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Frozen mammoth skin retained its chromosome structure

11 July 2024 at 17:58
Artist's depiction of a large mammoth with brown fur and huge, curving tusks in an icy, tundra environment.

Enlarge (credit: LEONELLO CALVETTI/SCIENCE PHOTO LIBRARY)

One of the challenges of working with ancient DNA samples is that damage accumulates over time, breaking up the structure of the double helix into ever smaller fragments. In the samples we've worked with, these fragments scatter and mix with contaminants, making reconstructing a genome a large technical challenge.

But a dramatic paper released on Thursday shows that this isn't always true. Damage does create progressively smaller fragments of DNA over time. But, if they're trapped in the right sort of material, they'll stay right where they are, essentially preserving some key features of ancient chromosomes even as the underlying DNA decays. Researchers have now used that to detail the chromosome structure of mammoths, with some implications for how these mammals regulated some key genes.

DNA meets Hi-C

The backbone of DNA's double helix consists of alternating sugars and phosphates, chemically linked together (the bases of DNA are chemically linked to these sugars). Damage from things like radiation can break these chemical linkages, with fragmentation increasing over time. When samples reach the age of something like a Neanderthal, very few fragments are longer than 100 base pairs. Since chromosomes are millions of base pairs long, it was thought that this would inevitably destroy their structure, as many of the fragments would simply diffuse away.

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DNA from mammoth remains reveals the history of the last surviving population

29 June 2024 at 11:15
A dark, snowy vista with a single mammoth walking past the rib cage of another of its kind.

Enlarge / An artist's conception of one of the last mammoths of Wrangel Island. (credit: Beth Zaiken)

A small group of woolly mammoths became trapped on Wrangel Island around 10,000 years ago when rising sea levels separated the island from mainland Siberia. Small, isolated populations of animals lead to inbreeding and genetic defects, and it has long been thought that the Wrangel Island mammoths ultimately succumbed to this problem about 4,000 years ago.

A paper in Cell on Thursday, however, compared 50,000 years of genomes from mainland and isolated Wrangel Island mammoths and found that this was not the case. What the authors of the paper discovered not only challenges our understanding of this isolated group of mammoths and the evolution of small populations, it also has important implications for conservation efforts today.

A severe bottleneck

It’s the culmination of years of genetic sequencing by members of the international team behind this new paper. They studied 21 mammoth genomesβ€”13 of which were newly sequenced by lead author Marianne Dehasque; others had been sequenced years prior by co-authors PatrΓ­cia PečnerovΓ‘, Foteini Kanellidou, and HΓ©loΓ―se Muller. The genomes were obtained from Siberian woolly mammoths (Mammuthus primigenius), both from the mainland and the island before and after it became isolated. The oldest genome was from a female Siberian mammoth who died about 52,300 years ago. The youngest were from Wrangel Island male mammoths who perished right around the time the last of these mammoths died out (one of them died just 4,333 years ago).

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