Medicine & USMLE

MicroRNA (miRNA)

  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)


MicroRNA, or miRNA, is a class of non-coding RNA that plays a role in the regulation of gene expression through silencing of complementary messenger RNA strands via translational repression. Translational repression occurs through microRNA binding to target messenger RNA and subsequently preventing it from binding to ribosomes which blocks translation. Translational repression does NOT require perfect complementarity between the microRNA strand and the target messenger RNA strand. miRNA is synthesized mainly by RNA polymerase II in the nucleus of eukaryotes and is not found in prokaryotes.

Key Points

  • microRNA (miRNA)
    • Naming
      • Very small (hence: “micro”) single-stranded RNA molecules
        • Each contain only ~19-25 nucleotides
    • Characteristics
      • Class of non-coding RNA
        • Does not code for proteins
        • Also known as functional RNA
      • Found in eukaryotes
        • Not known to be found in prokaryotes
    • Function
      • Plays a role in transcriptional and posttranscriptional regulation of gene expression
      • Silencing of complementary mRNA
        • Translational repression occurs by preventing the target mRNA from binding to ribosomes (blocking translation)
          • Main action of miRNA
          • Does NOT require perfect complementarity 
            • Binding and inhibition can occur without exact complementarity to the target mRNA
        • Target degradation occurs by promoting endonuclease activation and subsequent cleavage of the target mRNA
          • Rare action of miRNA
          • Only happens when there is a high level of complementarity
    • Synthesis
      • Eukaryotes
        • Transcribed mostly by RNA Polymerase II 
          • Partially by RNA Polymerase III
          • Occurs in the nucleus