A nucleolus is a dense, membrane-less structure found within the nucleus of eukaryotic cells. It is primarily involved in the production and assembly of ribosomes, which are essential for protein synthesis. The nucleolus synthesizes ribosomal RNA (rRNA) and combines it with proteins to form ribosomal subunits that are then transported to the cytoplasm for protein assembly. Additionally, the nucleolus plays a role in regulating various cellular processes, including stress responses and cell cycle progression.