Mitochondrial DNAA Rapidly Ticking Clock

Although mitochondria contain circular molecules of DNA reminiscent of bacterial chromosomes, the mitochondrial genome is much smaller. The mitochondrial DNA codes for a few of the proteins and ribosomal RNA of the mitochondrion, but most components are now encoded by the eukaryotic nucleus as already discussed in Chapter 19. What concerns us here is that the mitochondrial DNA of animals accumulates mutations much faster than the nuclear genes. In particular, mutations accumulate rapidly in the third codon position of structural genes and even faster in the intergenic regulatory regions. This means that mitochondrial DNA can be used to study the relationships of closely related species or of races within the same species. Most of the variability in human mitochondrial DNA occurs within the D-loop segment of the regulatory region. Sequencing this segment allows us to distinguish between people of different racial groups.

One apparent drawback to using mitochondrial DNA is that mitochondria are all inherited from the mother. Although sperm cells do contain mitochondria, these are not released during fertilization of the egg cell and are not passed on to the descendants. On the other hand, analysis of mitochondria gives an unambiguous female ancestry, as complications due to recombination may be ignored. Furthermore, a eukaryotic cell contains only one nucleus but has many mitochondria so there are often thousands of copies of the mitochondrial DNA. This makes extraction and sequencing of mitochondrial DNA easier from a technical viewpoint.

Mitochondrial DNA can sometimes be obtained from museum samples and extinct animals. Mitochondrial DNA extracted from frozen mammoths found in Siberia differed in four to five bases out of 350 from both Indian elephants and African elephants. The DNA analysis supports the three-way split proposed based on anatomical relationship. The quagga is an extinct animal, similar to the zebra. It grazed the plains of Southern Africa only a little over a hundred years ago. A pelt preserved in a

German museum has yielded muscle fragments from which DNA has been extracted and sequenced. The two gene fragments used were from the quagga mitochondrial DNA. The DNA from the quagga differed in about 5 percent of its bases from the modern zebra. The quagga and mountain zebra are estimated from this to have had a common ancestor about three million years ago.

DNA has also been successfully extracted from Egyptian mummies. Although the amounts of DNA obtained are only 5 percent or so of those from fresh, modern, human tissue, DNA sequences have been obtained from a mummy 2,400 years old. Although several thousand base pairs were sequenced, no actual human genes were identified. Since the DNA of higher animals consists mostly of non-coding sequences, this is hardly surprising. Nonetheless, the mummy DNA did contain Alu elements that are characteristic of human DNA.

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