What is the primary function of the nucleolus?

The primary function of the nucleolus is the synthesis of ribosomal RNA (rRNA), which is essential for the formation of ribosomes. The nucleolus is a dense, membrane-less structure found within the nucleus of eukaryotic cells, and it plays a critical role in ribosome biogenesis. Ribosomes are the cellular machinery responsible for protein synthesis, translating messenger RNA (mRNA) into proteins.

Within the nucleolus, rRNA is transcribed from DNA and then combined with ribosomal proteins, which are imported from the cytoplasm to form the subunits of ribosomes. These subunits are subsequently transported out of the nucleolus and join in the cytoplasm to enable protein synthesis. This highlights the crucial biochemical function of the nucleolus in the broader context of cellular function and gene expression.

Other functions such as lipid transport, protein modification for secretion, or digestion of macromolecules are carried out by different organelles and structures within the cell, such as the endoplasmic reticulum, Golgi apparatus, and lysosomes, respectively.