Where are ribosomes made?

Ribosomes are made in the nucleolus, a specialized sub-structure located within the nucleus of eukaryotic cells. The nucleolus is often referred to as the ribosome factory of the cell because it is the site where ribosomal RNA (rRNA) is synthesized and combined with ribosomal proteins to assemble the ribosomal subunits.

Step-by-Step Ribosome Formation Process

1. rRNA Synthesis in the Nucleolus

• The nucleolus contains DNA segments that code for ribosomal RNA (rRNA).
• RNA polymerase I transcribes these genes to produce rRNA, which forms the structural core of ribosomes.

2. Protein Import from Cytoplasm

• Ribosomal proteins are synthesized in the cytoplasm and transported back into the nucleolus through nuclear pores.
• Inside the nucleolus, these proteins combine with rRNA to form small and large ribosomal subunits.

3. Assembly of Ribosomal Subunits

• The eukaryotic ribosome is made of two subunits:
  • Small subunit (40S).
  • Large subunit (60S).
• These subunits are assembled separately in the nucleolus.

4. Export to Cytoplasm

• Once formed, the ribosomal subunits exit the nucleus via nuclear pores and enter the cytoplasm, where they remain free-floating or attach to the rough ER when protein synthesis is active.

Ribosomes in Prokaryotic Cells

In prokaryotes, ribosomes are synthesized directly in the cytoplasm since they lack a defined nucleus or nucleolus. Ribosome formation in prokaryotes involves the direct transcription and translation of rRNA and ribosomal proteins in the cytoplasm.

Importance of the Nucleolus

The nucleolus is:

• Dense and spherical, often visible under light microscopes.
• Critical for ribosome biogenesis, affecting protein production capacity.
• Larger and more prominent in actively dividing cells or secretory cells due to their high demand for protein synthesis.

Ribosomes are made in the nucleolus, where rRNA and ribosomal proteins are combined to form ribosomal subunits. These subunits are then transported to the cytoplasm where they become functional ribosomes, the cell’s protein synthesis machinery. This sophisticated assembly line is essential for life, powering all cellular activities through protein production.