Figure 1: Structure of circular mitochondrial DNA (mtDNA). A: The human mitochondrial genome (~17 kbp) encodes 13 subunits of respiratory enzymes which are indicated by areas filled with grey: NADH dehydrogenase (ND 1–6), cytochrome b (CYTB), cytochrome oxidase (COX 1-3), FoF1 ATP synthase (ATP6 and 8). MtDNA also encodes 12S and 16S rRNA genes which are indicated by areas filled with blue and 22 tRNA genes filled with green. Replication of the mtDNA occurs in the D-loop region which is indicated by areas filled with red. OH is the replication origin of the heavy chain (NCBI Reference Sequence: NC_012920.1). B: Free mtDNA and nucleoid. Several mtDNA molecules within a nucleoid associate with mitochondrial inner membrane through the complex formed with nucleoid proteins such as TFAM, mtSSB, Twinkle, POLG and so on.