Which structure serves as the site of ribosomal RNA synthesis?

The nucleus serves as the site of ribosomal RNA synthesis because it contains a substructure known as the nucleolus, where ribosomal RNA (rRNA) is produced. The nucleolus synthesizes rRNA and assembles it with proteins to form ribosomal subunits. Once these subunits are formed, they are then transported out of the nucleus into the cytoplasm, where they combine with messenger RNA (mRNA) to form functional ribosomes that play a crucial role in protein synthesis.

In contrast, while ribosomes are essential for protein synthesis, they do not synthesize rRNA; rather, they utilize rRNA to facilitate the translation of mRNA into proteins. The Golgi apparatus is primarily involved in modifying, sorting, and packaging proteins and lipids for secretion or delivery to other organelles. The endoplasmic reticulum plays a diverse role in protein and lipid synthesis but does not participate in ribosomal RNA synthesis. Therefore, the nucleus is specifically designated for the synthesis of rRNA and is fundamental to the formation of ribosomes.