Figure 1.

Organization of the human mitochondrial genome. The 16.5 kb genome encodes 37 genes including 22 for mitochondrial tRNA (20 standard amino acids, pink arrows) and 2 for the rRNA (black arrows). The rest of the 13 genes encode for subunits of the different respiratory complexes I to IV (11 genes) and ATP synthase (2 genes). Complex I (NADH dehydrogenase) is composed of 7 mitochondrial subunits (ND1, ND2, ND3, ND4, ND5, ND6,and ND4L) and 37 nuclear subunits. Complex-II (succinate dehydrogenase) is composed of 4 subunits (all being nuclear encoded) and is both a component of the ETC and an enzyme of the Krebs cycle. Complex III (cytochrome c reductase) is a complex of 11 subunits. Only subunit, cytochrome b (purple arrow), is encoded by mtDNA, the remainder 10 are nuclear encoded. Complex IV (cytochrome c oxidase) is a complex of 13 different subunits, 3 (red arrows) are encoded by mtDNA, and 10 are nuclear encoded. The ATP synthase family is composed of 14 subunits; 2 (blue arrows) are mtDNA encoded. The other 14 are nuclear encoded. The displacement loop (D-loop, orange arrows) is the main noncoding area of the mtDNA where replication occurs. The region contains promoters for the transcription of RNA from the 2 strands of mtDNA. The heavy H strand has higher guanine content, and is transcribed from the P_H promoter. The light L strand is transcribed from the P_L promoter. Replication of the heavy strand by DNA polymerase commences from the O_H replication origin; this eventually exposes the O_L origin allowing replication of the light strand.