First-ever fossilised ancient chromosomes discovered in 52,000-year-old woolly mammoth skin

In a first of its kind, fossil chromosomes have been discovered in a piece of woolly mammoth skin that was excavated from the Siberian permafrost, according to a recent study.

The 52,000-year-old remains were discovered in 2018 close to the northeastern Siberian settlement of Belaya Gora. The chromosomes, which are small thread-like structures that contain genetic material, or DNA, were preserved in perfect detail by the cold temperatures in this area.

Although samples of ancient DNA have been discovered previously, they are usually very fragmented and only contain a few hundred letters of genetic code. Millions of chromosomes can be found in fossils, providing a significantly more comprehensive image of an animal's genetic makeup.

Erez Lieberman Aiden, a professor of molecular and human genetics at the Baylor College of Medicine and co-corresponding author of the study published on Thursday in the journal Cell, stated, "Fossil chromosomes have never been found before."

According to earlier research, the pieces lacked a coherent framework as well, Lieberman Aiden continued. "The fragments are arranged here in a clear three-dimensional fashion, just as they were in the original mammoth chromosomes."

Assistant professor of molecular and human genetics at Baylor College of Medicine and co-first author Olga Dudchenko says that the chromosomes, which the researchers point out are "non-mineralized fossils, or subfossils," are in a state of preservation good enough to assemble the genome, or the sum of all the genetic material, of an extinct species.

Dudchenko, a senior investigator at Rice University's Centre for Theoretical Biological Physics, stated, "We strongly believe that this is not going to apply to just the mammoth or this particular mammoth but is basically opening up a new field that has tremendous potential

Diffusion of DNA

 
According to Kevin Campbell, a professor of environmental and evolutionary physiology at the University of Manitoba in Canada who was not involved in the study, DNA is arranged in distinct and specific 3D structures within different cell types that provide insight into the unique properties or traits of that cell type.

He said that after death, bodily cells break down rapidly, and this three-dimensional structure disappears in a few days at most. Due to the cold temperatures in the Arctic, the degradation of DNA is slowed down in animals like woolly mammoths, but it still occurs. Over extended periods of time, the DNA is predicted to lose its structure and biological characteristics.

 In an email, Campbell stated, "However, this study is the first to demonstrate that this is not necessarily always the case."

According to Dudchenko, "DNA is a very, very long molecule that gets chopped into shorter pieces once it sits there after an animal dies."

As for the individual parts, Dudchenko stated, "you normally expect them to start shifting in respect to each other and just kind of float away, losing any organisation that was there." "But it's evident from this sample that that didn't occur."

Food experts are familiar with how to stop this loss of structure, which they refer to as diffusion. They also mentioned that the process is not unlike from that of manufacturing beef jerky.

to combine cooling and dehydration to have something that is shelf stable for an extended period of time," the expert stated. "Food that is shelf stable but isn't canned is most likely in a state of arrest of diffusion."

The circumstances surrounding the death of the mammoth from which the skin sample was taken may have been ideal for initiating this process spontaneously. As per Dudchenko, "(The carcass) could have spontaneously undergone the same procedure that we currently use commercially all the time, right now we could read those pieces of chromosomes off 52,000 years after the fact by removing substantial amounts of water, arresting diffusion inside and locking those pieces in place."

Nevertheless, the DNA was not entirely intact despite its excellent preservation. In an email, Aiden stated, "Each chromosome, which was once one DNA molecule, has fragmented into millions of DNA molecules." However, even at the nanoscale scale, the molecules have not shifted all that much, which is why we refer to it as a fossil chromosome.

According to Lieberman Aiden, if this sample were a book, the binding would have been lost, leaving a significant number of loose pages—that is, DNA fragments. Diffusion can be compared to the wind carrying away pages and preventing them from being rearranged. However, in this example, the pages did not fly away; instead, they stayed in a tidy stack, exactly as they had been before the binding was lost.

Taking aim at beef jerky

 
Through trials on beef jerky, the researchers were able to verify this idea of preservation by determining the extent to which the meat snack could be mistreated before the chromosomes within it lost their structure.

As co-first author of the study and researcher at Rice's Centre for Theoretical Biological Physics and Baylor's Centre for Genome Architecture, Dr. Cynthia Pérez Estrada said, "We decided to test how far this beautiful molecule can resist stress and harm by having one of the Houston Astros pitchers throw a fastball and fire a shotgun at it."

“The beef jerky was breaki

breaking more and more, indicating to us that DNA is very resistant, especially in this kind of glass-like state (like in the sample), where the molecules are essentially frozen and acting like crystals," Pérez Estrada continued.

The researchers were able to ascertain for the first time that the woolly mammoth shared 28 pairs of chromosomes with current elephants because to the recently uncovered genetic information revealed in the skin samples.

However, the structure made it possible for them to go one step further and determine which specific genes the animal was using. According to Dudchenko, "we have some ideas thanks to the way that these chromosomes were preserved." "Everyone wants to know what exactly made it woolly."

CNN's Jacopo Prisco

source edition .cnn.com

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