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Did You Know?
In Did You Know? the National Geographic magazine team shares extra information we gathered to expand your knowledge of our featured subjects.

Scientists who trace modern human origins often talk about a special kind of DNA, mitochondrial DNA (mtDNA). But do you know what mitochondrial DNA is? And do you know what mitochondria do? Mitochondria are tiny structures found within the cell that resemble grains of rice. They produce the energy that cells need to function, and they have their own small complement of DNA, containing just 37 genes in humans. Compare this to the human nuclear genome (the entire set of genetic instructions found within the nuclei of all the cells), which contains some 30,000 genes. But mitochondrial DNA is especially powerful for studying the ancestry of modern humans. While the nuclear genome is reshuffled with each new generation as the father's and mother's DNA recombine, mitochondrial DNA is passed directly from a mother to her children. As a result it preserves patterns of ancient markers that thelp scientists map the path modern humans have taken around the world.

—Alice J. Dunn