Ribosomal subunits are assembled in which nuclear substructure?

• A. Cytoplasm
• B. Mitochondria
• C. Endoplasmic reticulum
• D. Nucleolus inside the nucleus

Answer: (D)

Ribosomal subunits are assembled in the nucleolus, a distinct region inside the nucleus where ribosomal RNA genes are transcribed, processed, and combined with ribosomal proteins to form the small and large subunits. Once assembled, these subunits are exported to the cytoplasm to participate in protein synthesis. The cytoplasm contains mature ribosomes for translation, mitochondria have their own ribosomes but not the site of nuclear ribosome subunit assembly, and the endoplasmic reticulum is a site for translating certain proteins rather than for ribosome biogenesis.