In which cellular structure does the assembly of ribosomal RNA occur?

The assembly of ribosomal RNA occurs in the nucleolus, which is a specialized structure located within the nucleus of the cell. The nucleolus is primarily responsible for synthesizing and processing ribosomal RNA (rRNA) and assembling it with proteins to form ribosomal subunits. These subunits are then transported out of the nucleolus and into the cytoplasm, where they combine to form functional ribosomes that play a critical role in protein synthesis.

Although the nucleus contains the genomic DNA and is essential for various cellular processes, it is specifically the nucleolus that specializes in the production of ribosomal components. The cytoplasm is where ribosomes function after they are assembled, and the Golgi apparatus is primarily involved in modifying, sorting, and packaging proteins for secretion or delivery to other organelles. Thus, the nucleolus is unique in its direct involvement in rRNA synthesis and ribosome assembly, making it the correct answer to the question.