6.2.2—Universal genetic code and codons

A codon is a triplet of bases on mRNA read during translation. Each codon specifies one amino acid or acts as a start or stop signal; it does not directly name a complete protein.

• Triplet and continuity: The mRNA sequence is read three bases at a time from a defined start point. The codons are read consecutively and do not overlap, so the reading frame determines which bases belong to each codon.
• Mapping: A codon maps to an amino acid, and the ordered amino acids form the polypeptide. A start codon establishes where the coding sequence is read; a stop codon signals termination and does not add an amino acid.
• Degeneracy: More than one codon can specify the same amino acid. This is degeneracy, not ambiguity: a given codon has one assigned meaning in the code.
• Near-universality: Almost all organisms use the same codon assignments, which is why the code is described as universal at A-Level. No special exception is needed to apply the core mapping here.

The triplet rule provides enough combinations to assign amino acids, while the fixed reading frame prevents the message from being regrouped at every step. Translation therefore follows: mRNA codons → amino-acid sequence → polypeptide, with start and stop signals defining the usable coding run.

A codon is read on mRNA, not as an untranslated DNA triplet, and it specifies an amino acid or signal rather than an entire protein. A stop codon ends translation but is not incorporated as an amino acid. Do not shift the reading frame or treat overlapping groups as the standard code.