Translation

Translation is the second and final stage of gene expression. It decodes the linear sequence of codons in mRNA into the linear sequence of amino acids of a polypeptide — converting nucleic-acid information into protein structure and ultimately into cellular function. Translation is the most chemically complex anabolic process in the cell: it involves the coordinated action of mRNA, tRNA, ribosomes (a ribonucleoprotein machine of ~80 components), ATP, GTP, dozens of accessory protein factors, and 20 different aminoacyl-tRNA synthetases. The fidelity of the process is extraordinary — about one error per 10⁴ amino acids — and it consumes a substantial fraction of the cell's energy budget.

Spec mapping: This lesson sits in AQA 7402 Section 3.4.2 — DNA and protein synthesis, with synoptic links to Section 3.6.2 (neurotransmitter receptors and ion channels as translated products) and Section 3.5 (ATP and energy currency). The relevant content covers the role of mRNA, tRNA and ribosomes; the mechanism of initiation, elongation and termination; the formation of peptide bonds; and the post-translational processing of polypeptides. (Refer to the official AQA specification document for exact wording.)

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Components of Translation

Translation requires four classes of molecule working in concert: the mRNA (the message), the tRNA (the adapter that links codon to amino acid), the ribosome (the workbench and the catalytic site), and a suite of accessory proteins (initiation factors, elongation factors, release factors). The free amino acids and ATP/GTP supply respectively the building blocks and the energy.

Messenger RNA (mRNA)

• mRNA carries the genetic information from the nucleus to the ribosomes in the form of codons — sequences of three bases read in non-overlapping triplets.
• Each codon specifies one amino acid (or a stop signal).
• mRNA is read in the 5′→3′ direction during translation, the same direction in which it was synthesised.
• Mature eukaryotic mRNA carries a 5′ cap and 3′ poly-A tail (both required for efficient initiation), and a single open reading frame from start to stop codon.
• The 5′ untranslated region (5′ UTR) lies between the cap and the start codon; the 3′ UTR lies between the stop codon and the poly-A tail. Both UTRs contain regulatory elements that influence mRNA stability and translational efficiency.

Transfer RNA (tRNA)

Key Definition: Transfer RNA (tRNA) is a small (~76–90 nucleotide) RNA molecule that carries a specific amino acid to the ribosome and matches it to the correct mRNA codon. Each tRNA has an anticodon that is complementary to a specific mRNA codon.

Key structural features of tRNA:

• Secondary structure: cloverleaf. When drawn in two dimensions, tRNA has three hairpin loops (D loop, anticodon loop, TΨC loop) and an acceptor stem. Base-pairing within the single strand generates the cloverleaf.
• Tertiary structure: L-shape. In three dimensions the cloverleaf folds into a compact L-shape with the anticodon at one end of the L and the amino-acid attachment site at the other. The two functional ends are therefore physically separated by about 7 nm, allowing the anticodon and the amino acid to occupy distinct regions of the ribosome simultaneously.
• Anticodon. A three-base sequence in the anticodon loop, complementary and antiparallel to the corresponding mRNA codon. Pairing follows standard A-U and G-C rules, with wobble permitted at the third codon position.
• Acceptor end. The 3′ end terminates in an invariant CCA sequence. The amino acid is attached as an ester bond to the 3′-OH of the terminal adenosine.
• Modified bases. tRNAs contain many unusual modified nucleotides (pseudouridine Ψ, inosine I, dihydrouridine D, ribothymidine rT) that stabilise the folded structure and tune anticodon-codon recognition.

Amino acid activation — "charging" the tRNA

Before translation, each amino acid must be attached to its correct tRNA. This process is amino acid activation (or tRNA charging) and is catalysed by aminoacyl-tRNA synthetase enzymes:

• There is at least one aminoacyl-tRNA synthetase for each of the 20 amino acids.
• The enzyme is doubly specific: it recognises both the amino acid (by its side-chain chemistry) and the correct tRNA (by its anticodon and by other identity-determining bases).
• The reaction has two steps. First, the amino acid reacts with ATP to form an aminoacyl-adenylate (amino acid–AMP), releasing pyrophosphate. Second, the activated amino acid is transferred to the 3′-OH of the terminal adenosine of the tRNA, releasing AMP.
• Net cost: one ATP is hydrolysed (to AMP + 2Pi) per amino acid attached — equivalent to two high-energy phosphate bonds.
• The product is an aminoacyl-tRNA, often called a "charged tRNA". The amino acid is held by a high-energy ester bond, the hydrolysis of which provides the energy for peptide bond formation later in the cycle.
• The synthetases have a proofreading site that hydrolyses incorrectly attached amino acids. The fidelity of charging is approximately one error per 10⁴–10⁵ — the major determinant of overall translational fidelity.

Ribosomes

Ribosomes are the workbench of translation. Each is a massive ribonucleoprotein particle composed of ribosomal RNA (rRNA) and tens of associated ribosomal proteins. The rRNA — not the protein — provides the catalytic activity that forms peptide bonds; the ribosome is therefore a ribozyme.

• Eukaryotic ribosomes are 80S (composed of a 60S large subunit and a 40S small subunit). The "S" stands for Svedberg units, a measure of sedimentation rate during ultracentrifugation; Svedberg values are not additive because they reflect both size and shape, which is why 40S + 60S = 80S rather than 100S.
• Prokaryotic ribosomes are 70S (50S large subunit + 30S small subunit). The structural difference between 70S and 80S ribosomes is exploited by antibiotics such as erythromycin, tetracycline, streptomycin and chloramphenicol, all of which selectively inhibit prokaryotic translation while sparing eukaryotic ribosomes.
• Mitochondrial and chloroplast ribosomes are 70S — a reflection of the endosymbiotic origin of these organelles from ancestral bacteria.
• Each ribosome has three tRNA binding sites:
  • A site (aminoacyl site) — receives the incoming aminoacyl-tRNA whose anticodon matches the codon currently displayed.
  • P site (peptidyl site) — holds the tRNA carrying the growing polypeptide chain.
  • E site (exit site) — transiently holds the deacylated tRNA before its release into the cytoplasm.
• The polypeptide emerges through an exit tunnel ~80 Å long that runs through the large subunit. Co-translational folding begins as soon as the nascent chain emerges from the tunnel.

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The Mechanism of Translation

Initiation

1. The small ribosomal subunit binds to the 5' end of the mRNA and moves along until it reaches the start codon (AUG).
2. The initiator tRNA (carrying methionine, Met) with the anticodon UAC binds to the start codon at the P site by complementary base pairing.
3. The large ribosomal subunit then associates with the small subunit, forming the complete ribosome with the initiator tRNA in the P site.
4. Initiation requires initiation factors (proteins) and GTP as an energy source.

Elongation

5. A second aminoacyl-tRNA enters the A site, with its anticodon complementary to the next codon on the mRNA.
6. A peptide bond forms between the amino acid in the P site and the amino acid in the A site. This reaction is catalysed by peptidyl transferase (a ribozyme — the catalytic activity resides in the rRNA of the large subunit, not a protein enzyme).
7. The ribosome translocates (moves) one codon along the mRNA in the 5' to 3' direction:
   • The tRNA in the A site (now carrying the growing polypeptide) moves to the P site.
   • The "empty" tRNA in the P site moves to the E site and is released.
   • The A site is now free for the next aminoacyl-tRNA.
8. Steps 5–7 repeat, and the polypeptide chain grows one amino acid at a time.

Termination

9. The ribosome reaches a stop codon (UAA, UAG, or UGA) on the mRNA.
10. No tRNA has an anticodon complementary to a stop codon. Instead, a release factor protein binds to the A site. Release factors are structural mimics of tRNA but are entirely proteinaceous.
11. The release factor stimulates hydrolysis of the bond between the polypeptide and the tRNA in the P site.
12. The completed polypeptide is released from the ribosome.
13. The ribosome dissociates into its two subunits, and the mRNA is released. The two subunits can re-enter the initiation cycle on a new (or the same) mRNA.

flowchart TD
  A["Initiation: small subunit + initiator tRNA scan mRNA; large subunit joins at AUG"] --> B["Elongation cycle:"]
  B --> C["1. Aminoacyl-tRNA enters A site (codon-anticodon pairing)"]
  C --> D["2. Peptidyl transferase forms peptide bond"]
  D --> E["3. Translocation: ribosome moves one codon 5'→3'; empty tRNA exits via E site"]
  E --> F["Cycle repeats until stop codon"]
  F --> G["Termination: release factor binds A site → polypeptide released"]
  G --> H["Subunits dissociate; mRNA released"]

Worked example — translating a short mRNA

Given mRNA sequence 5′-AUG GAA UUC GAU UCC UAA-3′, determine the polypeptide produced.

Codon	Anticodon (3′-5′) on tRNA	Amino acid
AUG	UAC	Methionine (Met) — start
GAA	CUU	Glutamic acid (Glu)
UUC	AAG	Phenylalanine (Phe)
GAU	CUA	Aspartic acid (Asp)
UCC	AGG	Serine (Ser)
UAA	—	Stop (release factor binds)

Polypeptide: Met–Glu–Phe–Asp–Ser

This 5-residue chain requires 5 aminoacyl-tRNAs (5 ATPs for charging), 4 peptide bonds formed (4 GTPs for codon recognition + 4 GTPs for translocation), and is released by release factor binding when the stop codon enters the A site. Total energy cost: ~20 high-energy phosphate bonds for a 5-residue peptide.

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Polysomes (Polyribosomes)

Key Definition: A polysome (polyribosome) is a cluster of multiple ribosomes translating the same mRNA molecule simultaneously. Each ribosome produces an independent copy of the polypeptide.

• Polysomes allow rapid and efficient production of multiple copies of the same protein from a single mRNA.
• Ribosomes at the 5′ end of the mRNA have shorter polypeptide chains (they started translation more recently), while those at the 3′ end have longer, nearly complete polypeptides.
• Polysomes are visible under the electron microscope as a string of ribosomes (each ~25 nm diameter) attached at intervals of ~80 nucleotides to a single mRNA strand — the spacing reflects the physical footprint of a ribosome on the message.
• Highly translated mRNAs (e.g. globin mRNAs in maturing erythrocytes) carry many ribosomes simultaneously, allowing a single mRNA molecule to direct the synthesis of hundreds of copies of the encoded protein per minute.
• The polysome configuration also helps "close the loop": poly-A-binding protein on the 3′ tail interacts with cap-binding initiation factors at the 5′ end, bringing the two ends together so that a ribosome reaching the stop codon can quickly recycle to the start.

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Post-Translational Modifications

The polypeptide produced by translation is rarely the final functional protein. Most polypeptides undergo one or more post-translational modifications (PTMs) before they can perform their cellular roles: