Mitochondrial genomes of eukaryotes, which encode approximately one and the same set of functions, are characterized by differences in the meanings of some codons, in the rules of anticodon-codon recognition, and significant differences in the general structural arrangement. A common property of all mitochondrial genomes is maximum structural compactness together with maximum information loading. It was achieved trough changes in a genetic code, which allowed decreasing the amount of tRNA necessary for read-out. Mitochondrial genetic codes in different organisms not only differ from the universal genetic code, but also differ from each other. Human mitochondrial genome is 16569 base pairs lengthwise. It contains information of the structure of 13 proteins, including three subunits of cytochrome-c-oxydase, cytochrome b, a subunit of a 6-mitochondrial ATPase, eight proteins with functions, for the present unknown. Besides, 12S- and 16S- rRNAs, 22 tRNAs are encoded. The genome achieves its economy not only owing to absence of introns in the structural genes, but also due to decreasing the set of tRNA to the necessary minimum, significant lessening of the rRNA size. Besides, human mitochondrial DNA does not contain spacer sections, and for multiple genes even terminating codons are not encoded. They are added on during post-transcriptional polyadenylation of the corresponding pre- mRNA.