Evolutionary changes in the genetic code.

1. The genetic code was thought to be identical ("universal") in all biological systems until 1981, when it was discovered that the coding system in mammalian mitochondria differed from the universal code in the use of codons AUA, UGA, AGA and AGG. 2. Many other differences have since been discovered, some in mitochondria of various phyla, others in bacteria, ciliated protozoa, algae and yeasts. 3. The original thesis that the code was universal and "frozen" depended on the precept that any mutational change in the code would be lethal, because it would produce widespread alterations in the amino acid sequences of proteins. Such changes would destroy protein function, and hence would be intolerable. 4. The objection was "by-passed" by nature. It is possible for a codon to disappear from mRNA molecules, often as a result of directional mutation pressure in DNA: thus all UGA stop codons can be replaced by UAA. 5. The missing UGA codon can then reappear when some UGG tryptophan codons mutate to UGA. The new UGA codons will be translated as tryptophan, as is the case in non-plant mitochondria and Mycoplasma. Therefore, no changes have taken place in the amino acid sequences of proteins. 6. Variations of this procedure have occurred, affecting various codons, and discoveries are still being made. The findings illustrate the evolutionary interplay between tRNA, release factors and codon-anticodon pairing.