What is produced in the nucleolus of a cell?

The nucleolus is a specialized structure within the nucleus of a cell that is primarily involved in the production of ribosomal RNA (rRNA). This is a crucial function because rRNA is a fundamental component of ribosomes, which are the cellular machinery responsible for protein synthesis.

During the process of ribosome biogenesis, the nucleolus synthesizes and assembles rRNA with proteins to form the two subunits of ribosomes. Once these subunits are assembled, they are transported out of the nucleolus into the cytoplasm, where they combine to form functional ribosomes that translate messenger RNA (mRNA) into proteins.

The other options—DNA, proteins, and enzymes—are not produced in the nucleolus. DNA replication occurs in the nucleus but not specifically within the nucleolus itself. Proteins, while involved in the formation of ribosomes, are synthesized in the cytoplasm and not produced in the nucleolus, and enzymes, which are types of proteins, are also synthesized in the cytoplasm. Therefore, the production of ribosomal RNA in the nucleolus is a distinct and vital cellular function that underscores the importance of this organelle in the cell's overall operation.