The following diagram shows an operon that is normally turned off but can be activated in the presence of a specific substrate (e.g., lactose). Which type of operon is depicted?

• A. Repressible operon
• B. Inducible operon
• C. Constitutive operon
• D. Attenuated operon

Answer: (B)

Verified step by step guidance

1. Understand the concept of an operon: An operon is a cluster of genes under the control of a single promoter and regulatory elements. It is a mechanism used by prokaryotes to regulate gene expression efficiently.
2. Learn the difference between types of operons: A repressible operon is typically active and can be turned off when a specific molecule binds to the repressor. An inducible operon is normally off and can be activated in the presence of a specific substrate. A constitutive operon is always active, and an attenuated operon involves regulation through transcription termination.
3. Analyze the problem statement: The operon described is normally turned off but can be activated in the presence of a specific substrate, such as lactose. This matches the definition of an inducible operon, which is activated by the presence of an inducer molecule.
4. Relate the example to a real biological system: The lactose (lac) operon in E. coli is a classic example of an inducible operon. It is activated when lactose is present, as lactose acts as an inducer by binding to the repressor and preventing it from blocking transcription.
5. Conclude the reasoning: Based on the description and the characteristics of inducible operons, the correct answer is 'Inducible operon.' This type of operon allows the cell to conserve energy by only expressing genes when the substrate is available.