Minor variations in the genetic code are found in mitochondria and certain microorganisms.
The Genetic Code Is Not "Universal"
The genetic code is not quite universal. Despite this, the term "universal genetic code" is used to refer to the codon table shown above (Fig. 8.02), which applies to almost all organisms. Rare exceptions are found in some protozoans and mycoplasmas and in the mitochondrial genome of animals and fungi (Table 8.04). Mycoplasmas are parasitic bacteria with unusually small genomes. Paramecium and Euplotes are ciliated protozoans and Candida is a yeast.
Note that there is no general mitochondrial genetic code. Although fungal and animal mitochondria share similarities (e.g., UGA = Trp), there are also differences. However, plant mitochondria and chloroplasts use the universal genetic code.
Unusual Amino Acids are Made in Proteins by Post-Translational Modifications
Although the genetic code has codons for only 20 amino acids, many other amino acids are occasionally found in proteins. Apart from selenocysteine and pyrrolysine (see below), these extra amino acids are made by modifying genetically encoded amino acids after the polypeptide chain has been assembled. This is known as post-translational modification.
An example of medical importance is diphthamide, which is derived from histidine by post-translational modification (Fig. 8.29). It is found only in elongation factor eEF2 of eukaryotes and archaebacteria, in a region of the amino acid sequence that is highly conserved. The corresponding bacterial factor, EF-G, does not contain diphthamide.
Selenocysteine: The 21st Amino Acid
Selenocysteine (Sec) is not one of the standard 20 amino acids and yet it is incorporated into a few rare proteins during translation of the mRNA by the ribosome. This occurs both in bacteria and in eukaryotes, including humans. Sequencing of the genes diphthamide Modified amino acid found only in eukaryotic elongation factor eEF2 that is the target for diphtheria toxin post-translational modification Modification of a protein or its constituent amino acids after translation is finished selenocysteine (Sec) Amino acid resembling cysteine but containing selenium instead of sulfur universal genetic code Version of the genetic code used by almost all organisms
After the polypeptide chain of eEF2 has been made on the ribosome one specific histidine residue is converted to diphthamide. Diphth eria toxin stops protein synthesis by adding an ADP-ribose group to the side chain of diphthamide. This inhibits the action of eEF2.
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