Problem 6 - Campbell Biology

Question

What would occur if the repressor of an inducible operon were mutated so it could not bind the operator? \begin{equation} \begin{array}{l}{\text { (A) irreversible binding of the repressor to the promoter }} \\ {\text { (B) reduced transcription of the operon's genes }} \\\ {\text { (C) buildup of a substrate for the pathway controlled by the }} \\\ {\text { operon }} \\ {\text { (D) continuous transcription of the operon's genes }}\end{array} \end{equation}

Step-by-Step Solution

Answer

(D) Continuous transcription of the operon's genes

Step 1: Understand the Components of an Inducible Operon

An inducible operon generally has a promoter, an operator, and one or more genes. The repressor protein can bind to the operator, preventing gene transcription.

Step 2: Consider What Happens When the Repressor Binds to the Operator

When the repressor binds to the operator, it blocks the RNA polymerase from transcribing the genes in the operon, leading to no gene expression.

Step 3: Examine the Effect of a Mutated Repressor

If the repressor is mutated and cannot bind to the operator, it cannot prevent the RNA polymerase from transcribing the operon's genes.

Step 4: Conclusion from the Mutation

Since the repressor cannot bind the operator, the operon's genes are continuously transcribed because there's nothing stopping the RNA polymerase. Hence, the correct effect is continuous transcription of the operon's genes.

Step 5: Choose the Correct Option

From the analysis, the best choice is: (D) continuous transcription of the operon's genes.

Key Concepts

repressor protein

Repressor proteins play a crucial role in regulating gene transcription. They are usually encoded by regulatory genes and have the ability to bind to specific DNA sequences known as operators. The main function of a repressor protein is to prevent the transcription of genes in an operon by blocking the attachment of RNA polymerase to the promoter.

If a repressor protein binds to the operator region, it essentially acts as a blockade, stopping the RNA polymerase from moving forward and starting transcription. Without this intervention, the genes would be continuously active.

In an inducible operon, a repressor protein keeps the genes turned off until a specific inducer molecule binds to the repressor, causing it to change shape and release its hold on the operator. This allows for gene transcription to occur, but only when needed. In the absence of a properly functioning repressor, the operon's genes would be continuously transcribed, as there would be no roadblock to stop RNA polymerase from accessing the DNA sequence.

gene transcription

Gene transcription is the process by which the genetic code from DNA is copied into mRNA (messenger RNA), which is then used to produce proteins. This is a vital step in gene expression. The process begins when RNA polymerase binds to a promoter region upstream of the gene.

While this sounds straightforward, multiple regulatory mechanisms come into play to ensure that genes are transcribed only when necessary. In the case of an inducible operon, transcription is usually turned off unless specific conditions prompt it to turn on. This on-off switch is tightly controlled by proteins that interact with the DNA sequences near the gene.

The presence of the repressor protein on the operator prevents RNA polymerase from starting the transcription process. However, if the operator is not blocked due to a malfunctioning repressor, RNA polymerase has free access to the DNA and can begin transcription, leading to continuous production of mRNA and, subsequently, proteins.

operator binding

The operator is a DNA sequence found within operons that serves as a binding site for regulatory proteins, such as repressors. Basically, the operator acts as a gatekeeper, determining whether the downstream genes would be transcribed.

In an inducible operon, the operator is usually located between the promoter and the genes it controls. When the repressor protein is bound to the operator, RNA polymerase is physically hindered from moving past the promoter region to start gene transcription.

However, if the repressor is mutated or otherwise unable to bind to the operator, this checkpoint is effectively bypassed. The operator then remains unoccupied, allowing RNA polymerase unfettered access to the genes, leading to continuous transcription. This lack of regulation can result in an unnecessary buildup of proteins or other gene products, which might be detrimental to the cell's overall functioning.