Nukleolen

The nucleolus (German: Nukleolen) is a prominent, non-membrane-bound structure within the nucleus of eukaryotic cells. It forms around ribosomal DNA (rDNA) repeats and is the site of ribosomal RNA synthesis and early ribosome assembly.

The nucleolus consists of three main substructures: fibrillar centers (FC) containing active rDNA; the dense fibrillar component (DFC) where transcription and early rRNA processing occur; and the granular component (GC) where later ribosomal subunit assembly takes place. These components organize around nucleolar organizer regions on certain chromosomes that carry rDNA repeats.

Functionally, the nucleolus is the site where RNA polymerase I transcribes rRNA genes in the FC/DFC region; pre-rRNA is processed in the DFC; ribosomal proteins imported from the cytoplasm assemble with rRNA to form preribosomal particles in the GC, which mature and are exported as ribosomal subunits to the cytoplasm for final assembly.

Dynamics and formation are characterized by its non-membrane-bound nature and its responsiveness to cellular state. Its size and activity reflect cellular growth and proliferation, and it disassembles during mitosis, then reassembles in daughter nuclei during telophase. Stress, nutrient availability, and DNA damage can alter nucleolar morphology and function.

Clinical and biological significance includes the correlation between nucleolar size and cellular proliferation, with nucleolar hypertrophy often observed in rapidly dividing cells and many cancers. Disruption of ribosome biogenesis, or nucleolar stress, can activate p53 signaling and affect cell-cycle progression, making the nucleolus a focus in studies of cancer biology and antiviral responses.