Final answer:
The nucleolus assembles ribosomal RNA and proteins into ribosomes, which later function in protein synthesis in the cytoplasm.
Step-by-step explanation:
The function of the nucleolus is to assemble ribosomal RNA (rRNA) and proteins into ribosomes. Within the nucleus of a cell, some chromosomes have specific sections of DNA that encode rRNA. The nucleolus is identifiable as a darkly staining area within the nucleus and it functions as the site where rRNA is synthesized and combined with proteins to form preliminary ribosomal subunits. These subunits are then transported out of the nucleus through the pores in the nuclear envelope and into the cytoplasm where they are finalized into functional ribosomes that are essential for protein synthesis.