mRNA Processing & Translation

• No mRNA processing in prokaryotes

  • in Prokaryotes transcription and translation occur simultaneously
  • Translation begins before mRNA transcription is even completed
• In eukaryotic cells, mRNAs must first be processed and transported to the cytoplasm before they are translated.
• RNA Processing includes 3 main steps:

  1. A nucleotide “cap” is covalently attached to the 5’ end
  2. A poly(A) “tail” is added to the 3’ end
  3. Introns are spliced out
• Once the 3 steps of RNA processing are complete, RNA becomes a mature messenger RNA (mRNA)
• '5 Cap

  • gives mRNA stability
  • provides a recognition site for ribosomes to bind
• 3' PolyA tail

  • gives mRNA stability
  • provides a recognition site for “exporter” proteins to bind, to export mRNA from nucleus to cytoplasm
• A gene includes coding regions called exons and non-coding regions called introns
• The whole gene will be transcribed during transcription

  • Thus, the resulting pre-processed (“primary”) RNA transcript contains both exonic and intronic regions as well
  • During splicing, introns are removed from RNA transcript and exons are joined together
• Introns are spliced out of RNA transcript by a protein called the spliceosome
• • Transcripts can be spliced in different ways (to include different combinations of exons)
  • This allows a single gene to be edited in various ways to make lots of different proteins
• Our mature mRNA carries the information (“blueprint”) for a protein in its nucleotide sequence (“genetic code”)
• The same nucleotide amino acid translation is used by most prokaryotes, eukaryotes and viruses
• • The genetic code is read 3 nucleotides at a time.
  • Each triplet of nucleotides (called a “codon”) can be translated into a specific amino acid
• This includes a universal start codon AUG (which translates to methionine, or Met), signals the start of mRNA’s coding region
• There are also 3 codons that don’t encode any amino acid, and therefore end or STOP the protein’s synthesis (“stop codons”)
• General Rules of Translation

  1. Read the mRNA 5’ to 3’
  2. Start at the start codon (AUG)- this assures the correct “reading frame"
  3. Once you reach a stop codon (UAA, UAG, or UGA), no more amino acids are added and the polypeptide ends