The nucleolus is found inside the nucleus of a eukaryotic cell. Eukaryotic cells are cells that contain membrane-bound organelles, such as the nucleolus. Prokaryotic cells are cells that do not contain such organelles. Therefore, a nucleolus would not be present in a eukaryotic cell.
When looking under a microscope at a stained cell, the nucleolus appears as a dark, central circle that takes up about twenty-five percent of the volume of the nucleus. The main components of the nucleolus are deoxyribonucleic acid (DNA), ribonucleic acid (RNA), and proteins. The function of the nucleolus is to use DNA to transcribe ribosomal subunits from proteins. These ribosomal subunits are later used to make ribosomal-RNA (rRNA). For this reason, the nucleolus is sometimes referred to as the “ribosome factory of the cell”.