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Mammoths Mitochondrial Genome Deciphered
March 14, 2006 15:20

Report about an outstanding scientific achievement of Russian scientists is published in PloS Biology magazine: they have deciphered full sequence of mitochondrial DNA of the mammoth, who lived 30 000 years ago.

Many researchers have tried to extract DNA from mammoths' bones and soft tissues found in permafrost. They mainly succeed in extracting very short fragments of DNA, which are almost impossible to be combined in longer fragments. German scientists have recently reported about deciphering 5000-nucleotide-long mammoth DNA fragment, combined from numerous short blocks.

Professor Evgeniy Rogaev and his colleagues from Russia and USA achieved some weighty results: they succeeded in deciphering full sequence of mammoth's mitochondrial DNA, which contains 16842 nucleotides. In their studies scientists used perfectly conserved mammoth's leg, which was found in 1986 in Chukotka permafrost in the valley of river Enmynveem. Frozen mammoth was conserved well enough to allow scientists distinguishing nuclear (chromosomal) DNA in cell nuclei. It was the first time ever nuclear DNA was discovered in such an ancient animal. The results of radiocarbon analysis showed that Chukchi mammoth lived 32 850 +/- 900 years ago. Other mammoths, from which DNA was extracted, are much younger.

May be nuclear DNA of this mammoth will also be deciphered, but today we have only mitochondrial DNA sequence. Mitochondrial DNA is very convenient for evolutional reconstruction, because it transfers through mother lines and never recombinates (because father's genes never mix with mother's genetic material). Moreover, mitochondrial DNA has much more copies in the cell than nuclear DNA, which is essential for genetic analysis of ancient animal fossils.

Scientists extracted unusually long mitochondrial DNA fragments from mammoth's leg - up to 1200-1700 nucleotides long. Two laboratories, Russian and American ones, performed independent DNA extraction, sequencing (defining nucleotide sequence) and "assembly" of mitochondrial genome from sequenced fragments. Usual assembly procedure required overlapping of sequenced fragments, moreover its accuracy increased with overlapping growth. In said case overlapping was about 30 nucleotides, sometimes reaching 200 nucleotides. Every fragment of mitochondrial genome was sequenced minimum 9 times in Russian laboratory and 5 times in American one. All their work resulted in nucleotide construction, which exceeded previous deciphering of mammoth mitochondrial DNA nucleotide sequences in accuracy and safety.

Scientists compared mammoth's mitochondrial DNA with mitochondrial DNA of modern elephants and confirmed existing hypothesis of German scientists, stating close relations of mammoths rather with Indian elephants than with African ones. Lines of Indian and African elephants have separated 6 million years ago. Separation time of the first line (mammoth and Indian elephant) was measured according incomplete mitochondrial DNA sequence analysis and was about 5.5 million years; data of Russian scientists show that separation happened about 4 million years ago.

For the first time Russian scientists estimated genetic variability of Siberian mammoths. They have compared all existing sequenced fragments of mitochondrial DNA of mammoths, who lived in various regions of Siberia about 32-33 and 12 thousand years ago and concluded that mammoth population, which lived in Siberia in late Pleistocene, was genetically homogeneous. Modern populations of Indian and African elephants are much more polymorphous (which means they have more variations in mitochondrial DNA nucleotide sequence). Unlike modern elephants, Siberian mammoths might have had simple population structure, which looked like a large family without separate clans or subpopulations.


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