Codon
A codon is a three-nucleotide unit in DNA or RNA that tells the translation machinery which amino acid to add, or when to stop building a protein.
What a codon is
A codon is a sequence of three nucleotide bases that is read as one unit during protein synthesis. In messenger RNA, codons are written with the letters A, U, C, and G. Each codon corresponds to an amino acid or to a signal that tells the ribosome to stop translation.
Why three bases are used
Cells need enough code words to represent 20 common amino acids plus stop signals. A one-base code would provide only four possibilities, and a two-base code would provide sixteen. A three-base code provides sixty-four possible codons, enough to encode amino acids with redundancy.
Reading frame
Codons are read in a fixed reading frame, three bases at a time. If translation starts at the wrong base, every downstream triplet changes, often producing a very different and usually nonfunctional protein. This is why insertions or deletions that are not multiples of three can cause frameshift mutations.
Start and stop codons
The codon AUG usually acts as the start codon and codes for methionine. Stop codons, commonly UAA, UAG, and UGA in the standard code, do not specify amino acids. Instead, they recruit release factors that help end translation and release the completed polypeptide.
Degeneracy of the code
The genetic code is degenerate, meaning several codons can specify the same amino acid. For example, some amino acids are encoded by multiple codons that differ only at the third position. This redundancy helps explain why some DNA or RNA changes are silent at the protein level.
Codons and tRNA
During translation, transfer RNAs carry amino acids to the ribosome. Each tRNA has an anticodon that pairs with a matching codon in the mRNA. Aminoacyl-tRNA synthetases attach the correct amino acids to tRNAs, making the codon-to-amino-acid relationship work in the cell.
Codon usage
Organisms do not always use synonymous codons equally. Codon usage can vary among species, genes, and cellular conditions, partly reflecting tRNA abundance and evolutionary history. In biotechnology, codon optimization may adjust a gene sequence so a host organism can translate it more efficiently.
Why it matters
Codons are the small units that connect nucleic acid sequences to proteins. They make the genetic code readable by ribosomes, shape how mutations affect proteins, and give biologists a practical language for interpreting genes, designing experiments, and comparing life across species.