The structural gene in a transcription unit could be said as polycistronic

Options

A. Mostly in prokaryotes
B. Mostly in eukaryotes
C. Both
D. None

Answer

To answer the question, "The structural gene in a transcription unit could be said as polycistronic," we need to analyze the concept of polycistronic genes and their occurrence in different types of organisms.

### Step-by-Step Solution:

1. **Understanding the Term 'Gene':**
- A gene is a segment of DNA that contains the information necessary to produce a functional product, typically a protein. It can also code for functional RNA molecules like tRNA or rRNA.

**Hint:** Remember that genes are segments of DNA that code for proteins or RNA.

2. **Defining 'Cistron':**
- A cistron is a segment of DNA that codes for a single polypeptide chain. In other words, it is synonymous with a structural gene.

**Hint:** Cistrons are specific segments of DNA that lead to the production of polypeptides.

3. **Monocistronic vs. Polycistronic:**
- A monocistronic transcription unit contains a single cistron (structural gene) and is typical in eukaryotes (u-carriers).
- A polycistronic transcription unit contains multiple cistrons (structural genes) and is characteristic of prokaryotes (pro-carriers).

**Hint:** Eukaryotes usually have monocistronic genes, while prokaryotes often have polycistronic genes.

4. **Occurrence in Organisms:**
- Monocistronic genes are predominantly found in eukaryotic organisms, where each gene is transcribed separately.
- Polycistronic genes are primarily found in prokaryotic organisms, such as bacteria, where multiple genes can be transcribed together from a single promoter.

**Hint:** Think about the differences in gene organization between prokaryotes and eukaryotes.

5. **Conclusion:**
- Since the question asks about the structural gene in a transcription unit being polycistronic, the correct answer is that this characteristic is mostly found in prokaryotes.

**Final Answer:** The structural gene in a transcription unit could be said as polycistronic mostly in prokaryotes.

### Summary:
The correct option is that polycistronic structural genes are primarily found in prokaryotes, while eukaryotes typically have monocistronic genes.