Polycistronic mesenger RNA (mRNA) usually occurs in

Options

A. bacteria
B. prokaryotes
C. eukaryotes
D. both a and b

Explanation

**Step-by-Step Solution:**

1. **Understanding mRNA Types**:
- There are two types of messenger RNA: monocistronic and polycistronic.
- Monocistronic mRNA contains the coding information for a single gene, while polycistronic mRNA contains the coding information for multiple genes.

2. **Definition of Polycistronic mRNA**:
- Polycistronic mRNA is formed when several genes (also known as cistrons) are transcribed into a single mRNA molecule. This means that multiple proteins can be synthesized from one mRNA strand.

3. **Occurrence of Polycistronic mRNA**:
- Polycistronic mRNA is typically found in prokaryotes, which are organisms that lack a defined nucleus. Examples of prokaryotes include bacteria.

4. **Comparison with Eukaryotic mRNA**:
- In contrast, eukaryotic mRNA is usually monocistronic, meaning it typically encodes only one protein per mRNA molecule.

5. **Conclusion**:
- Since polycistronic mRNA is characteristic of prokaryotes (like bacteria), the correct answer to the question is that polycistronic mRNA usually occurs in prokaryotes, which includes bacteria.

6. **Final Answer**:
- Therefore, the correct option is "both A and B" (which refers to bacteria and prokaryotes).

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