Which organelle is primarily responsible for producing ribosomal subunits in eukaryotic cells?

• A. Nucleolus
• B. Mitochondrion
• C. Endoplasmic Reticulum
• D. Golgi apparatus

Answer: (A)

Ribosome production for the cell’s protein-building machinery happens in the nucleolus, a specialized region inside the nucleus. Here, ribosomal RNA genes are transcribed and processed, and ribosomal proteins that were made in the cytoplasm are imported back into the nucleus to assemble with the rRNA into the small and large ribosomal subunits. Once assembled, these subunits are exported to the cytoplasm where they come together to form functional ribosomes during translation. The mitochondrion contains its own small set of ribosomes but not the cytosolic ribosomal subunits, and the endoplasmic reticulum and Golgi apparatus handle protein folding, modification, and trafficking rather than ribosome assembly. So the nucleolus is the site primarily responsible for producing ribosomal subunits.