inducible system of regulation for lactose. (2)

Now let say that if the repressor gene lac I is deleted or mutated ( lac I-). In a mutated lac I- cell, there is no repressor so the transcription of the structural gene is "always on" regardless of whether there is an inducer present or not. (1) This unregulated expression of the structural genes as a result of the mutation in the repressor gene lac I- is known as "constitutive expression". (1)

This diploid genotype mean that a cell has one lac operon with a normal gene I and one lac operon with mutated lac I- gene. (1) The state of transcription in the presence of lactose is that lactose will bind to the repressor produced by the normal lac I gene and inactivate it. (1) Transcription of the structural genes will happen normally for both the wildtype lac I and mutant lac I-. The "good" or wildtype lac I gene will produce enough repressors to compensate for the bad mutated repressor lac I- on the other lac operon. (1) The normal repressor will travel to the operator on the other mutated lac I operon and bind to it. As a result, the expression of both lac operons will be normally control. (1)

In the absence of the lactose, the "good" lac I gene will encode enough repressors to bind to both of the (lac O) operators on each lac operons. So each of the lac operons (wildtype and mutated) will control expression normally. (1) So the state of transcription in the presence and absence of lactose in cell with the diploid genotype of wildtype lac I/ mutant lac I- is that transcription will happen normally because the normal lac I will encode enough repressor to compensate for the mutant lac I-. Thus, the expression of both operons will be control normally. As mentioned, the function of lac I is that it encodes for the repressor protein that binds to the operator on the lac operon; and repress transcription of the structural genes Z-Y-A. The function of lac I- is that it is a mutation in the lac I gene, which prevent production of repressor. Lac I- does not produce repressor protein. As a result, the lac operon will always be on and transcription of gene Z-Y-A is always on. The function of lac I- is to constitutively expressed the lac Z-Y-A on the lac operon. (1)

References:
1. http://www.cliffsnotes.com/study_guide/The-Lactose-OperonA-Look-at-Regulation.topicArticleId-24594,articleId-24575.html
2. Brock et al. Biology of Microorganisms. 7th edition.

Solution Preview

The lac operon encodes a set of genes for the metabolism of lactose. There are three genes encode by the lac operon: lac Z gene, lac Y gene, and lac A gene. (1 )
The lac Z gene translate into B-galactosidase. (1)
The lac Y gene translate into lactose permease. (1)
The lac A gene translate into lactose transacetylase. (1)

These three genes are the structural genes of the lac operon. (1,2) The direction of the structural gene on the lac operon is Z-Y-A. (1, 2)

The lac I gene is close to the structural genes, and it encodes for the lac repressor. (1, 2) The function of the lac repressor is that it can bind to the sequence of DNA called the operator (lac O), and stop transcription of the structural genes Z-Y-A. (1, 2) The repressor has another function in that it can bind to the inducer, which is a metabolite of lactose called allolactose. (1, 2) When the repressor bind to the inducer allolactose or lactose, it becomes inactive and cannot bind to the operator. Therefore, transcription of the structural genes B-galactosidase to ...

Solution Summary

The lac operon encodes a set of genes for the metabolism of lactose. There are three genes encode by the lac operon: lac Z gene, lac Y gene, and lac A gene. (1 ) The lac Z gene translate into B-galactosidase. (1) The lac Y gene translate into lactose permease. (1) The lac A gene translate into lactose transacetylase. (1) The lac I gene is close to the structural genes, and it encodes for the lac repressor. (1, 2) The function of the lac repressor is that it can bind to the sequence of DNA called the operator (lac O), and stop transcription of the structural genes Z-Y-A. (1, 2) The repressor has another function in that it can bind to the inducer, which is a metabolite of lactose called allolactose. (1, 2) When the repressor bind to the inducer allolactose or lactose, it becomes inactive and cannot bind to the operator. Therefore, transcription of the structural genes B-galactosidase to metabolize lactose is induced when there is no repressor. (2) This induction of transcription only occur in the present of lactose, so the inducer is lactose.
On the other hand, if the inducer allolactose or lactose is absent, then it cannot bind to repressor. The repressor will stay active and bind to the operator. Binding of repressor to the operator will prevent transcription of lac Z-Y-A. (1) As a result, there is no B-galactosidase production because lactose is absent. Thus, the lac operon is an inducible system of regulation for lactose. (2)

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