Which part of the cell is known as the site of ribosome production?

The correct answer is Nucleolus. The nucleolus is a dense, specialized region within the nucleus of eukaryotic cells where ribosomal RNA (rRNA) is synthesized and ribosome subunits are assembled — effectively making it the "factory" of ribosomes.

Ribosome Biogenesis in the Nucleolus

• Function of the Nucleolus: It transcribes ribosomal DNA (rDNA) into rRNA, processes the rRNA, and combines it with ribosomal proteins imported from the cytoplasm. The assembled small and large ribosomal subunits are then exported through nuclear pores into the cytoplasm for final maturation.
• Why Not the Nucleus?: While the nucleolus is located inside the nucleus, the general nucleus as a whole is not the specific site of ribosome production. The ribosome assembly line is localized within the nucleolus.
• Cytoplasm and Endoplasmic Reticulum: Cytoplasm is where ribosomes function during protein synthesis, but not where they are manufactured. The Endoplasmic Reticulum (especially rough ER) is studded with ribosomes, but again, it does not build them.
• Significance: Actively growing cells, such as those in embryos or cancer cells, often have prominent and enlarged nucleoli due to high demand for protein synthesis and thus ribosome production.

Hence, the factory of ribosomes within the cell is the nucleolus.