Messenger RNAs are read in the 5' to 3' direction, which is the same direction in which are synthesized. Proteins are synthesized from the amino terminus to the carboxyl terminus, meaning the amino-terminal amino acid is added first. The genetic code consists of three-base sequences called codons in mRNA, which instruct the ribosome to incorporate specific amino acids into a polypeptide. The code nonoverlapping, meaning each base is part of only one codon, and it lacks gaps or commas, with every base in the coding region of an mRNA being part of a codon. There are 64 codons in total, three of which are stop signals, while the remaining codons encode amino acids, making the code highly degenerate. The degeneracy of the genetic code is partially managed by isoaccepting tRNA species that bind the same amino acid but recognize different codons. Additionally, wobble pairing allows the third base of a codon to deviate slightly from its normal position, forming non-Watson–Crick base pairs with the anticodon. This enables a single aminoacyl-tRNA to pair with multiple codons. Wobble pairs include G–U (or I–U) and I–A. The genetic code is not strictly universal. In certain eukaryotic nuclei, mitochondria, and at least one bacterium, codons that serve as termination signals in the standard genetic code can instead encode amino acids such as tryptophan and glutamine. In some mitochondrial genomes, the meaning of codons is altered, switching from one amino acid to another. Despite these deviations, the altered codes remain closely related to the standard genetic code from which they likely evolved. Elongation occurs in three steps: (1) EF-Tu, bound with GTP, delivers an aminoacyl-tRNA to the ribosomal A site. (2) Peptidyl transferase forms a peptide bond between the peptide in the P site and the newly arrived aminoacyl-tRNA in the A site, extending the peptide by one amino acid and shifting it to the A site. (3) EF-G, in conjunction with GTP, translocates the growing peptide.

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