The genetic code

Molecules (Continuity and change) · Protein synthesis · note 3 of 4

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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.

Part of the mRNA codon table (practice extract)
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).

Exam tip:

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 SL specification · Updated October 2026

Frequently asked questions

What is the difference between transcription and translation?

Transcription is the synthesis of RNA using a DNA template, carried out by RNA polymerase in the nucleus of eukaryotes. Translation is the synthesis of a polypeptide from mRNA at a ribosome, where tRNA anticodons pair with mRNA codons and amino acids are joined by peptide bonds. Transcription copies the sequence; translation decodes it.

Why is the genetic code a triplet code?

The genetic code is a triplet code because there are four bases and twenty amino acids. Pairs of bases would give only 4² = 16 combinations, too few for twenty amino acids. Triplets give 4³ = 64 codons, enough for every amino acid plus start and stop signals, which is why the code is also degenerate.

How does sickle-cell anaemia change the haemoglobin protein?

Sickle-cell anaemia is caused by a base substitution in the haemoglobin beta-chain gene. The mRNA codon changes from GAG to GUG, so valine replaces glutamic acid at the sixth amino acid. This hydrophobic valine makes haemoglobin S molecules stick together into fibres at low oxygen, distorting red blood cells into sickle shapes.