codon
/ KOH-don /
If the genetic code is the dictionary, a codon is a single word in that dictionary. When the cell reads a messenger RNA molecule, it does not read one base at a time; it reads them in fixed groups of three. Each of those three-base groups is a codon, and it is the smallest unit of meaning in the message — like reading a sentence three letters at a time and looking up what each chunk stands for.
A codon is a sequence of three consecutive bases (chosen from A, U, G, C) in messenger RNA. Each codon specifies either one amino acid to be added to the growing protein chain, or a command to stop. Because the bases are read three at a time without gaps or punctuation, where you begin reading sets the 'reading frame' — the particular way the long string of bases is sliced into triplets. Slip by even one base and every codon downstream changes, scrambling the whole protein. The cell keeps the frame correct by always beginning at a defined start codon and then stepping exactly three bases at a time.
Codons matter because they are the actual unit the protein-building machinery counts in. They also explain certain genetic diseases: a mutation that inserts or deletes a single base shifts the reading frame and garbles everything after it (a frameshift), which is usually far more damaging than swapping one base for another. A frequent point of confusion is between a codon, which lives on the messenger RNA, and an anticodon, the matching three-base group on the transfer RNA that pairs with it.
The codon UUU codes for the amino acid phenylalanine — in fact UUU was the very first codon ever deciphered, in 1961, when scientists fed ribosomes an artificial RNA of pure U's and got a chain of pure phenylalanine.
Cracking the meaning of UUU was the first step in reading the whole genetic code.
A codon is on the mRNA; its partner is the anticodon on a tRNA. Mixing the two up — and forgetting that there is no punctuation between codons, only a fixed reading frame — are the two most common slip-ups.