The primary role of the nucleolus in a cell is to produce ribosomal RNA (rRNA) and assemble ribosome subunits. Located within the nucleus, the nucleolus is a dense region where rRNA is synthesized and combined with proteins to form the subunits that are essential for protein synthesis. These ribosomal subunits are then transported to the cytoplasm, where they combine to form functional ribosomes, critical for translating mRNA into proteins.