What is the nucleolus?

• A. The site within the nucleus where ribosomal RNA is rewritten and proteins are assembled.
• B. The energy-producing organelle.
• C. A small body within the nucleus that contains RNA and is composed of protein.
• D. The cell's cytoskeleton.

Answer: (C)

The nucleolus is a small, dense region inside the nucleus made of RNA and protein where ribosome components are assembled. Its main job is ribosome biogenesis: ribosomal RNA genes are transcribed there, and ribosomal proteins from the cytoplasm join with the rRNA to form ribosomal subunits. That makes it a ribonucleoprotein body within the nucleus. It isn’t an energy-producing organelle like mitochondria, nor part of the cytoskeleton, and the idea of RNA being “rewritten” isn’t the precise way transcription works, though the essential point is that this region is RNA-rich and protein-containing for building ribosomes.