nucleoluss

The nucleolus is a prominent, membrane-less substructure within the nucleus of eukaryotic cells. It forms around nucleolar organizer regions that contain tandem repeats of ribosomal RNA (rRNA) genes. The term nucleoluss appears to be a misspelling; the standard term is nucleolus, with the plural nucleoli.

Structurally, the nucleolus comprises three main regions: the fibrillar center (FC), the dense fibrillar component (DFC), and the granular component (GC). Ribosomal RNA transcription by RNA polymerase I occurs in the FC/DFC region, where rDNA is actively expressed and early processing begins. The GC hosts late assembly steps where ribosomal proteins join the rRNA to form ribosomal subunits.

Functionally, the nucleolus is the site of ribosome biogenesis. It coordinates transcription of rRNA, processing of precursor transcripts, and assembly of ribosomal proteins into pre-ribosomal particles. These particles mature into the small (40S) and large (60S) ribosomal subunits, which are then exported to the cytoplasm for final assembly into functional ribosomes.

Beyond ribosome production, the nucleolus participates in other cellular processes, including regulation of the cell cycle and responses to stress. It can influence p53 signaling through the release of certain ribosomal proteins that interact with MDM2, stabilizing p53 under stress conditions. Nucleolar activity generally reflects cellular growth and proliferation rates and can change in disease states.

In humans, nucleolar organizer regions reside on several chromosomes, notably the five acrocentric chromosomes 13, 14, 15, 21, and 22. The nucleolus can be observed as a distinct nuclear body under light microscopy and is dynamically reorganized during the cell cycle, disassembling in mitosis and reassembling in daughter cells.