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Mitochondrial DNA is found in the mitochondria and is believed to have originated from ancient bacteria. It is smaller than nuclear DNA and only undergoes mutation, making it a useful genetic marker for studying human ancestry and migration patterns. Analysis of mitochondrial DNA supports the Out-of-Africa theory and rules out the multiregional hypothesis.
Mitochondrial DNA are small loops of DNA found within the organelles in the cell, the mitochondria. Mitochondria serve as the cell’s “powerhouse” and are believed to be descendants of ancient bacteria that participated so symbiotically with ancient cells that they integrated into them as organelles. The vast majority of DNA in all animals is found in the nucleus and is known as nuclear DNA, while mitochondrial DNA is the only DNA located outside the nucleus.
Unlike the DNA in the nucleus, which contains about 20,000 protein-coding genes over 3 billion base pairs, mitochondrial DNA is relatively small, consisting of only 13 protein-coding genes over 15,000-17,000 base pairs. Unlike nuclear DNA, which consists of linear DNA, mitochondrial DNA is found in ring format, just like the DNA found in bacteria. Genes in mitochondria express proteins that help build mitochondria, although over billions of years of evolution, nuclear DNA has taken over most of mitochondrial DNA’s roles in building mitochondria.
Unlike nuclear DNA, which undergoes recombination from generation to generation due to the sex between a male and female, mitochondrial DNA only undergoes recombination with DNA from the same organelle, greatly limiting genetic change. Therefore, the only factor that introduces genetic changes is mutation, rather than mutation plus recombination, as is the case with nuclear DNA. This makes mitochondrial DNA a very useful genetic marker that can be used to compare different lineages, including different human lineages.
Analysis of human mitochondrial DNA has proved crucial in determining the ancestry of various human groups and the early migration patterns of humans around the world. These studies supported the Out-of-Africa theory, the theory that the human species originated in Africa about 250,000 years ago, then spread around the world. The main competing theory is the multiregional hypothesis, which assumes that different human races around the world evolved independently from hominid precursors. Due to the evidence from mitochondrial DNA analysis, this hypothesis has essentially been ruled out.
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