(a) The respiratory chain. Nuclear DNA-encoded subunits are light gray; mtDNA-encoded subunits (see panel b) are dark gray. (b) Map of the human mitochondrial genome. Polypeptide-coding gene products (outside the circle) specify 7 subunits of NADH dehydrogenase-CoQ oxidoreductase (ND), 1 subunit of CoQ-cytochrome b oxidoreducase (Cyt b), 3 subunits of COX, and 2 subunits of ATP synthase (A). Protein synthesis gene products (inside the circle) specify 12S and 16S rRNAs, and 22 tRNAs (one-letter code). Figure modified from Schon and Manfredi (10).