Mitochondrial DNA (mtDNA) is characterized by:
Mitochondrial DNA illustrates its bacterial endosymbiont origin and maintains partial organelle autonomy despite extensive nuclear contribution. Human mtDNA is a small, covalently closed circular double-stranded molecule about 16,569 base pairs in length, present in 2 to 10 copies per mitochondrion and organized into nucleoids with transcription factor A mitochondrial, TFAM, high mobility group proteins, and Twinkle helicase rather than histones. It is intron-less and extremely densely coded with limited noncoding displacement-loop control region containing promoters and origin of replication. It encodes 13 core subunits of Complex I, III, IV and ATP synthase, plus 22 tRNAs and 2 rRNAs required for organellar translation using slightly non-universal genetic code. Replication occurs via asynchronous strand-displacement model using polymerase gamma, helicase Twinkle and mitochondrial single-stranded binding protein, with separate origins for heavy and light strands, OH and OL. Inheritance is predominantly maternal, mutation rate is elevated tenfold over nuclear due to reactive oxygen exposure and limited excision repair. Most mitochondrial proteins, over 1000, remain nuclear-encoded, translated cytosolically and imported via TOM and TIM complexes.
Ref: NCBI Bookshelf, Molecular Biology of the Cell, Section: Organization and Inheritance of Mitochondrial DNA.