**3. Mitochondria and genetics of mitochondria-related diseases**

The mitochondrial genome is a multicopy, double-stranded circular DNA molecule, which is 16.6 kb in human [11]. This genome encodes 13 essential proteins for the OXPHOS system and 24 components of the RNA machinery: 2 rRNAs and 22 tRNAs [11]. It is intronless and the only noncoding region is the displacement region (D-Loop), a region of 1.1 kb. It contains both the replication origins and the transcriptional promoters. Although mitochondria are genetically controlled by both mitochondrial and nuclear genomes, mtDNA is only maternally inherited [3]. Mitochondrial genetics differ greatly from Mendelian genetics in size, number of encoded genes, number of DNA molecules per cell, lack of introns, gene density, replication, transcription, recombination, and mode of inheritance. The 13 proteins include 7 subunits of NADH Dehydrogenase (complex I: ND1, ND2, ND3, ND4, ND4L, ND5 and ND6), Cytochrome b (subunit of complex III), 3 subunits of Cytochrome c oxidase or complex IV (COI, COII and COIII), and 2 subunits of F0F1 ATPase (ATPase 6 and ATPase 8). They are all encoded by mtDNA and synthesized in the organelle. While, complex II (Succinate Dehydrogenase) and the remaining subunits of complexes I, III, IV, and V are entirely encoded by the nuclear genome. These nuclear-encoded proteins are synthesized on cytosolic ribosomes and subsequently transported into the mitochondria.
