Messenger RNA (mRNA)

Summary

Messenger RNA, or mRNA, is a class of coding RNA responsible for carrying instructions for protein synthesis as codons. Codons are organized into sequences of three bases, each of which specify a particular amino acid. Before translation mRNA must mature through three posttranscriptional modifications including the addition of a 5’ 7-methylguanosine cap and a 3’ poly-A tail as well as splicing out introns and leaving only exons. mRNA is synthesized mainly by RNA polymerase II in the nucleus of eukaryotes and by RNA polymerase in the cytoplasm of prokaryotes.

Key Points

• Messenger RNA (mRNA)
  • Naming
    • Carries a written message from DNA to ribosomes
  • Characteristics
    • Class of coding RNA
      • Only type of RNA that codes for proteins
    • Found in eukaryotes and prokaryotes
  • Function
    • Carries instructions for protein synthesis as codons
      • Information initially copied from DNA
      • Codons organized into sequences of three bases, each of which specify a particular amino acid
      • There are 20 amino acids in eukaryotic proteins, each of which is represented by at least one codon.
    • Read by ribosomes to produce chains of amino acids (proteins)
  • Synthesis
    • Eukaryotes
      • Pre-mRNA transcribed by RNA Polymerase II in the nucleus
      • Matures through three posttranscriptional modifications
        • 7-methylguanosine cap added to the 5′ end
          • Recognized by ribosomes as the binding site for later translation
          • Prevents degradation of mRNA in the cytoplasm
        • Chain of adenine nucleotides (poly-A tail) added to 3’ end
          • Fosters transport of mRNA transcript from nucleus to cytoplasm
          • Prevents degradation of mRNA in the cytoplasm
        • Splicing out introns, leaving only exons
    • Prokaryotes
      • Transcribed by RNA Polymerase in the cytoplasm
      • Note that transcription and translation can occur simultaneously due to the lack of a nuclear membrane