which statement s about repressible operons is are correct

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19-02-2025

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Which Statement(s) About Repressible Operons Is/Are Correct?

Repressible operons are a fascinating aspect of bacterial gene regulation. Understanding how they function requires a grasp of their core components and their responses to environmental cues. This article will explore the characteristics of repressible operons and clarify which statements about them are accurate. We'll examine several common statements and determine their validity.

Understanding Repressible Operons

Before diving into specific statements, let's establish a foundational understanding of repressible operons. Unlike inducible operons (like the lac operon), which are usually "off" and switched "on" by a specific inducer molecule, repressible operons are typically "on" and are switched "off" in response to a specific signal. This "off" switch is usually a repressor protein, activated by a corepressor molecule.

The core components of a repressible operon include:

• Structural genes: These genes code for enzymes involved in a specific metabolic pathway.
• Promoter: The region where RNA polymerase binds to initiate transcription.
• Operator: A DNA sequence where the repressor protein binds, blocking RNA polymerase.
• Repressor gene: Codes for a repressor protein that can bind to the operator.
• Corepressor: A molecule that binds to the repressor protein, causing a conformational change that allows it to bind to the operator and repress transcription.

Evaluating Statements About Repressible Operons

Now, let's analyze some common statements regarding repressible operons and determine their accuracy:

Statement 1: Repressible operons are usually "on" in the absence of a corepressor.

Correct. In the absence of the corepressor, the repressor protein is inactive and cannot bind to the operator. This allows RNA polymerase to transcribe the structural genes, resulting in the production of the enzymes encoded by those genes.

Statement 2: The corepressor binds to the promoter region, inhibiting transcription.

Incorrect. The corepressor binds to the repressor protein, not the promoter region directly. This binding changes the repressor's shape, enabling it to bind to the operator and block transcription. The promoter remains unaffected by the corepressor itself.

Statement 3: Repressible operons are involved in anabolic pathways.

Correct. Repressible operons typically regulate anabolic pathways – those that synthesize molecules the cell needs. When the end product of the pathway (the corepressor) is abundant, it signals the cell to stop producing more, preventing wasteful overproduction. The trp operon, which synthesizes tryptophan, is a classic example.

Statement 4: The repressor protein is always active and binds to the operator even in the absence of the corepressor.

Incorrect. This statement describes a constitutively repressed operon, not a typical repressible operon. In repressible operons, the repressor is inactive in the absence of the corepressor.

Statement 5: Transcription of the structural genes is increased in the presence of the corepressor.

Incorrect. The presence of the corepressor decreases transcription of the structural genes. The corepressor activates the repressor protein, leading to repression of gene expression.

Statement 6: The lac operon is an example of a repressible operon.

Incorrect. The lac operon is a classic example of an inducible operon, not a repressible one. It's regulated by the presence or absence of lactose, not by the accumulation of a metabolic end-product.

Summary

In conclusion, statements 1 and 3 are correct regarding repressible operons. They are typically "on" unless a corepressor is present to activate the repressor protein and block transcription. These operons primarily regulate anabolic pathways, ensuring efficient resource use. Understanding the differences between repressible and inducible operons is crucial for comprehending the intricate regulatory mechanisms within bacterial cells. Further study of specific operons like the trp operon can provide a deeper understanding of these processes.