First discovered Mumming-Chromosomes
Researchers have only been studying small fragments of ancient DNA until now. But they have now discovered chromosomes in the hide of a Woolly Mammoth, which contain more than a million letters. The ancient genetic material remains have been preserved in a "glass-like" state.
In the 52,000-year-old remains of a Woolly Mammoth, a research group has found fossil chromosomes that are significantly larger than all previously known forms of ancient DNA. This is likely due to the Siberian permafrost, which kept the hide with the chromosomes frozen and dry.
When people think of fossils, most likely imagine fossilized bones or insects preserved in amber. However, the traces of the past can also be much smaller units of life - namely, when it comes to fossil DNA.
The international research group has now discovered fossilized chromosomes in the hide of a Woolly Mammoth, which was found in Siberia in 2018 - after 52,000 years. This find is particularly noteworthy because genetic material traces usually only survive in the form of small DNA fragments.
Entire chromosome structure preserved
The now-described fossil chromosomes are much larger than previously known forms of ancient DNA. "We already knew that small fragments of ancient DNA could survive over long periods of time," says co-author Marcela Sandoval-Velasco from the University of Copenhagen in a statement. "But what we have found here is a sample where the three-dimensional arrangement of these DNA fragments was frozen for decades, allowing the structure of the entire chromosome to be preserved."
A chromosome of this type reveals significantly more about the extinct organisms that once inhabited the Earth than individual DNA fragments, which are rarely longer than a few hundred letters of the genetic code, and thus much smaller than the complete genetic sequence of organisms with billions of letters. In contrast, fossil chromosomes can contain hundreds of millions of genetic letters.
Comparing these fossil sequences with the DNA of modern species allows the changes in the genetic code over the course of evolution to be traced. "Fossil chromosomes are a turning point," says co-author Olga Dudchenko. "If you know the shape of an organism's chromosomes, you can reconstruct the entire DNA sequence of extinct organisms. This provides insights that were previously impossible."
Exciting counting up to 28
Among the extinct organisms are also Woolly Mammoths, whose chromosomes the research group is now studying. "We found out that they had 28 chromosome pairs, which is very logical since modern elephants have the same number, and they are the closest living relatives of the Woolly Mammoth," says co-author Juan Antonio Rodríguez. "It was extremely exciting to be able to count the chromosomes of an extinct animal for the first time."
The analysis of the fossil chromosomes also revealed many structural features of modern chromosomes, including tiny so-called chromatin loops, which are important for the organization and function of the genetic material in cells. "The survival of these loops in these ancient chromosomes is perhaps the most impressive thing," comments co-author Marc Martí-Renom. "DNA loops that are only 50 nanometers in size are important because they bring activating DNA sequences close to their targets. These fossils show us not only which genes were active but also why."
## Preserved like Tortilla-Chips and Jerky
According to the research group, the fossil chromosomes have survived for thousands of years due to the fact that they were in a "glass-like" state. Such a state is also used to make foodstuffs preservation-ready, namely through a combination of cooling and drying. The result are foodstuffs like Tortilla-Chips and Jerky, which are brittler than the original food, but last much longer. The research team rather vaguely explains that the chromosomes were essentially encapsulated in freeze-dried Woolly Mammoth Jerky.
To test their theory on the astonishing survival of the chromosomes, the group conducted some innovative experiments with freeze-dried sausages, as described by co-author Cynthia Pérez Estrada. "We shot at it with a shotgun. We ran over it with a car. A former pitcher for the Houston Astros threw a fastball at it", Pérez Estrada lists. Each time, the freeze-dried sausage shattered into tiny glass-like pieces. "But on the nanoscale, the chromosomes were intact and unchanged", the scientist explains. "That's why these fossils have survived, having been around for 52,000 years and waiting to be discovered by us."
This groundbreaking discovery of fossil chromosomes in the Woolly Mammoth's hide has significant implications for our understanding of ancient heritage. For the first time, scientists have encountered a well-preserved chromosome structure with over a million genetic letters, which is a testament to the preserving effects of Siberian permafrost. The ancient Mammoth-Chromosomes could potentially unlock new insights into the evolutionary history of extinct species, including Woolly Mammoths, by allowing researchers to compare their genetic sequences with those of modern organisms.
