nucleolus is a machine for producing ribosomal precursors in the nucleus. The transcription of ribosomal RNA(rRNA) in all eukaryotes is completed in the nucleolus, and the process is that rDNA is transcribed into rRNA, which is then combined with protein from cytoplasm, processed and transformed into ribosomal precursors, and then exported to cytoplasm. Nucleolar components include rRNA, rDNA and ribonucleoprotein. Nucleolar is the place where rRNA gene is stored, rRNA is synthesized and processed, and ribosome subunits are assembled.