Translation: making the polypeptide
In translation, ribosomes read mRNA codons and join the amino acids brought by charged tRNAs. It runs from the start codon AUG to a stop codon.
What translation needs
Translation joins amino acids into a polypeptide in the order set by the mRNA codons. The amino acids are linked by peptide bonds. First, each amino acid is activated using ATP and attached to its own tRNA: this is charging or aminoacylation of tRNA. When two charged tRNAs are held close together, forming a peptide bond becomes energetically favourable. The ribosome is the cell's protein factory. It has a small and a large subunit; the large subunit has two sites where neighbouring tRNAs bind. The ribosome is also the catalyst: in bacteria, 23S rRNA acts as a ribozyme. The part of mRNA that is translated, from the start codon to the stop codon, is the translational unit. Extra sequences before AUG and after the stop codon are untranslated regions (UTRs).
The steps of translation
- 1Charging: each amino acid is activated by ATP and linked to its specific tRNA (aminoacyl-tRNA).
- 2Initiation: the small subunit meets the mRNA. The ribosome binds at the start codon AUG, which is recognised only by the initiator tRNA carrying methionine.
- 3The large subunit joins, giving two side-by-side sites for charged tRNAs.
- 4★Elongation: the next charged tRNA pairs its anticodon with the next codon. A peptide bond forms between the two amino acids.
- 5The ribosome moves one codon along the mRNA. Amino acids are added one by one in the order of the codons.
- 6Termination: a release factor binds the stop codon. The finished polypeptide is released, and the ribosome subunits separate.
★ key step
Translation at a glance
Reading an mRNA into a polypeptide
An mRNA reads 5′-GCAUGGCUUCUAAAUGAGC-3′. (a) Find the translational unit. (b) How many amino acids does the polypeptide have, and which are they? (c) What are the UTRs?
- Scan from the 5′ end for the first AUG: GC, then AUG.
- Read triplets from AUG: AUG GCU UCU AAA UGA.
- AUG = methionine, GCU = alanine, UCU = serine, AAA = lysine, UGA = stop.
- Translational unit: AUGGCUUCUAAAUGA.
- The bases before AUG (GC) and after UGA (GC) are not translated.
(a) AUGGCUUCUAAAUGA. (b) 4 amino acids: Met-Ala-Ser-Lys. (c) 5′ UTR: GC; 3′ UTR: GC.
How to draw translation on a ribosome
- Draw a horizontal mRNA strand; label its 5′ and 3′ ends.
- Draw the small subunit as an oval below the mRNA and the large subunit as a bigger oval above it.
- Inside the large subunit, draw two tRNAs side by side, each with its anticodon paired to a codon.
- Show a growing polypeptide chain of beads attached to one tRNA, and label a peptide bond.
- Draw an arrow along the mRNA for the direction of ribosome movement (5′ to 3′).
Antibiotics such as streptomycin and tetracycline stop bacterial ribosomes from translating, but leave our ribosomes largely alone. That is how they stop germs from growing without stopping our own protein making.
2 marks: “What is meant by charging of tRNA? Why is it needed?” Activation of an amino acid using ATP and its attachment to its specific tRNA (aminoacylation). It brings each amino acid to the ribosome with the right anticodon, and makes peptide bond formation energetically favourable.
2 marks: “What are UTRs? Where are they found?” Untranslated regions of mRNA: before the start codon at the 5′ end and after the stop codon at the 3′ end. They are needed for efficient translation.
5 marks: “Describe the process of translation in bacteria.” Charging of tRNA with ATP; ribosome subunits; initiation at AUG with initiator tRNA; elongation with anticodon-codon pairing, peptide bonds catalysed by 23S rRNA, ribosome moving codon by codon; termination by release factor at stop codon; UTRs.
C-I-E-T: Charge, Initiate, Elongate, Terminate.