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.
Ref: NCBI Bookshelf, Molecular Biology of the Cell, Section: Organization and Inheritance of Mitochondrial DNA.