Ribosomes are produced in the nucleolus inside the nucleus of eukaryotic cells

. Ribosomes consist of RNA and proteins, making them ribonucleoprotein complexes

. The nucleolus acts as a ribosome production factory to generate the large amounts of rRNAs and to assemble ribosomal subunits needed by the cell

. The stages of ribosome production:

• rRNA Transcription The rRNA genes are in the fibrillar centers of the nucleolus, but transcription happens mostly at the border of the fibrillar centers and the dense fibrillar component

• Pre-rRNA Processing Processing of the pre-rRNA starts in the dense fibrillar component and continues in the granular component, where the rRNA and ribosomal proteins assemble to form preribosomal subunits that are ready to be exported to the cytoplasm

• Ribosomal Protein Synthesis Ribosomal protein genes are transcribed outside the nucleolus by RNA polymerase II, which yields mRNAs that are then translated on cytoplasmic ribosomes. These ribosomal proteins are transported from the cytoplasm to the nucleolus to assemble with rRNAs to form preribosomal particles

• 5S rRNA Assembly Although the genes for 5S rRNA are also transcribed outside the nucleolus by RNA polymerase III, 5S rRNAs are assembled into preribosomal particles inside the nucleolus

• Ribosome Maturation After the preribosomal particles are exported to the cytoplasm, the final stages of ribosome maturation occur, creating the active 40S and 60S subunits of eukaryotic ribosomes