Medicine & USMLE

Ribosomal RNA (rRNA)

  1. Messenger RNA (mRNA)
  2. Ribosomal RNA (rRNA)
  3. Transfer RNA (tRNA)
  4. Small Nuclear RNA (snRNA)
  5. MicroRNA (miRNA)
  6. Small Interfering RNA (siRNA)


Ribosomal RNA, or rRNA, is a type of non-coding RNA which functions as the main building block of ribosomes. rRNA is synthesized mainly by RNA polymerase I in the nucleolus of eukaryotes, and by RNA polymerase in the cytoplasm of prokaryotes.

Key Points

  • Ribosomal RNA (rRNA)
    • Characteristics
      • Class of non-coding RNA 
        • Does not code for proteins
        • Also known as functional RNA
      • Found in both prokaryotes and eukaryotes
    • Function
      • Forms ribosomes
        • Combines with proteins to form molecular machinery used to assemble proteins from mRNA in the cytoplasm 
    • Synthesis
      • Eukaryotes
        • Transcribed mostly by RNA Polymerase I
          • Occurs in the nucleolus
          • Partially by RNA Polymerase III in the nucleus
      • Prokaryotes
        • Transcribed by RNA Polymerase in the cytoplasm