Ribosome units are synthesised are considered to be endosymbionts of cell because they

Options

A. Nucleus
B. Nucleolus
C. Nuclear pore complex
D. Cytoplasm

Answer

**Step-by-Step Solution:**

1. **Understanding Ribosomes**: Ribosomes are cellular structures responsible for protein synthesis. They are composed of two subunits, a large and a small subunit, and are made of ribosomal RNA (rRNA) and proteins.

2. **Location of Ribosome Synthesis**: Ribosomes are synthesized in a specific region of the cell known as the nucleolus, which is located within the nucleus. The nucleolus is the site where rRNA is transcribed and combined with proteins to form ribosomal subunits.

3. **Endosymbiont Concept**: The term "endosymbiont" refers to a theory that certain organelles in eukaryotic cells, such as mitochondria and chloroplasts, originated from free-living prokaryotes that were engulfed by ancestral eukaryotic cells. This concept can also extend to ribosomes because they share similarities with prokaryotic ribosomes in terms of structure and function.

4. **Nucleolus and Ribosome Formation**: The nucleolus plays a crucial role in the assembly of ribosomes. It synthesizes rRNA and combines it with proteins to create the ribosomal subunits, which are then transported out to the cytoplasm where they function in protein synthesis.

5. **Conclusion**: Given that ribosomes are synthesized in the nucleolus and that they share characteristics with prokaryotic cells, they can be considered as endosymbionts of the cell. Therefore, the correct answer to the question is option B: nucleolus.

**Final Answer**: B. Nucleolus

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