Genetic code mitochondrial

Telford, M.J., Herniou, E.A., Russell, R.B. and Littlewood, D.T.J. (2000) Changes in mitochondrial genetic codes as phylogenetic characters two examples from the flatworms. Proceedings of the National Academy of Sciences USA 97, 1 1 359-11 364. 

Reassignment of the platyhelminth mitochondrial genetic code from the standard inverte-... 

Table 2.7. Platyhelminth mitochondrial genetic code assignment. 

Ohama, T., Osawa, S., Watanabe, K. and Jukes, T.H. (1990) Evolution of the mitochondrial genetic code. IV. AAA as an asparagine codon in some animal mitochondria. Journal of Molecular Evolution 30, 329-332. 

In mammalian cells, some 1% of the total cellular DNA is found in the mitochondria. This DNA is double stranded, circular, and small, with a molecular weight of about 10 million, which is in the same range as that of viral DNAs. Some four to ten molecules of DNA per mitochondrion, along with some ribosomes, are found in the matrix space. DNA replication, transcription, and synthesis of some mitochondrial proteins take place in the matrix space. This protein synthesis very much resembles that of bacteria. The mitochondrial genetic code differs from the "universal" genetic code (Chapter 12) used for nuclearly encoded proteins and bacteria. The reasons for this are unknown. 

Mitochondrial Genetic Codes Differ from the Standard Nuclear Code... 

Tlie suitability of using the tRNA import pathway for the collection of respiratoiy deficiencies in the mitochondrial DNA and the applicability of this system for human therapeutic application were investigated. Mitochondrial import of nuclear encoded tRNAs has been described in yeasts, plants and protozoans. Tlie complexity of the imported tRNA pool varies among organisms, from a complete set required for reading all codons of the mitochondrial genetic code in trypanosomatids to a single tRNA in the yeast S. cerevisiae. 

The BioSomce includes information on the source organism (scientific name and common name), its lineage in the NCBI integrated taxonomy, and its nuclear and (if appropriate) mitochondrial genetic code. It also includes information on the location of the sequence in the cell (e.g., nuclear genome or mitochondrion) and additional modifiers (e.g., strain, clone, isolate, chromosomal map location). 

Mitochondria contain the only extranuclear genomic DNA (mtDNA) and it encodes 13 proteins using a genetic code different from that in the nucleus. These proteins are key components of OXPHOS I, III, IV and V, but not complex II, which is solely encoded by nuclear DNA (nuDNA). Inhibition of mtDNA transcription as well as expression of mitochondrial proteins will therefore lead to loss of OXPHOS function. Probably the best known drugs that inhibit mtDNA synthesis are the nucleotide... 

Of the very few variations in the genetic code that we know of, most occur in mitochondrial DNA (mtDNA), which encodes only 10 to 20 proteins. Mitochondria have their own tRNAs, so their code variations do not affect the much larger cellular genome. 

As often happens, a well-established conclusion may have to be modified. There are exceptions to the universal genetic code in mitochondrial DNA and in some protozoa (Chapter 5).45... 

The genetic code differs very little between species. By contrast, considerable differences occur between species in the anticodon translation system of tRNA, as evidenced by the mitochondrial tRNA system. In all systems the bases in the anticodon-codon complex run antiparallel, as in standard double-helix pairing, and in all cases only Watson-Cricklike base pairing occurs between the first two bases in the codon and the opposing bases in the anticodon segment of the tRNA. However, for the 3 base in the codon, the rules for pairing vary with the species and with the base in question. These rules, summarized in table 29.4, are as follows. 

The genetic code is not universal but is the same in most organisms. Exceptions are found in mitochondrial genomes where some codons specify different amino acids to that normally encoded by nuclear genes. In mitochondria, the UGA codon does not specify termination of translation but instead encodes for tryptophan. Similarly, in certain protozoa UAA and UAG encode glutamic acid instead of acting as termination codons. 

Mitochondria of various organisms use different genetic code. Search the Internet site to obtain information for the mitochondrial codons from either vertebrates or invertebrates. Discuss usage differences between the standard codons and mitochondrial codons. 

Kurland CG (1992) Evolution of mitochondrial genomes and the genetic code. BioEssays 14 709-714... 

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