Universality of the Genetic Code,Termination Codons,Mitocondrial Systems,Selenocysteine,Nuclear Genes

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	Universality of the Genetic Code - A vast amount of data suggest that the genetic code is universal. It was postulated that the genetic code must be frozen and unable to evolve because a change in a codon meaning would cause almost every protein in the cell to be altered. Experiments with cell free system with mammalian extract yielded same results as that of E. coli. The major exception to the universality of the code occurs in mitochondria of humans, yeast and several other species where UGA is a tryptophan codon. UGA is a termination codon in the non mitochondrial system.
Also, in yeast mitochondria, CUA specifies threonine instead of the usual leucine and in mammalian mitochondria, AUA specifies methionine instead of the usual isoleucine. Still more surprising is the discovery that the meaning of a codon can vary from gene to gene in the same organism. In human nuclear genes UGA is frequently used as a termination codon in accordance with its standard meaning. However, in at least two human genes, for the enzymes glutathione peroxidase and iodothyronine 5 -deiodinase, UGA specifies the unusual amino acid called selenocysteine (a cysteine in which the sulphur atom is replaced by selenium).
It appears that the mRNA transcribed from these genes have special stem loop structures in their trailer regions, and that these stem loops play some role during translation, ensuring that the UGA triplet is recognized as a selenocysteine codon rather than a termination signal.