The organelle that makes ribosomes is the nucleolus. The nucleolus is a dense structure found within the nucleus of eukaryotic cells where ribosomal RNA (rRNA) genes are transcribed and ribosomal subunits are assembled. After assembly in the nucleolus, ribosomal subunits are transported to the cytoplasm to participate in protein synthesis. In prokaryotic cells, which lack a defined nucleus and nucleolus, ribosome assembly occurs in the cytoplasm near the rRNA transcription sites. Thus, the nucleolus is the primary site of ribosome production in eukaryotic cells.