Ribosomal RNA is synthesized and attached to proteins in the _____, which produces eukaryotic ribosomal subunits.

• A. Nucleolus
• B. Cytoplasm
• C. Nucleus
• D. Mitochondrion

Answer: (A)

Ribosomal RNA synthesis and the initial assembly of ribosomal subunits take place in the nucleolus, a specialized region inside the nucleus dedicated to ribosome production. In this area, rRNA genes are transcribed by RNA polymerase I to produce a large precursor rRNA, which is processed into the mature rRNAs. Ribosomal proteins, made in the cytoplasm, are imported into the nucleolus where they combine with the rRNA to form the small and large ribosomal subunit precursors. These subunits are then exported to the cytoplasm to complete assembly and participate in protein synthesis. The other compartments aren’t where this initial production and assembly happens: transcription and processing of rRNA are localized to the nucleolus, whereas translation occurs in the cytoplasm, and mitochondria have their own distinct ribosomes.