Nucleolus contains
**Core Concept**
The nucleolus is a region within the nucleus of eukaryotic cells where ribosome synthesis occurs. It contains a complex array of RNA and protein molecules essential for ribosome biogenesis.
**Why the Correct Answer is Right**
The nucleolus is a site of active RNA synthesis, where ribosomal RNA (rRNA) is transcribed from the nucleolar organizer regions of the chromosomes. This rRNA is then processed and assembled with proteins to form ribosomal subunits. The correct answer is related to the primary function of the nucleolus, which is the synthesis of ribosomal components. The nucleolus is the site where the rRNA genes are transcribed, processed, and assembled into ribosomal subunits.
**Why Each Wrong Option is Incorrect**
**Option A:** Mitochondrial DNA is not directly associated with the nucleolus, as it is located in the mitochondria, separate from the nucleus.
**Option B:** The endoplasmic reticulum (ER) is involved in protein synthesis and folding but is not a component of the nucleolus.
**Option C:** The Golgi apparatus is involved in protein modification, sorting, and packaging but is not directly associated with the nucleolus.
**Clinical Pearl / High-Yield Fact**
The nucleolus is essential for cell growth and proliferation, as ribosome synthesis is crucial for protein production. Disruptions in nucleolar function can lead to cellular dysregulation and potentially contribute to various diseases, including cancer.
**Correct Answer:** D. Ribosomal RNA (rRNA)