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.

Aubrey David Nicholas Jasper de Grey (born 20 April 1963) is an English author and theoretician in the field of gerontology, and the Chief Science Officer of the SENS Foundation. He is editor-in-chief of the academic journal Rejuvenation Research, author of The Mitochondrial Free Radical Theory of Aging (1999) and co-author of Ending Aging (2007). He is perhaps best known for his view that human beings could, in theory, live to lifespans far in excess of that which any authenticated cases have lived to today.

De Grey's research focuses on whether regenerative medicine can thwart the aging process. He works on the development of what he calls "Strategies for Engineered Negligible Senescence" (SENS), a tissue-repair strategy intended to rejuvenate the human body and allow an indefinite lifespan. To this end, he has identified seven types of molecular and cellular damage caused by essential metabolic processes. SENS is a proposed panel of therapies designed to repair this damage.

De Grey is a fellow of the Gerontological Society of America, the American Aging Association, the Institute for Ethics and Emerging Technologies, and an advisor to the Singularity Institute for Artificial Intelligence. He has been interviewed in recent years in a number of news sources, including CBS 60 Minutes, the BBC, The New York Times, Fortune Magazine, The Washington Post, TED, Popular Science, The Colbert Report, Time, and the Skeptics' Guide to the Universe.