Question

All of the following are part of an operon except

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

Answer 1

The Correct Option is B

An operon is a functional unit of genomic DNA that serves as a major element in the regulation of gene expression, particularly in prokaryotes like bacteria. It contains several key components that work together to control the transcription of structural genes into messenger RNA (mRNA).

Let us examine each part mentioned in the question in the context of an operon:

1. An operator: This is a segment of DNA to which a repressor protein can bind. The binding can either block or facilitate the transcription of the genes in the operon. Therefore, the operator is indeed part of an operon.
2. Structural genes: These genes encode proteins that are required for the operon's function. They are transcribed together from a single mRNA transcript. Thus, structural genes are definitely a component of an operon.
3. A promoter: This is a sequence of DNA where RNA polymerase binds to initiate transcription of the downstream genes. As it is essential for gene transcription, a promoter is part of an operon.
4. An enhancer: An enhancer is a short region of DNA that can be bound by proteins to increase the likelihood of transcription of particular genes. Enhancers are typically not part of prokaryotic operons. They are more commonly associated with eukaryotic gene regulation where they can act at a considerable distance from the genes they regulate. Therefore, an enhancer is not part of an operon.

In conclusion, the correct answer is "an enhancer" because it is not part of an operon.