The Code Is Not Quite Universal

Many of the known variations in the genetic code are found in genes of mitochondria and chloroplasts. It is easy to see why these genetic systems might be more plastic, since they frequently encode only 10-20 proteins. The remainder of the organellar proteins are derived by importing nuclear gene products. 

Another peculiarity of the mitochondrial code emerged from a study of yeast codon usage. By comparing tables 29.3 and 29.1, you will see that the mitochondrial code has several differences in code word meaning. The codons beginning with CU represent Thr instead of Leu, the AUA codon represents Met instead of He, and the UGA codon represents Trp rather than a stop signal. 

Snurce From S. G. Bonitz et al., Codon recognition rules in yeast mitochondria, in Proc. Natl. Acad. Sci. USA 77 3167, 1980. 

Two tRNAs for methionine have been found. One is used in initiation and one is used for internal methionines. hAlthough an Arg tRNA has been found in yeast mitochondria, the extent to which the CGN codons are used is not clear. 

The Rules Regarding Codon-Anticodon Pairing Are Species-Specific 

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The Code Is Not Quite Universal The Rules Regarding Codon-Anticodon Pairing Are Species-Specific The Steps in Translation... 

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