7.4 Translation

7.4.1  Explain that each tRNA molecule is recognised by a tRNA-activating enzyme that binds a specific amino acid to the tRNA using ATP for energy

  • Each different tRNA molecule has a unique shape and chemical composition that is recognised by a specific tRNA-activating enzyme
  • The enzyme (aminoacyl-tRNA synthetase) first binds the amino acid to a molecule of ATP (to form an  amino acid-AMP complex linked by a high energy bond)
  • The amino acid is then transferred to the 3'-end of the appropriate tRNA, attaching to a terminal CCA sequence on the acceptor stem and releasing the AMP molecule
  • The tRNA molecule with an amino acid attached is thus said to be 'charged' and is now capable of participating in translation
  • The energy in the bond linking the tRNA molecule to the amino acid will be used in translation to form a peptide bond between adjacent amino acids

Structure of tRNA

7.4.2  Outline the structure of ribosomes, including protein and RNA composition, large and small subunits, three tRNA binding sites and mRNA binding sites

  • 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)

7.4.3  State that translation consists of initiation, elongation, translocation and termination

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

7.4.4  State that translation occurs in a 5' - 3' direction

  • The start codon (AUG) is located at the 5' end of the mRNA sequence and the ribosome moves along it in the 3' direction
  • Hence translation occurs in a 5' - 3' direction

7.4.5  Draw and label a diagram showing the structure of a peptide bond between two amino acids

7.4.6  Explain the process of translation, including ribosomes, polysomes, start codons and stop codons


  • Specific tRNA-activating enzymes catalyse the attachment of amino acids to tRNA molecules, using ATP for energy


  • The small ribosomal subunit binds to the 5' end of mRNA and moves along it until it reaches the start codon (AUG)
  • Next, the appropriate tRNA molecule binds to the codon via its anticodon (according to complementary base pairing)
  • Finally, the large ribosomal subunit aligns itself to the tRNA molecule at its P-site and forms a complex with the small ribosomal subunit


  • 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


  • The ribosome moves along one codon position, the deacylated tRNA moves into the E-site and is released, while the tRNA bearing the dipeptide moves into the P-site
  • Another tRNA molecules attaches to the next codon in the newly emptied A-site and the process is repeated
  • The ribosome moves along the mRNA sequence in a 5' - 3' direction, synthesising a polypeptide chain
  • Multiple ribosomes can translate a single mRNA sequence simultaneously (forming polysomes)


  • 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

Overview of the Process of Translation

7.4.7  State that free ribosomes synthesise proteins for use primarily within the cell, and that bound ribosomes synthesise proteins primarily for secretion or for lysosomes

  • Ribosomes floating freely in the cytosol produce proteins for use within the cell
  • Ribosomes attached to the rough ER are primarily involved in producing proteins to be exported from the cell or used in the lysosome
  • These proteins contain a signal recognition peptide on their nascent polypeptide chains which direct the associated ribosome to the rough ER