Codon ChartNames and abbreviations
Protein sequences usually use one-letter amino-acid codes. Three-letter abbreviations are useful when discussing individual residues: alanine is Ala or A, lysine is Lys or K, and tryptophan is Trp or W. The table above lists the 20 standard amino acids and the codons that specify them in the selected genetic code. Search by a full name, three-letter abbreviation or one-letter code to highlight matching codons.
These single letters describe amino acids, not nucleotide ambiguity codes. For example, R in a protein means arginine; R in a DNA sequence means either A or G. The alphabet and the kind of sequence matter. Use the DNA-to-protein tool when converting nucleotide letters into a protein sequence.
Side-chain groups
The chart uses a broad teaching classification: nonpolar, polar, acidic and basic. These groups describe common side-chain properties, rather than a complete prediction of a residue's behavior in a folded protein. Protonation depends on pH and local environment. Tyrosine and cysteine are placed in the polar group here; other teaching charts sometimes divide aromatic or sulfur-containing residues into separate categories.
Stop signals are not amino acids, so they do not occupy a row in this table. You can inspect them on the complete codon chart. The reference covers the usual 20-residue alphabet, rather than recoding rules for selenocysteine or pyrrolysine.
Why some amino acids have several codons
There are 64 possible three-base combinations, but only 20 standard amino acids. Multiple codons therefore map to the same residue in the standard code. The assignments are not identical in every genetic code: changing the NCBI table updates the codons beside each amino acid. A dagger marks context-dependent termination. Read the genetic-code guide before interpreting a sequence from an unfamiliar organism or organelle.