Life's dictionary

The universality of the genetic code is one of the best bits of evidence that all life shares a single common ancestor.

There are a number of exceptions to the code, mostly in organelles with small genomes, and in most cases the alterations involve stop codons.

Prokaryotes: AUG codes for N-formyl-methionine in the start position, not methionine.
Widespread: UGA may also code for selenocysteine, instead of a stop (the ribosomes distinguish the two cases).
Archaea: UAG codes for pyrrolysine, instead of a stop.
Animal and fungal (opisthokont) mitochondria: UGA codes for tryptophan, not a stop codon.
Yeast mitochondria: CUN is threonine, not leucine. CUG is often threonine in nuclear genes too.
Animal mitochondria: AUA (and sometimes AUG) usually code for methionine, not isoleucine.
Vertebrate mitochondria: AGA and AGG are stop codons, not arginine. The former is also found in some Gram positive bacteria.
Arthropod mitochondria: AGG often codes for lysine rather than serine.
Platyhelminth mitochondria: AAA often codes for asparagine rather than lysine, and UAA for tyrosine rather than stop.
Ciliates: UAA and UGA code for glutamine (UGA may also code for cysteine), instead of stop.
Mollicute bacteria: UGA codes for tryptophan, not stop; and CGG is a stop codon, rather than arginine.

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