- As cells complete mitosis and enter the early G1 phase, one of the first and most essential events in nuclear reorganization is the reassembly of the nucleolus. This process is tightly coupled to the reactivation of ribosomal RNA (rRNA) transcription at specific chromosomal sites known as nucleolar organizer regions (NORs).
- These NORs harbor tandemly repeated clusters of ribosomal DNA (rDNA) genes, which encode the precursor rRNAs necessary for ribosome biogenesis.
- The nucleolus forms exclusively around these rDNA loci, making them the structural and functional foundation of nucleolar architecture.
- During mitosis, the nucleolus disassembles as cells enter prophase. This disassembly is a coordinated process triggered by chromatin condensation and the cessation of rRNA transcription.
- The components of the nucleolus, including RNA polymerase I, transcription factors, and processing machinery, become dispersed throughout the cell. This dismantling is necessary to allow equitable distribution of chromosomes and nucleolar components into daughter cells.
- As cells exit mitosis and transition into telophase and early G1 phase, chromatin decondensation permits the reactivation of transcription at NORs. RNA polymerase I resumes synthesis of the 47S precursor rRNA, marking the onset of nucleolar reassembly. The newly transcribed rRNA acts as a scaffold that recruits RNA processing factors, small nucleolar RNAs (snoRNAs), and ribosomal proteins. Together, these elements drive the stepwise reformation of nucleolar substructures: the fibrillar center (associated with rDNA transcription), the dense fibrillar component (involved in early rRNA processing), and the granular component (site of ribosome assembly).
- The formation of functional nucleoli at rDNA clusters is essential for cell growth and proliferation. Ribosome production must quickly resume following mitosis to meet the high demand for protein synthesis in daughter cells. Importantly, not all NORs are transcriptionally active in every cell cycle, and the decision to activate a particular NOR can influence nucleolar number and size.
- In summary, post-mitotic nucleolar reassembly is a highly regulated process that depends on the spatial and functional organization of rDNA gene clusters. The NORs serve not only as genomic landmarks but also as hubs for the regeneration of nucleolar function, ensuring that ribosome production recommences promptly after cell division.
