kjarnakorn

kjarnakorn, known in English as the nucleolus, is a prominent, membrane-less region within the cell nucleus of most eukaryotes. It forms around ribosomal DNA repeats located in nucleolar organizing regions on certain chromosomes and functions as the primary site of ribosome biogenesis. The nucleolus is dynamic, changing in size and composition in response to cellular growth, metabolism, and stress.

Anatomically, the nucleolus comprises three main subcompartments: the fibrillar center, the dense fibrillar component, and the granular component. The fibrillar center is associated with rDNA loci and hosts the initiation of transcription by RNA polymerase I. The dense fibrillar component contains early processing of precursor ribosomal RNA, while the granular component is involved in the later stages of ribosome assembly, incorporating ribosomal proteins and small nucleolar RNAs. This substructure underpins the coordinated transcription of rRNA and the maturation of ribosomal subunits.

Functionally, the nucleolus concentrates the machinery for producing ribosomes, starting with rRNA transcription, through processing, to assembly of ribosomal subunits that will later exit the nucleus to form ribosomes in the cytoplasm. In addition to ribosome production, the nucleolus participates in other cellular processes such as regulation of the cell cycle, stress responses, and maintenance of genome stability. Nucleolar size and activity often reflect cellular growth and proliferative state, and changes in the nucleolus are commonly observed in diseases, particularly cancer, where enlarged nucleoli are frequently associated with high ribosome biogenesis. During mitosis, the nucleolus disassembles and reassembles in daughter cells, a dynamic that reflects broader changes in chromatin organization during cell division.