Protein biosynthesis (translation)
DNA's message is copied into mRNA, then ribosomes read it three bases at a time, joining amino acids carried by tRNA into a protein.
From gene to protein
A codon is a set of three mRNA bases that stands for one amino acid. Each tRNA has a matching anticodon and carries one amino acid. Step 0 happens in the nucleus; the rest happen on ribosomes in the cytoplasm.
- 0DNA → mRNA (transcription)RNA polymerase · nucleus · mRNA is capped, given a poly-A tail and spliced (introns removed), then leaves the nucleus
- 1★Amino acid + tRNA + ATP → aminoacyl-tRNA + AMP + PPiAminoacyl-tRNA synthetase · activation · one specific enzyme for each amino acid · 2 high-energy bonds
- 2★Small subunit + mRNA + Met-tRNA find AUG; large subunit joins (80S)Initiation factors (eIFs) + GTP · initiation · Met-tRNA sits in the P site (N-formylmethionine in bacteria)
- 3Next aminoacyl-tRNA enters the A siteEF-1 (EF-Tu in bacteria) + GTP · elongation · anticodon pairs with codon
- 4★Peptide bond forms between the P-site and A-site amino acidsPeptidyl transferase · an rRNA of the large subunit (a ribozyme, an RNA that acts as an enzyme)
- 5Ribosome moves three bases along the mRNA (translocation)EF-2 (EF-G in bacteria) + GTP · empty tRNA leaves via the E site; steps 3–5 repeat
- 6★Stop codon (UAA, UAG or UGA) reaches the A site → chain releasedRelease factors + GTP · termination · ribosome splits into its subunits
- 7New chain → folded, active proteinChaperones and modifying enzymes · post-translational modification
★ one-way, controls the pace
Features of the genetic code
- Triplet: three bases (one codon) code for one amino acid; there are 64 codons in all.
- Degenerate: 61 codons code for 20 amino acids, so most have more than one codon (leucine has 6; methionine and tryptophan have only 1).
- Universal: the same code works in nearly all living things (mitochondria show a few differences).
- Unambiguous: one codon always stands for the same amino acid.
- Non-overlapping and comma-less: read continuously, three bases at a time, with no gaps.
- Start and stop: AUG starts (methionine); UAA, UAG and UGA stop (non-sense codons).
- Wobble: the third base pairs loosely, so one tRNA can read more than one codon.
Energy count per amino acid added
Post-translational modification
- Folding into the correct 3-D shape, helped by chaperones.
- Trimming: cutting off a signal piece or pro-piece (proinsulin → insulin).
- Adding groups: phosphate (phosphorylation) or sugars (glycosylation).
- Hydroxylation of proline and lysine in collagen, which needs vitamin C.
- Carboxylation of glutamate in clotting factors, which needs vitamin K.
- Disulphide bridges between cysteines, and attaching groups such as haem.
The cell needs all 20 amino acids present together. If an essential one runs short (the limiting amino acid), synthesis slows or stops; this is why protein quality matters and why cereals and pulses are combined. Vitamin C (collagen hydroxylation; lack causes scurvy) and vitamin K (clotting factor carboxylation) are needed for key modifications. In kwashiorkor the liver makes less albumin, which leads to oedema.
2 marks: “Name the stop codons.” UAA, UAG and UGA; they code for no amino acid and are recognised by release factors.
5 marks: “List the features of the genetic code.” Triplet, degenerate, universal, unambiguous, non-overlapping, comma-less and wobble; AUG start, three stop codons.
10 marks: “Describe the steps of protein biosynthesis.” Brief transcription, then activation, initiation, elongation, termination and post-translational modification, with a ribosome diagram and the energy count.
Stop codons: UAA 'U Are Away', UAG 'U Are Gone', UGA 'U Go Away'. The start codon AUG codes for methionine.