A nucleolus (plural: nucleoli) is the part of a eukaryotic cell where ribosomes are made. Seen under a microscope, the nucleolus is a dark spot inside the cell's nucleus. It is made up of dense RNA and proteins. Prokaryotic cells also have ribosomes, but they don't make as many as eukaryotes do, and there is no visible nucleotatius. There is usually more than one nucleolus in a eukaryote cell. The nucleolus is not bounded by a membrane. 