Final answer:
The nucleolus in eukaryotic cells is responsible for the assembly of ribosomal subunits using ribosomal proteins from the cytosol and ribosomal RNA from the nucleus.
Step-by-step explanation:
In eukaryotic cells, the nucleolus is the site of ribosomal subunit assembly, using ribosomal proteins from the cytosol and ribosomal RNA from the nucleus.