7.4 Translation

Question 1

Explain how a tRNA molecule is joined to an amino acid

Answer 1

• Each different tRNA molecule has a unique shape and chemical composition that is recognised by a specific tRNA-activating enzyme
  
  • The enzyme first binds the amino acid to a molecule of ATP, forming an amino acid-AMP complex
  • The amino acid is then transferred to the 3’ end of the appropriate tRNA, attaching to a terminal CCA sequence (attachment site) 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

Question 2

Outline the structure of ribosomes

Answer 2

• Ribosomes are made of protein (for stability) and ribosomal RNA (rRNA) for catalytic activity
• They consist of two subunits:
– The small subunit (30S in eukaryotes) contains an mRNA binding site
– The large subunit (50S in eukaryotes) 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)

Question 3

Outline the 4 steps of translation

Answer 3

• 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

Question 4

State which direction translation occurs and why

Answer 4

• 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

Question 5

Explain the initiation phase of translation

Answer 5

• A tRNA molecule with the anticodon for the start codon AUG (UAC) binds to a small ribosome subunit
  
  • The small ribosomal subunit binds to the 5’ end of mRNA and moves along it until it reaches the start codon (AUG)
  • The 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

Question 6

Explain the elongation phase of translation

Answer 6

• 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

Question 7

Explain the translocation phase of translation

Answer 7

• The ribosome moves along one codon position, the deactivated 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)

Question 8

Define polysome

Answer 8

Polysomes are a group of ribosomes moving along the same mRNA molecule at the same time

Question 9

Explain the termination phase of translation

Answer 9

• 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 large ribosome unit detaches, then the polypeptide, and finally the mRNA and small ribosome subunit

Question 10

Distinguish between free and bound ribosomes

Answer 10

• Free ribosomes synthesise proteins for use primarily within the cell
  
  • Bound ribosomes synthesise proteins primarily for secretion or for lysosomes