nucleolus

The nucleolus is a distinct, membrane-free region within the nucleus of eukaryotic cells where ribosome biogenesis begins. It forms around nucleolar organizer regions that harbor tandem repeats of ribosomal RNA genes on certain chromosomes; its size reflects transcriptional activity, and it disassembles during mitosis and reassembles in daughter cells during early interphase.

Nucleolus has three major structural components: fibrillar centers, the dense fibrillar component, and the granular component.

Its primary functions include transcription of ribosomal RNA by RNA polymerase I, processing of pre-rRNA, and assembly with ribosomal proteins to form preribosomal particles that mature into ribosomal subunits exported to the cytoplasm.

Beyond ribosome production, the nucleolus participates in cellular stress responses and cell-cycle regulation by sequestering regulatory proteins; disruption of nucleolar function can influence p53 pathways.

In brief, the nucleolus is the nuclear hub of ribosome biogenesis, organizing rRNA transcription, RNA processing, and early ribosome assembly within a distinct, dynamic substructure.