Codon ChartThree bases make one codon
A nucleotide sequence is read in groups of three bases during translation. Each group is a codon. RNA has four usual bases, A, U, G and C, giving 4 × 4 × 4 = 64 possible three-base combinations. In the standard genetic code, 61 combinations specify amino acids and three are termination signals. The interactive codon chart shows the complete mapping, while the codon wheel arranges the same information from the center outward.
The mapping is redundant: several codons can specify the same amino acid. This is often called degeneracy of the genetic code. It does not mean that all mutations are harmless. A change can produce a different residue, introduce a stop signal or affect features outside the amino-acid assignment itself. A codon chart answers a specific question about translation; it does not summarize every effect of a DNA change.
The reading frame changes the groups
Consider a sequence that begins ATGAAACCC. Reading from the first base produces ATG, AAA and CCC. Starting one base later produces TGA and AAC, with two bases left over. Starting two bases later produces GAA and ACC, with one base left over. These are the three forward reading frames. Each groups the same letters differently and can give a different protein result.
Double-stranded DNA also has the opposite strand. To describe that strand in the usual 5′ to 3′ direction, calculate the reverse complement, then consider its three reading frames. This gives six possible frames in total. The DNA-to-protein translator can show them together. Selecting a frame is an assumption about how the sequence is read, not evidence that the frame represents an expressed gene.
Starts and stops depend on more than a lookup
AUG specifies methionine in the standard code and is a familiar initiation codon. Initiation, however, requires the appropriate biological context. An internal AUG is generally a methionine codon, rather than the start of a new protein. Some organisms also use alternative initiation codons. NCBI's tables distinguish ordinary amino-acid assignments from the codons recorded as possible starts.
The usual standard-code termination signals are UAA, UAG and UGA. A stop terminates ordinary translation rather than adding a standard amino-acid residue. An asterisk is a common representation in translated output. Special biological recoding can change the interpretation of particular codons, so a simple sequence-only translator should state its assumptions. This site's translator applies table assignments and does not infer specialized recoding from surrounding signals.
The genetic code has variants
The standard code is widely used, but it is not universal. Mitochondria and several other biological systems use alternative assignments. For example, UGA specifies tryptophan in the vertebrate mitochondrial code, whereas it is a stop in the standard code. The correct table depends on the organism and the sequence's genomic compartment. Choosing a table because it gives a longer translation is not a substitute for identifying the sequence's origin.
NCBI's genetic-code reference provides the 27 tables available in this site's selector. Some tables contain codons that can specify an amino acid or termination depending on context. The charts mark these assignments explicitly. The translator uses X where the input and selected table cannot uniquely establish an amino acid, including these context-dependent cases.
Ambiguous bases do not always mean ambiguous residues
Sequence data can contain IUPAC ambiguity letters. N means any nucleotide, while R means A or G. A codon containing ambiguity can sometimes still resolve to a single amino acid. GCN gives alanine under the standard code because GCA, GCC, GCG and GCT all encode alanine. By contrast, a set of possible codons with different assignments cannot be reduced to one residue without additional information.
The reverse complement tool preserves ambiguity by using the corresponding IUPAC pairing rules. The amino acid chart explains the separate one-letter protein alphabet. Keep those alphabets distinct: the same letter can have different meanings in nucleotide and protein sequences.
A useful checking order
Establish the sequence's origin, strand direction, reading frame and translation table. Check whether the sequence is complete and whether the annotation describes a coding sequence. Then inspect internal stops, incomplete codons and ambiguous positions. These checks make a computed translation interpretable. They do not establish expression, protein function or the success of a laboratory experiment.
See sources and methods for data provenance and the exact behavior of these tools.