Nukleolus

The nukleolus, also spelled nucleolus in many languages, is a prominent, membrane-less nuclear substructure found within eukaryotic cell nuclei. It forms around ribosomal DNA repeats known as nucleolar organizer regions and is the site of ribosomal RNA gene transcription and early ribosome assembly.

The nucleolus comprises three main subcomponents: the fibrillar centers, the dense fibrillar component, and the granular component. The fibrillar centers contain rDNA and transcription machinery; the dense fibrillar component is where transcription and early processing of pre-rRNA occur; the granular component contains assembling ribosomal subunits.

Nucleolar activity is cyclical with the cell cycle and it disassembles during mitosis and reassembles in daughter cells during telophase. In humans, the nucleolus forms around multiple active rDNA repeats on the five acrocentric chromosomes.

Because nucleolus reflects cellular growth, its size and number correlate with transcriptional activity and proliferation. Enlarged nucleoli are often observed in cancer cells and other conditions with high protein synthesis. Nucleolar stress from disrupted ribosome biogenesis can activate p53-dependent pathways, linking nucleolar function to cell cycle control.