Codon Chart: The Standard Genetic Code
Every mRNA codon, mapped to the amino acid it codes for, in the classic first-base × second-base × third-base layout -- plus the start and stop codons.
| 1st | 2nd base | 3rd | |||
|---|---|---|---|---|---|
| U | C | A | G | ||
| U | UUU Phe | UCU Ser | UAU Tyr | UGU Cys | U |
UUC Phe | UCC Ser | UAC Tyr | UGC Cys | C | |
UUA Leu | UCA Ser | UAA Stop | UGA Stop | A | |
UUG Leu | UCG Ser | UAG Stop | UGG Trp | G | |
| C | CUU Leu | CCU Pro | CAU His | CGU Arg | U |
CUC Leu | CCC Pro | CAC His | CGC Arg | C | |
CUA Leu | CCA Pro | CAA Gln | CGA Arg | A | |
CUG Leu | CCG Pro | CAG Gln | CGG Arg | G | |
| A | AUU Ile | ACU Thr | AAU Asn | AGU Ser | U |
AUC Ile | ACC Thr | AAC Asn | AGC Ser | C | |
AUA Ile | ACA Thr | AAA Lys | AGA Arg | A | |
AUG Met start | ACG Thr | AAG Lys | AGG Arg | G | |
| G | GUU Val | GCU Ala | GAU Asp | GGU Gly | U |
GUC Val | GCC Ala | GAC Asp | GGC Gly | C | |
GUA Val | GCA Ala | GAA Glu | GGA Gly | A | |
GUG Val | GCG Ala | GAG Glu | GGG Gly | G | |
Read a codon by combining row and column: first base (left) + second base (top) + third base (right). AUG (highlighted green) is both the start codon and methionine; the three red cells are stop codons.
How to Read This Chart
Find a codon's amino acid by locating its first letter down the left side, its second letter across the top, and its third letter down the right side -- the cell where that row and column meet shows the codon and the amino acid it codes for. For example, the codon GGC: first base G (bottom row group), second base G (last column), third base C, gives glycine.
Reading the table the other direction -- from a known amino acid back to its possible codons -- is exactly what happens in reverse translation, and it's why an amino acid like leucine (6 codons) gives far less certainty about the original DNA sequence than tryptophan (only 1 codon, UGG) does.
Worked example: translating a short mRNA sequence
Translate the mRNA sequence AUG-GCU-CAU-UAA.
AUG = Met (start) → GCU = Ala → CAU = His → UAA = Stop.
The finished peptide is Met-Ala-His (M-A-H), three amino acids long -- translation stops at UAA before adding a fourth.
Common mistake
Reading a DNA sequence directly as if it were mRNA without converting T to U, or reading the template strand instead of the coding strand. DNA's coding strand and mRNA have the same sequence (T↔U aside); the template strand is the complementary, reverse strand actually transcribed from.
Keep going
- Prefer the classic circular layout instead of a grid? This same standard genetic code, read center-outward by first, second, and third base, is also available as the Codon Wheel
- Every 3-letter code in the table above (Ala, His, Met...) is one of the 20 standard amino acids. Look up each one's full name, side-chain classification, and molecular weight in the Amino Acid Chart
- Once you've translated a codon sequence into a chain of amino acids, you can look up each one's molecular formula on the Amino Acid Chart and total it up with the Molar Mass Calculator
- If you're studying gene expression alongside general chemistry, preparing a solution of a specific translated amino acid uses the same concentration math as the Molarity Calculator
Frequently Asked Questions
What is a codon?
A codon is a sequence of three consecutive nucleotide bases in mRNA (like AUG or GGC) that the ribosome reads as a single unit during translation. With 4 possible bases in each of the 3 positions, there are 4^3 = 64 possible codons -- more than enough to specify the 20 standard amino acids, which is why most amino acids have more than one codon.
Why does this chart use U instead of T?
Codons are read from messenger RNA (mRNA), which uses uracil (U) instead of the thymine (T) found in DNA. If you're looking at a DNA sequence instead of mRNA, swap every T for a U (and read the coding, non-template strand) to get the same codons shown here.
What is the start codon, and why is it also an amino acid?
AUG is the near-universal start codon -- the ribosome uses it to identify where to begin translating a protein -- and it also codes for methionine. That's why almost every newly made protein starts with a methionine at its very first (N-terminal) position, though it's sometimes removed afterward by cellular enzymes.
What do the stop codons do?
UAA, UAG, and UGA don't code for any amino acid. When the ribosome reaches one, it releases the finished protein chain instead of adding another amino acid. They're sometimes called by historical nicknames -- UAA is 'ochre', UAG is 'amber', and UGA is 'opal' -- names from early genetics research, not anything to do with actual color.
Why do some amino acids have 6 codons and others only 1?
The genetic code is redundant (biologists call this 'degenerate') but not random -- amino acids that are chemically similar tend to share codons that differ only in the third position. Methionine (AUG) and tryptophan (UGG) are the only amino acids with just one codon each; leucine, serine, and arginine each have 6, the most of any amino acid.
Is the genetic code the same in every organism?
Almost, but not perfectly. This table is the 'standard' genetic code (NCBI translation table 1), used by the vast majority of nuclear genomes including humans. A handful of exceptions exist -- for example, human mitochondrial DNA reassigns a few codons, including reading AGA and AGG as stop codons instead of arginine.
Source: NCBI Genetic Codes -- The Standard Code (translation table 1) (https://www.ncbi.nlm.nih.gov/Taxonomy/Utils/wprintgc.cgi), fetched 2026-09-25. Codons converted from NCBI's T-based DNA notation to U-based mRNA notation.
Related Calculators
- Molarity Calculator — Solutions & Concentration
- Dilution Calculator — Solutions & Concentration
- Molality Calculator — Solutions & Concentration
- Molar Mass Calculator — Atomic & Molecular