start codon

The start codon is the “begin reading here” mark on an mRNA. It tells the ribosome exactly where to start translating and, just as importantly, which reading frame to use — like the capital letter and first word that tell you where a sentence begins.

In almost all organisms the start codon is AUG, which also codes for the amino acid methionine; so newly made proteins typically begin with methionine (often removed later). The ribosome scans or binds the mRNA, finds this AUG in the right context, and sets the frame from that point.

Choosing the start point is not trivial: an mRNA may contain many AUG triplets, but only one in the right surrounding sequence is selected as the true start. Once it is, every following codon is read in step until a stop codon is reached, which is why the start codon anchors the entire protein's correct reading.