The nucleolus is a small, dense structure found within the nucleus of eukaryotic cells. It is primarily responsible for the synthesis and assembly of ribosomal RNA (rRNA) and the formation of ribosome subunits. The nucleolus plays a crucial role in protein synthesis by facilitating the production of ribosomes, which are essential for translating messenger RNA (mRNA) into proteins. Its size and prominence can vary depending on the cell's metabolic activity and growth state.