What is the significance of the nucleolar organizer region (NOR)?

• A. NORs are sites of DNA replication origins.
• B. NORs regulate transcription of mRNA in the cytoplasm.
• C. NORs assemble ribosomes directly in the cytoplasm.
• D. NORs contain ribosomal gene clusters that form the nucleolus and drive rRNA transcription by RNA polymerase I.

Answer: (D)

NORs are regions on the chromosomes that house clustered ribosomal RNA genes. These ribosomal gene repeats organize to form the nucleolus, the nucleus’s dedicated hub for making ribosomal RNA. The key activity here is transcription of rRNA by RNA polymerase I, producing the precursor rRNA that is then processed and combined with ribosomal proteins to build ribosomal subunits. This ribosome biogenesis happens inside the nucleolus and is essential for supplying the components needed for protein synthesis in the cytoplasm. So, NORs’ significance lies in containing the rRNA gene clusters that drive nucleolus formation and rRNA transcription by RNA polymerase I. They aren’t DNA replication origins, they don’t regulate mRNA transcription in the cytoplasm, and they don’t directly assemble ribosomes in the cytoplasm.