American Society for Clinical Pathology (ASCP) Histotechnologist Practice Test

Which structure in the nucleus is responsible for producing most ribosomal RNA?

1. Chromatin

2. Nucleolus

3. Nuclear envelope

4. Nuclear matrix

The correct answer is: Nucleolus

The nucleolus is the structure in the nucleus that is primarily responsible for the synthesis of ribosomal RNA (rRNA). It is a dense, membrane-less organelle where the transcription of rRNA genes occurs and where the initial stages of ribosome assembly take place. The nucleolus assembles rRNA molecules along with ribosomal proteins imported from the cytoplasm to form the ribosomal subunits, which are then transported out to the cytoplasm for protein synthesis. Chromatin consists of DNA and proteins that make up the chromosomes; while it plays a crucial role in gene expression and the regulation of cellular activities, it is not specifically focused on producing ribosomal RNA. The nuclear envelope serves as a barrier that encloses the nucleus and regulates the transport of materials into and out of the nucleus, but it does not directly contribute to rRNA synthesis. The nuclear matrix provides structural support and organization within the nucleus but does not participate in the production of ribosomal RNA. This makes the nucleolus the correct answer, as it is specifically designed for the production and initial processing of rRNA, crucial for ribosome formation and, consequently, protein synthesis.