Mitochondrial DNA (mtDNA or mDNA) is the DNA located in organelles called mitochondria, structures within eukaryotic cells that convert the chemical energy from food into a form that cells can use, adenosine triphosphate (ATP). Most other DNA present in eukaryotic organisms is found in the cell nucleus.

Mitochondrial DNA can be regarded as the smallest chromosome, and was the first significant part of the human genome to be sequenced. In most species, including humans, mtDNA is inherited solely from the mother.

The DNA sequence of mtDNA has been determined from a large number of organisms and individuals (including some organisms that are extinct), and the comparison of those DNA sequences represents a mainstay of phylogenetics, in that it allows biologists to elucidate the evolutionary relationships among species. It also permits an examination of the relatedness of populations, and so has become important in anthropology and field biology.

Nuclear and mitochondrial DNA are thought to be of separate evolutionary origin, with the mtDNA being derived from the circular genomes of the bacteria that were engulfed by the early ancestors of today's eukaryotic cells. This theory is called the endosymbiotic theory. Each mitochondrion is estimated to contain 2-10 mtDNA copies. In the cells of extant organisms, the vast majority of the proteins present in the mitochondria (numbering approximately 1500 different types in mammals) are coded for by nuclear DNA, but the genes for some of them, if not most, are thought to have originally been of bacterial origin, having since been transferred to the eukaryotic nucleus during evolution.

Lynn Margulis (born Lynn Alexander) (March 5, 1938 – November 22, 2011) was an American biologist and University Professor in the Department of Geosciences at the University of Massachusetts Amherst. She is best known for her theory on the origin of eukaryotic organelles, and her contributions to the endosymbiotic theory, which is now generally accepted for how certain organelles were formed. She is also associated with the Gaia hypothesis, based on an idea developed by the English environmental scientist James Lovelock.

Lynn Margulis attended the University of Chicago, earned a master's degree from the University of Wisconsin-Madison in 1960, and received her Ph.D. in 1963 from UC Berkeley. In 1966, as a young faculty member at Boston University, she wrote a theoretical paper entitled The Origin of Mitosing Eukaryotic Cells. The paper however was "rejected by about fifteen scientific journals," Margulis recalled. It was finally accepted by The Journal of Theoretical Biology and is considered today a landmark in modern endosymbiotic theory. Although it draws heavily on symbiosis ideas first put forward by mid-19th century scientists and by Merezhkovsky (1905) and Ivan Wallin (1920) in the early-20th century, Margulis's endosymbiotic theory formulation is the first to rely on direct microbiological observations (as opposed to paleontological or zoological observations which were previously the norm for new works in evolutionary biology). Weathering constant criticism of her ideas for decades, Margulis is famous for her tenacity in pushing her theory forward, despite the opposition she faced at the time.