Which of the following structure is present only in prokaryotes ?

Options

A. Mesosome
B. Ribosome
C. Nucleus
D. Both (2) and (3)

Answer

To determine which structure is present only in prokaryotes, we need to analyze the options provided: mesosome, ribosome, nucleus, and both second and third (which refers to ribosome and nucleus).

### Step-by-Step Solution:

1. **Understand Prokaryotic and Eukaryotic Cells**:
- Prokaryotic cells are simpler and do not have a defined nucleus or membrane-bound organelles. Examples include bacteria and archaea.
- Eukaryotic cells are more complex, with a defined nucleus and various membrane-bound organelles. Examples include plant and animal cells.

2. **Analyze Each Option**:
- **Mesosome**: This is a structure found in prokaryotic cells. It is an infolding of the cell membrane and is involved in various functions such as DNA replication and respiration.
- **Ribosome**: Ribosomes are present in both prokaryotic and eukaryotic cells. They are the sites of protein synthesis.
- **Nucleus**: The nucleus is a membrane-bound organelle found only in eukaryotic cells. Prokaryotic cells do not have a nucleus; instead, their genetic material is located in a nucleoid region.
- **Both second and third**: This option refers to ribosomes and nuclei, both of which are not exclusive to prokaryotes.

3. **Conclusion**:
- The only structure that is present exclusively in prokaryotes is the **mesosome**. Ribosomes are found in both types of cells, and the nucleus is exclusive to eukaryotes.

### Final Answer:
The structure that is present only in prokaryotes is **mesosome**.

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