In eukaryotic cells, what is the primary function of the nucleolus?

The primary function of the nucleolus is the assembly of ribosome subunits. The nucleolus is a dense region within the nucleus where ribosomal RNA (rRNA) is synthesized and combined with ribosomal proteins to form the subunits of ribosomes. These subunits are then transported out of the nucleus into the cytoplasm, where they combine to create functional ribosomes essential for protein synthesis in the cell.

In contrast, the storage of genetic material pertains to the broader function of the nucleus, which contains DNA. Gene expression regulation is also a function primarily associated with the nucleus, particularly involving various factors and RNA polymerase during transcription. The production of cellular energy is mainly carried out by mitochondria, not the nucleolus, as they are responsible for cellular respiration and ATP generation. Thus, the nucleolus’s specific role in ribosome assembly distinguishes it from these other functions.