Nucleoli are small, dense structures found within the nucleus of eukaryotic cells. They are primarily involved in the production and assembly of ribosomes, which are essential for protein synthesis. Nucleoli are made up of RNA, DNA, and proteins, and their size and number can vary depending on the cell's activity and type. During the cell cycle, nucleoli can change in size and prominence. They are most visible during interphase, when the cell is not dividing, and may disappear during cell division. The presence of nucleoli indicates active ribosome production, which is crucial for cellular function and growth.