Stop Codons — TAA, TAG, TGA (UAA, UAG, UGA)
Translation stops at 3 codons — TAA, TAG, TGA in DNA, UAA, UAG, UGA in RNA. No tRNA reads them; a release factor binds and the finished polypeptide is let go.
It is three-fold degenerate, and positions 2 and 3 both differ across the set. Which one a host actually uses is a different question, and the eight organisms below split across 2 different favourites.
Stop codon usage by organism
Each number is that codon's share of all stop codons in that organism, so every column sums to 100%. The highest in each column is the preferred codon a codon optimizer would pick.
| DNA | RNA | E. coli | Human | Yeast | CHO | Pichia | Insect | Arabidopsis | Zebrafish |
|---|---|---|---|---|---|---|---|---|---|
| TAA | UAA | 64% | 30% | 47% | 26% | 51% | 63% | 36% | 36% |
| TAG | UAG | 7% | 24% | 23% | 24% | 29% | 18% | 20% | 18% |
| TGA | UGA | 29% | 47% | 30% | 50% | 20% | 18% | 44% | 46% |
Reference approximations derived from the Kazusa Codon Usage Database — the same tables the codon optimizer runs on. Verify against your own expression system for critical work.
Preferred stop codon by host
Use this
Frequently asked questions
Which codons stop translation?
Three: TAA, TAG, TGA in DNA, written UAA, UAG, UGA in RNA. None of them is read by a tRNA — a release factor binds instead and the finished chain is let go.
Which stop codon should I use?
It depends on the host, and the eight organisms here do not agree — they split across 2 different favourites (TAA, TGA). E. coli (K-12) prefers TAA, Human (H. sapiens) prefers TGA, Yeast (S. cerevisiae) prefers TAA. The table below gives every fraction.
Does TAA behave the same in every organism?
No — it is the codon that varies most across these eight. E. coli (K-12) uses it for 64% of its stop codons, CHO (C. griseus) for 26%. That gap is the reason a gene that expressed well in one host can stall in another without a single amino acid changing.
Is one stop codon better than another?
TAA is the most used in most of the organisms here and is generally the safest single choice; read-through is lowest when the stop is followed by a purine. Many expression constructs use two stops in a row for that reason. The fractions below are what each organism actually does, not a recommendation.
Where do these usage numbers come from?
They are reference approximations derived from the Kazusa Codon Usage Database and are the same tables the codon optimizer on this site runs on. For critical work, check them against your own expression system rather than treating any single table as definitive.
Codons for the other amino acids
Or go back to the full codon table for the codon-to-amino-acid direction.