codon

A codon is a single word in the genetic message: a run of exactly three consecutive bases in mRNA that the ribosome reads as one unit. Each codon specifies one amino acid to add to the protein, or signals where to start or stop.

Why three? With four possible bases, pairs would give only 16 combinations — too few for 20 amino acids — while triplets give 64, comfortably enough. The cell reads codons consecutively, without gaps or overlaps, so a sequence like AUG-GCA-UUU is read as three distinct codons in order.

Some codons have special jobs. AUG usually means “start here” (and also codes for the amino acid methionine), while three codons — UAA, UAG, and UGA — mean “stop,” telling the ribosome to release the finished chain. The rest simply name amino acids, many of them more than once.