The nucleolus is a large organelle within the nucleus of eukaryotic cells. In fact, it is the largest structure within the cell nucleus. The main function of the nucleolus is the assembly of ribosomes (organelle that act as a site for protein synthesis). Other functions of the nucleolus include rRNA (ribosomal RNA) transcription and processing. Since ribosomes have to be generated each time the cell divides and also because a cell contains millions of ribosomes, the nucleolus has a significant role and function in a eukaryotic cell. There are three distinct regions in the nucleolus, each corresponding to progressive stages of nucleolus functions:
- fibrillar center: location of rRNA genes
- Dense fibrillar component: rRNA transcription occurs at the boundary of fibrillar center and dense fibrillar component
- granular component: location of assembly of ribosome; from here it is transported to the cytoplasm.
Hope this helps.