In eukaryotes, where is mitochondrial DNA located?

In eukaryotes, mitochondrial DNA is located inside the mitochondria. These organelles contain their own small genome in addition to the much larger nuclear genome housed in the cell nucleus.

Mitochondrial DNA is generally a circular DNA molecule, a feature it shares with bacterial chromosomes. This similarity supports the endosymbiotic theory, which proposes that mitochondria evolved from bacteria that entered into a long-term partnership with ancestral eukaryotic cells.

Human mitochondrial DNA contains 16,569 base pairs and encodes 37 genes. These include genes for several proteins involved in oxidative phosphorylation, as well as ribosomal RNAs and transfer RNAs needed for mitochondrial protein production. Most mitochondrial proteins, however, are encoded by nuclear DNA and imported into the organelle.

Mitochondria sit in the cytoplasm, so mitochondrial DNA is outside the nucleus but not simply loose in the cytoplasm. In humans, mitochondrial DNA is typically inherited through the mother, making it useful for studying maternal ancestry and certain inherited mitochondrial disorders.