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The codon

A codon is three bases in a row on mRNA, and it codes for one amino acid. There are four bases, so the number of possible codons is four to the power of three. That is far more codons than the twenty common amino acids. Several codons can therefore give the same amino acid, and some are stop signals. The start codon AUG codes for methionine and starts translation. The stop codons UAA, UAG and UGA do not code for an amino acid. Translation happens in the ribosome, after the mRNA has been made in the nucleus in eukaryotes.

Symbols

Example

Three positions and four bases: $4^3 = 64$ codons.

Only 20 common amino acids, so several codons share an amino acid.

From amino acids to bases →

Part of Biology 2: From gene to protein.