Where is rRNA produced?

• A. Nucleolus
• B. Nucleoplasm
• C. Cytoplasm
• D. Ribosome

Answer: (A)

rRNA is produced in the nucleolus. This region within the nucleus contains the ribosomal RNA genes arranged in chromosomal clusters (the nucleolar organizer regions) and hosts the transcription of the 45S pre-rRNA by RNA polymerase I. The pre-rRNA is then processed and combined with ribosomal proteins brought in from the cytoplasm to begin assembling the ribosomal subunits. The nucleolus is essentially the ribosome biogenesis hub, so it’s the site where rRNA transcription and early processing occur. The nucleoplasm is the general interior of the nucleus where DNA resides and much transcription happens, but rRNA transcription specifically happens in the nucleolus. The cytoplasm is where translation occurs, and the ribosome is the complex that carries out translation, not the site of rRNA production.