Here's why:
* Ribosomal RNA (rRNA) synthesis: rRNA, a major component of ribosomes, is synthesized in the nucleolus, a specialized region within the nucleus.
* Ribosomal protein synthesis: The proteins that make up ribosomes are synthesized in the cytoplasm by ribosomes themselves.
* Assembly of ribosomal subunits: The rRNA and ribosomal proteins then move to the cytoplasm where they assemble into the two ribosomal subunits (large and small) in a process guided by specific proteins.
Therefore, while the initial steps of ribosomal subunit biosynthesis occur within the nucleus (rRNA synthesis), the final assembly and maturation of the subunits happen in the cytoplasm.
