Translation and the genetic code

The genetic code uses 64 triplet codons: 61 specify the 20 standard amino acids and 3 (UAA, UAG, UGA) are stop signals; AUG (methionine) is the usual start codon. The code is degenerate (most amino acids have several codons, differing mainly at the third base), unambiguous (each codon means one amino acid), non-overlapping and read without punctuation from a fixed start, and almost universal (mitochondria have a few differences, e.g. UGA = tryptophan).

Transfer RNAs (≈75–90 nucleotides, cloverleaf secondary structure, L-shaped in 3-D) carry an amino acid on their 3′ CCA end and an anticodon loop that base-pairs antiparallel with the codon. Flexible "wobble" pairing at the third codon position lets one tRNA read several codons. Aminoacyl-tRNA synthetases attach the correct amino acid to each tRNA using ATP and proofread their work — the true "translators" of the code.

Initiation: the 40S subunit with eIFs and the initiator Met-tRNA binds the 5′ cap and scans to the first AUG in a good (Kozak) context; the 60S subunit joins, leaving Met-tRNA in the P site. Elongation: an aminoacyl-tRNA delivered by eEF1A-GTP enters the A site; the peptidyl transferase centre (28S rRNA — a ribozyme) forms the peptide bond; eEF2-GTP translocates the ribosome one codon, moving the empty tRNA to the E site. Chains grow from N-terminus to C-terminus as mRNA is read 5′ → 3′. Termination: a release factor recognises the stop codon and the polypeptide is released. Each peptide bond costs ≈4 high-energy phosphate bonds (2 for tRNA charging, 2 GTP for elongation). Proteins then fold (chaperones) and may be modified (phosphorylation, glycosylation, cleavage, ubiquitination).