All of the following are part of an operon except:

Options

A. an operator
B. a promoter
C. an enhancer
D. structural genes

Answer

To solve the question "All of the following are part of an operon except:", we will analyze the components of an operon and identify which one does not belong.

### Step-by-Step Solution:

1. **Understand the Definition of an Operon**:
An operon is a cluster of genes under the control of a single promoter and is primarily found in prokaryotic organisms (like bacteria). It consists of multiple components that work together to regulate gene expression.

2. **Identify the Components of an Operon**:
The main components of an operon include:
- **Promoter**: The region where RNA polymerase binds to initiate transcription.
- **Operator**: A segment of DNA that a repressor can bind to, blocking RNA polymerase from transcribing the genes.
- **Structural Genes**: The genes that are transcribed into mRNA and ultimately translated into proteins.

3. **Consider the Options Given**:
The options provided in the question are:
- Operator
- Promoter
- Enhancer
- Structural Genes

4. **Determine Which Component is Not Part of an Operon**:
- The **promoter**, **operator**, and **structural genes** are all integral parts of an operon.
- The **enhancer**, however, is not a part of the operon structure. Enhancers are regulatory sequences that can increase the transcription of genes but are not directly involved in the operon mechanism itself.

5. **Conclusion**:
Therefore, the correct answer to the question is that the component that is not part of an operon is the **enhancer**.

### Final Answer:
**Enhancer**

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