Researchers have made a discovery by unearthing fossil chromosomes from a 52,000-year-old woolly mammoth skin sample preserved in Siberian permafrost. This find was published in the journal Cell.
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Approximately 50,000 years ago, a woolly mammoth perished on the frozen tundra of Siberia. In its well-preserved skin samples, researchers discovered chromosomes in their original 3D configuration.
Scientists previously believed that the 3D structure of DNA could not survive over such extensive periods. This research overturns that assumption showing the potential for ancient DNA to retain its complex spatial organization.
The study is the first instance of reconstructing the 3D structure of an ancient genome revealing critical aspects of the mammoth’s cell biology and gene activity.
By analyzing the spatial arrangement of the woolly mammoth’s DNA, researchers identified active genes including those responsible for the animal’s characteristic woolly fur.
Ancient DNA typically exists in fragmented forms making it challenging to reconstruct a genome. The mammalian genome is vastly larger than typical ancient DNA fragments.
The discovery of nearly intact chromosomes which are longer than most ancient DNA fragments.
The research team conducted a nine-year search for well-preserved ancient samples leading to the discovery of the mammoth skin sample from Siberian permafrost.
The team described the discovery as mind-blowing, given the assumptions about DNA degradation over time.
Researchers proposed that the woolly mammoth’s chromosomes were preserved in a glass-like state due to a dehydration process similar to freeze-drying.
This process resulted in densely packed, immobile DNA molecules. Experiments with freeze-dried beef jerky subjected to extreme tests confirmed the theory, as the jerky shattered like glass while its chromosomes remained intact.
Comparisons with modern elephants, the closest living relatives of mammoths showed similarities in chromosome number and structure but differences in gene activity related to hair growth and cold adaptation.
The woolly mammoth skin was excavated in 2018 near Belaya Gora in northeastern Siberia. The permafrost’s freezing temperatures preserved the chromosomes in detail.
Unlike previous ancient DNA samples which were typically fragmented, these fossil chromosomes contained millions of genetic code letters.
The fossil chromosomes described as non-mineralized fossils or subfossils retained their 3D structure closely resembling their original state in the living woolly mammoth.
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The preservation quality was sufficient to assemble the genome of the extinct woolly mammoth. This level of preservation allowed researchers to see which individual genes were active.
Lieberman Aiden, a professor of molecular and human genetics at Baylor College of Medicine addressed that fossil chromosomes had never been found before.
Although freezing temperatures in Arctic regions slow down DNA degradation, it was surprising to find that the mammoth’s DNA retained its structure over 52,000 years.
Researchers simulated the preservation process by experimenting on beef jerky, finding that DNA structures could withstand stress and damage while remaining intact at the nano-scale.
For the first time, researchers determined that the woolly mammoth had 28 pairs of chromosomes similar to modern elephants.
The study revealed which genes were active in the mammoth, providing clues about traits like woolly fur and other adaptations.
An international team of scientists from labs in Houston, Copenhagen, Barcelona and other locations worked together on this study pooling their expertise and resources.
The team employed experimental techniques to probe the 3D architecture of the mammoth’s genome building on previous advancements in genomic mapping.
The researchers faced numerous challenges including high contamination rates and difficulties in obtaining viable samples.
Despite many failed attempts the team persisted, succeeding with the well-preserved mammoth sample.
The study utilized the “Hi-C” technique adapted for ancient specimens as “paleoHi-C” to map the 3D structure of the woolly mammoth’s genome.
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