Translation


Translation is the process of protein synthesis in which the genetic information encoded in mRNA is translated into a sequence of amino acids in a polypeptide chain, and occurs at one of two locations:

  • Free ribosomes in the cytosol for intracellular proteins 
  • ER-bound ribosomes (i.e. rough ER) for extracellular proteins


The Genetic Code

The genetic code is the set of rules by which information encoded in mRNA sequences is converted into proteins (amino acid sequences) by living cells

  • Codons are a triplet of bases which encodes a particular amino acid
  • As there are four bases, there are 64 different codon combinations (4 x 4 x 4 = 64)
  • The order of the codons determines the amino acid sequence for a protein 
  • The coding region always starts with a START codon (AUG) and terminates with a STOP codon


                                                                                              The Genetic Code


The genetic code has the following features:

  • It is universal - every living thing uses the same code (there are only a few rare and minor exceptions)
  • It is degenerate - there are only 20 amino acids but 64 codons, so more than one codon may code for the same amino acid (this allows for silent mutations whereby a change in the DNA sequence does not affect the polypeptide sequence)


Structure of the Ribosome

  • Ribosomes are made of protein (for stability) and ribosomal RNA (rRNA - for catalytic activity)
  • They consist of two subunits:
    • The small subunit contains an mRNA binding site
    • The large subunit contains three tRNA binding sites - an aminacyl (A) site, a peptidyl (P) site and an exit (E) site
  • Ribosomes can be either found freely in the cytosol or bound to the rough ER (in eukaryotes)
  • Ribosomes differ in size in eukaryotes and prokaryotes (eukaryotes = 80S ; prokaryotes = 70S)


Process of Translation

Translation occurs in four main steps:

  • Initiation:  Involves the assembly of an active ribosomal complex
  • Elongation:  New amino acids are brought to the ribosome according to the codon sequence
  • Translocation:  Amino acids are translocated to a growing polypeptide chain
  • Termination:  At certain "stop" codons, translation is ended and the polypeptide is released


The Four Stages of Translation


Initiation:

  • The small ribosomal subunit binds to the mRNA and moves along it in a 5' - 3' direction until it reaches a start codon (AUG)
  • The tRNA molecule with the appropriate anticodon will align opposite the start codon according to complementary base pairing 
  • Each tRNA molecule carries a specific amino acid according to the genetic code (e.g. AUG = Met)
  • The large ribosomal subunit aligns itself to the tRNA molecule at its P-site and forms a complex with the small ribosomal subunit


Elongation:

  • A second tRNA molecule pairs with the next codon in the ribosomal A-site
  • The amino acid in the P-site is covalently attached via a peptide bond to the amino acid in the A-site


Translocation:

  • The ribosome moves along one codon position (in a 5' - 3' direction)
  • The tRNA molecules that was in the P site is now in the E site and, having transferred it's amino acid, is released
  • The tRNA that was in the A site is now in the P site and has two amino acids ready to be transferred
  • A new tRNA molecule binds to the now empty A site and the elongation process is repeated


Termination: 

  • Elongation and translocation continue until the ribosome reaches a stop codon
  • These codons do not code for any amino acids and instead signal for translation to stop
  • The polypeptide is released and the ribosome disassembles back into subunits
  • The polypeptide may undergo post-translational modification prior to becoming a functional protein
  • Multiple ribosomes can translate a single mRNA sequence simultaneously (forming polysomes)


Simplified Overview of the Process of Translation