Question

For the lac operon, what would be the expected effects of a mutation in the operator site that prevented the binding of the repressor protein? a. The operon would always be turned on. b. The operon would always be turned off. c. The operon would always be turned on, except when glucose is present. d. The operon would be turned on only in the presence of lactose. e. The operon would be turned on only in the presence of lactose and the absence of glucose.

Short answer

The correct answer is a. The operon would always be turned on.

Step-by-step solution

Understand the Normal Lac Operon Operation

The Lac operon is a system that responds to the presence or absence of lactose in a bacterial cell. Normally, the Lac repressor protein binds to the operator site, blocking the transcription of downstream genes (lacZ, lacY, lacA) when lactose isn't present. These genes, when expressed, produce proteins responsible for breaking down lactose.

Understand the Mutation's Outcome

Now, we're dealing with a mutation that prevents the repressor protein from binding to the operator site. This means that the repressor protein can no longer block the expression of downstream genes, and these genes will no longer be controlled by the repressor protein regardless of the presence or absence of lactose.

Relate the Outcome to the Given Options

We can conclude because the mutation prevents the repressor protein from binding to the operator site, the lac operon (genes) would always be turned on because they're no longer being repressed. This aligns with option a.

Key concepts

Mutation Effects

Mutations are changes in the DNA sequence that can affect how genes function. In the context of the lac operon, a mutation in the operator site has significant implications. Normally, the operator is a critical site for controlling gene expression, specifically within the lac operon. When a mutation occurs here, it can prevent the repressor protein from effectively binding to the operator. This faulty binding means that the normal regulation of genes involved in lactose metabolism is disrupted. For the lac operon, if a mutation in the operator site prevents repressor binding, the genes responsible for metabolizing lactose are continually expressed and are constantly turned on because the blockage that normally stops their transcription is removed. Such mutations thus lead to unregulated gene expression, highlighting the pivotal role of precise DNA interactions in cellular control mechanisms.

Repressor Protein

The repressor protein is a key player in managing gene expression. In the lac operon, it is produced by the regulator gene and its main role is to prevent transcription of the lactose-metabolizing genes by binding to the operator site.

• When lactose is absent, the repressor binds to the operator, acting like a barricade that blocks RNA polymerase from transcribing the downstream genes.
• In the presence of lactose, the repressor protein changes shape when lactose or its isomer, allolactose, binds to it. This shape change releases the repressor from the operator site, allowing transcription.

A mutation that affects the repressor protein's ability to bind to the operator can disrupt this regulation. Without the repressor binding, even in the absence of lactose, the genes will be transcribed, leading to unnecessary production of enzymes.

Gene Expression Regulation

Regulation of gene expression is essential for cellular efficiency and adaptation. In the lac operon model, this regulation determines whether or not proteins involved in lactose metabolism are synthesized. Efficient regulation means that enzymes are produced only when necessary, namely in the presence of lactose and absence of glucose.

• The primary regulatory mechanism involves a repressor protein that inhibits transcription by binding to the operator site under normal conditions.
• Additional regulation is provided by the presence of glucose, which through catabolite repression, can further inhibit the lac operon, ensuring that lactose is used only when needed.

When mutations inhibit this precise control, gene expression may occur regardless of substrate availability, resulting in wasted cellular resources.

Operator Site

The operator site is a short region of DNA crucial for the regulation of gene expression in the lac operon. It is located between the promoter and the genes of the operon and acts as a binding site for the repressor protein.

• Under normal conditions, the operator site ensures that the repressor can effectively bind and block transcription when lactose is absent.
• When the operator is mutated, it can no longer act as a binding site for the repressor, thereby leaving the genes within the operon perpetually active.

The functionality of the operator site is vital for maintaining balance and order in cellular processes. Mutations that disrupt its operation can lead to persistent gene expression, which might lead to energy inefficiency and imbalance in the cell.