The Genetic Code

Key points

• • The code is read in non-overlapping triplets called codons; each codon of three bases specifies one amino acid, and there are 64 possible codons.
• • AUG codes for methionine and acts as the start codon, while three codons (UAA, UAG, UGA) are stop codons that end translation and code for no amino acid.
• • The code is degenerate (redundant): most amino acids are specified by more than one codon, which cushions the effect of some point mutations.
• • The code is nearly universal, meaning almost all organisms use the same codons, which is why a human gene can be read correctly by a bacterium.
• • The third base of a codon is often the least important; flexible pairing there, called wobble, lets one transfer RNA read several codons for the same amino acid.
• • Because the code is read in fixed triplets, inserting or deleting a number of bases that is not a multiple of three shifts the reading frame and garbles everything downstream.