Mitochondrion genetic code

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. 

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). 

Ered Sanger, a double Nobel Prize winner, sequenced the human mitochondrial genome back in 1981. This genome codes for 13 proteins and the mitochondrion possesses the genetic machinery needed to synthesize them. Thus, the mitochondria are a secondary site for protein synthesis in eukaryotic cells. It turns out that the 13 proteins coded for by the mitochondrial genome and synthesized in the mitochondria are critically important parts of the complexes of the electron transport chain, the site of ATP synthesis. The nuclear DNA codes for the remainder of the mitochondrial proteins and these are synthesized on ribosomes, and subsequently transported to the mitochondria. 

Mitochondrial DNA is small and codes for relatively few mitochondrial proteins. Although mitochondria contain their own protein synthesis machinery, the majority of the hundreds of mitochondrial proteins are coded for by nuclear genes. These proteins are synthesized in the cytoplasm and imported into the mitochondria. Plastid DNA is somewhat larger than that of the mitochondrion and contains the genetic information for more chloroplast proteins. However, as is the case for mitochondria, most of the proteins in a chloroplast are coded by nuclear genes... 

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