Mitochondria are cellular organelles in eukaryotic cells. They convert chemical energy from food into a form that cells can use, adenosine triphosphate (ATP). Mitochondrial DNA is only a small part of the DNA in a eukaryotic cell. Most of the DNA is in the cell nucleus on chromosomes. In plants, the chloroplasts have DNA as well.
In humans, mitochondrial DNA codes for 37 genes and with about 16,600 base pairs in a circle. Human mitochondrial DNA was the first significant part of the human genome to be sequenced. The mtDNA in plants is much larger: Arabidopsis has 367 kilobytes.
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Electron microscopy reveals mitochondrial DNA in discrete foci. Bars: 200 nm. (A) Cytoplasmic section after immunogold labelling with anti-DNA; gold particles marking mtDNA are found near the mitochondrial membrane. (B) Whole mount view of cytoplasm after extraction with CSK buffer and immunogold labelling with anti-DNA; mtDNA (marked by gold particles) resists extraction. From Iborra et al., 2004.
Mitochondrial DNA Facts for Kids. Kiddle Encyclopedia.