Which organelle is most directly involved in the assembly of ribosomes?

The nucleolus is the organelle most directly involved in the assembly of ribosomes. It is a dense region within the nucleus where ribosomal RNA (rRNA) is synthesized and combined with proteins to form the basic structure of ribosomes. The nucleolus serves as the site of ribosome biogenesis, where the rRNA is transcribed and processed, and then joined with ribosomal proteins imported from the cytoplasm. Once assembled, the ribosomal subunits are then transported out of the nucleolus into the cytoplasm, where they combine to form functional ribosomes that play critical roles in protein synthesis.

In contrast, the nucleus contains the genetic material and governs cellular activities, but it is the nucleolus specifically that focuses on ribosome production. The cytoplasm is the site of ribosomal function and protein synthesis but does not have a role in ribosome assembly. Lastly, the endoplasmic reticulum is involved in protein synthesis and processing but is not the primary location for ribosome assembly. Thus, the nucleolus is uniquely suited for this specific function within the cell.