What is the start codon?

AUG is the standard start codon. It signals the ribosome to begin translation and also codes for methionine, so most proteins begin with a methionine residue that is often removed afterward. A few alternative start codons exist in some organisms.

Which codons are stop codons?

UAA, UAG, and UGA are the three stop codons in the standard genetic code. They do not encode an amino acid; instead they signal release factors to terminate translation and free the finished polypeptide from the ribosome.

Why do multiple codons code for the same amino acid?

The genetic code is degenerate: most amino acids are specified by two to six codons. This redundancy, concentrated in the third codon position (the wobble position), buffers against many point mutations that would otherwise change the protein.

What is the difference between DNA and RNA codons?

RNA codons use uracil (U) where DNA uses thymine (T). Codon tables are written for messenger RNA, so this tool uses U. To convert a DNA coding strand, replace every T with U before looking up the codon.

What does the wobble position mean?

The third base of a codon often pairs loosely with the tRNA anticodon, a flexibility called wobble. It explains why changing the third base frequently encodes the same amino acid, and why a single tRNA can read several codons.

What is the Genetic Codon Table Reference?

Look up any of the 64 RNA codons and the amino acid it encodes using the standard genetic code, with start and stop codons highlighted. Type or build a codon and translate it instantly. Runs entirely in your browser. It runs free in your browser on Gera Tools, with nothing uploaded.

Genetic Codon Table Reference

Name: Genetic Codon Table Reference
Creator: Gera Tools
License: https://creativecommons.org/licenses/by/4.0/

The genetic code maps each three-base RNA codon to an amino acid (or a stop signal). This reference translates any of the 64 codons using the standard genetic code and highlights the start and stop codons.

How it works

Messenger RNA is read in non-overlapping triplets called codons. Each of the three positions can be A, U, G, or C, giving 4 × 4 × 4 = 64 possible codons. Of these:

AUG is the start codon and also encodes methionine.
UAA, UAG, and UGA are stop codons and encode no amino acid.
The remaining 60 codons encode the other amino acids, with most amino acids specified by several synonymous codons.

This tool looks up a codon in the standard code table and returns the amino acid, or flags start/stop.

Why there are only 20 amino acids for 64 codons

The genetic code has 64 possible codons but only 20 standard amino acids plus three stop signals. This means most amino acids are encoded by more than one codon — a property called degeneracy (or redundancy). The pattern is not random:

Amino acids with more codons tend to be either common in proteins or structurally simple. Leucine and serine each have six codons; methionine and tryptophan each have only one.
The third codon position (the “wobble position”) is the most redundant. Changing the third base often encodes the same amino acid — for example, GCU, GCC, GCA, and GCG all encode alanine. This is not an accident: the wobble position pairs more loosely with the tRNA anticodon, so the ribosome tolerates mismatches there.
The first two positions are more deterministic. Changing the first base almost always changes the amino acid; changing the second base frequently does too.

This degeneracy provides a natural buffer against mutations: many single-nucleotide changes in the wobble position produce a synonymous codon that encodes the same amino acid, leaving the protein unchanged.

Working with the table: DNA to RNA conversion

The codon table is written in RNA notation using uracil (U). If you are working from a DNA sequence, you need to convert:

DNA coding strand: ATG TTC GAA → RNA (replace T with U): AUG UUC GAA
Then look up each codon: AUG = Met (start), UUC = Phe, GAA = Glu

Note that this is the coding strand (same sequence as the mRNA, except T→U). The template strand read 3’→5’ would be the complement.

Reading frames and why they matter

mRNA is read in non-overlapping triplets starting from the AUG start codon. The reading frame — where you start counting — determines the entire translation:

For the sequence AUGUUCGAA:

Frame 1: AUG | UUC | GAA → Met-Phe-Glu (the correct reading frame if AUG is at position 1)
Frame 2: A | GUU | CGA | A → nonsense (shifted by 1)
Frame 3: AU | GUU | CGA | A → different result (shifted by 2)

A frameshift mutation — an insertion or deletion that is not a multiple of 3 — shifts the reading frame and typically produces a completely different and usually nonfunctional downstream protein. This is why frameshift mutations are generally more disruptive than point mutations.

Tips and example

AUG → Methionine (Met, M) and marks the start of translation. The initiator methionine is often removed from the finished protein.
The code is degenerate: UUU and UUC both encode phenylalanine. UUA, UUG, CUU, CUC, CUA, and CUG all encode leucine.
For DNA, replace T with U first — the coding strand ATG becomes the codon AUG.
The standard genetic code is used by most organisms, but mitochondria use a slightly variant code, and a few unusual organisms have reassigned one or two codons. This table implements the standard (universal) code.