start codon

Imagine being handed a long ribbon of letters with no spaces and told to read it three at a time. Your first question is obvious: where do I begin? Begin one letter too early or too late and the whole message turns to nonsense. The start codon is the cell's answer to that question — it is the marker that says 'reading the protein begins exactly here.'

The start codon is almost always the three-base sequence AUG on the messenger RNA. It does double duty: it both marks the spot where the ribosome should begin building the protein and codes for the first amino acid, methionine. By choosing this fixed starting point, the cell locks in the correct reading frame for everything that follows — every codon downstream is now measured in steps of three from this anchor. The cell does not just stumble onto the AUG; small proteins called initiation factors and special features of the messenger RNA help the ribosome find the right one (in bacteria a sequence just upstream guides it; in our cells the ribosome usually scans from the capped front end).

The start codon matters because translation is meaningless without a defined beginning — it is what converts the abstract genetic code into a concrete, correctly-framed protein. A useful caveat: because AUG also means 'methionine' everywhere it appears, not every AUG in a message is a start codon. The very first appropriately positioned AUG is the start; later AUGs in the middle of the message simply add another methionine to the growing chain. Also, that initial methionine is often trimmed off the finished protein afterward.