How to Read a Codon Table Using an mRNA Sequence

Translating an mRNA molecule into a protein is one of the core operations of molecular biology. The bridge between the nucleotide language of RNA and the amino‑acid language of proteins is the genetic code, which is most often presented as a codon table. This guide shows how to use that table with an mRNA sequence.

What a Codon Table Is

A codon is a group of three nucleotides in messenger RNA (mRNA) that specifies a single amino acid or a translation stop signal. Because there are four possible bases (A, U, C, G), three‑base combinations yield 4³ = 64 possible codons, enough to encode the 20 standard amino acids plus start and stop signals. The codon table arranges these 64 triplets in a grid: the first base selects a row block, the second base selects a column, and the third base selects a cell within that column.⁴

Why Codons Are Read in Triplets

The three‑base “word” model was established in the early 1960s when Nirenberg, Khorana and others demonstrated that a single nucleotide or a pair could not generate enough distinct combinations to represent all amino acids. A triplet provides 64 combinations, which is both sufficient and redundant (different codons can code for the same amino acid). This redundancy is called a degenerate code.⁴

Step‑by‑Step: Translating an mRNA Sequence

1. Write the mRNA sequence in the 5’→3′ direction. Example: 5'-AUGGCCUUAACG-3'.
2. Divide the sequence into consecutive, non‑overlapping triplets. Using the example: AUG | GCC | UUA | ACG. If the last group has fewer than three bases, it is ignored for translation.
3. Locate each codon in the table. For each triplet, find the first base on the left side of the table, the second base on the top row, and the third base in the rightmost column. The intersecting cell gives the amino‑acid abbreviation (e.g., AUG → Met, GCC → Ala).²
4. Write the amino‑acid sequence. Concatenate the symbols in order. The example becomes Met‑Ala‑Leu‑Thr.
5. Stop at a stop codon. If a codon such as UAA, UAG, or UGA appears, translation ends and the protein chain is released.

Worked Example

Translate the following mRNA strand:

5'-AUG UUU GGC CGA UAA-3'

Follow the steps:

1. Split into codons: AUG | UUU | GGC | CGA | UAA.
2. Look up each codon:
   • AUG – first base A (row), second base U (column), third base G → Methionine (Met), also the start codon.
   • UUU – U‑U‑U → Phenylalanine (Phe).
   • GGC – G‑G‑C → Glycine (Gly).
   • CGA – C‑G‑A → Arginine (Arg).
   • UAA – U‑A‑A → Stop signal.
3. Combine until the stop codon: Met‑Phe‑Gly‑Arg. Translation stops at UAA, so the final protein is a four‑amino‑acid peptide.

Common Misconceptions

Self‑Check

Translate the mRNA sequence 5'-AUG CCG UUU UGA-3'. Write the amino‑acid chain.

Answer: AUG → Met, CCG → Pro, UUU → Phe. The fourth codon UGA is a stop, so the peptide is Met‑Pro‑Phe.

If you obtained a different result, review the reading frame and ensure each codon is read from left to right without overlap.

Next Steps

After mastering the codon table, explore how ribosomes physically read mRNA, how tRNA anticodons match codons, and how mutations in codons can alter protein function.