The genetic codeSpec D1.2.8, D1.2.9
In short
The genetic code is the set of rules by which mRNA codons are translated into amino acids. It is a triplet code because 4³ = 64 combinations are enough for 20 amino acids, whereas pairs give only 16. It is degenerate, as most amino acids have several codons, and universal, as almost all organisms use the same codons.
Why a triplet code
There are four bases in mRNA and 20 amino acids in proteins. A code of single bases would give 4 combinations, and pairs of bases would give 4² = 16, which is not enough. Triplets give ==4³ = 64 codons==, which is more than enough for 20 amino acids plus start and stop signals.
- Degeneracy
- More than one codon can code for the same amino acid. For example, GAA and GAG both code for glutamic acid. Of the 64 codons, 61 code for amino acids and 3 are stop codons.
- Universality
- The same codons code for the same amino acids in almost all living organisms, with only minor variations. This is evidence for common ancestry and allows genes to be transferred between species.
- Start codon
- AUG, which codes for methionine and marks where translation begins.
- Stop codons
- UAA, UAG and UGA, which do not code for an amino acid and signal the end of translation.
Using a table of mRNA codons
Codon tables give the amino acid for each mRNA codon. Read the mRNA from the start codon in groups of three, without overlapping, and look up each codon. If you are given DNA, first work out the mRNA: it is complementary to the template strand, and the same as the coding strand with U in place of T.
| Codon | Amino acid | Codon | Amino acid |
|---|---|---|---|
| AUG | Met (start) | GAA | Glu |
| GCU | Ala | GAG | Glu |
| AAA | Lys | GUG | Val |
| UGG | Trp | UUU | Phe |
| CAU | His | UAA | stop |
Maths skill: deducing an amino acid sequence
The template strand of a short DNA sequence reads TAC CGA TTT ACC CTT ATT. Deduce the amino acid sequence.
- Write the complementary mRNA (A→U, T→A, C→G, G→C): AUG GCU AAA UGG GAA UAA.
- Look up each codon: AUG = Met, GCU = Ala, AAA = Lys, UGG = Trp, GAA = Glu, UAA = stop.
Answer: Met – Ala – Lys – Trp – Glu (then stop).
Check whether a sequence is DNA (contains T) or mRNA (contains U) before using a codon table. Codon tables are always written for mRNA.
Written and checked against the IB Biology HL specification · Updated October 2026