Codon usage and the codon table

A coding sequence is read three bases at a time. Each triplet — acodon — specifies one amino acid, and tabulating which codons a gene uses tells you both what it encodes and how well it might express.

Reading in triplets

Starting at the first base, split the sequence into non-overlapping codons and translate each with the standard codon table. ATG GCA AAA GCA GCA TAA reads as M-A-K-A-A-stop → the protein MAKAA* (stops shown as *). Here GCA appears three times, making it the most-used codon in this short example.

The frame is everything

Shift the start by one base and every codon downstream changes — a different protein, or none at all. Codon counting assumes your sequence begins at the first base of a codon (frame 1). Any 1–2 bases left over at the end don’t complete a codon and are reported as a remainder, not translated; a non-zero remainder often means the sequence isn’t a clean coding region.

Why usage matters

Most amino acids have several synonymous codons, and organisms don’t use them equally — each has a preferred set matched to its abundant tRNAs.Rare codons in a gene expressed in a foreign host can stall the ribosome and lower yield, which is why codon optimisation is common when expressing a gene in E. coli or yeast.

What this tells you (and doesn’t)

Counting codons profiles the sequence you give it: the codon and amino-acid tallies, the translated protein, and the most-used codons. It does not compare against a species’ codon-usage table or generate an optimised sequence — it describes what is there, which is the first step before deciding whether to optimise.