The nucleolus is the site of ribosomal RNA (rRNA) synthesis and ribosome assembly. It is organised around the nucleolus organiser regions (NORs) — chromosomal loci containing tandem repeats of rRNA genes transcribed by RNA polymerase I. The nucleolus contains: fibrillar centres (rRNA gene location), dense fibrillar component (active rRNA transcription), and granular component (ribosome subunit assembly). Ribosome subunits are exported to the cytoplasm for final assembly.