nucleolar

The nucleolus is a prominent, membrane-free structure within the nucleus of eukaryotic cells. It forms around clusters of ribosomal RNA (rRNA) genes, known as nucleolar organizer regions, which reside in the rDNA repeats on several chromosomes. The nucleolus is the primary site of ribosome biogenesis, coordinating transcription of rRNA, processing of precursor transcripts, and assembly of ribosomal subunits.

Anatomically, the nucleolus comprises three major subcompartments: the fibrillar center (FC), the dense fibrillar component (DFC), and the granular component (GC). The FC contains rDNA genes and transcription factors; the DFC is the site of early rRNA transcription and processing; the GC hosts late processing steps and assembly of ribosomal proteins into preribosomal particles.

Transcription of rRNA genes is carried out by RNA polymerase I, generating a 47S (in humans) or 45S pre-rRNA transcript that is processed into the mature 18S, 5.8S, and 28S rRNAs. These rRNAs combine with ribosomal proteins imported from the cytoplasm to form the small (40S) and large (60S) ribosomal subunits, which then exit to the cytoplasm for assembly into functional ribosomes.

The nucleolus is dynamic, dissolving during mitosis and reassembling in daughter nuclei. Its size and activity reflect cellular growth conditions and metabolic state. Increased nucleolar activity is often associated with rapid cell proliferation and cancer, where nucleolar hypertrophy is a common feature. Conversely, perturbations of ribosome biogenesis can trigger nucleolar stress, stabilizing p53 and inducing cell cycle arrest or apoptosis. Beyond ribosome production, the nucleolus participates in the assembly of other ribonucleoproteins and in regulation of cellular stress responses.