DNA with preserved spatial structure was found in Siberian mammoths.
A large group of researchers from the University of Copenhagen, Rice Universitythe Institute of Molecular and Cellular Biology of the Siberian Branch of the Russian Academy of Sciences and other scientific centers write in Cellthat they managed to study the chromosomes of a woolly mammoth that had lain in Siberian permafrost for 52,000 years. We are not just talking about DNA, but about chromosomes that were frozen in their lifetime spatial configuration.
As we know, ancient DNA – that is, DNA from prehistoric bones of people and animals, and not only from bones, but even from soil – has long been read. Reading such DNA has given many discoveries, including those concerning mammoths. In a living cell, some genes are active, while others, on the contrary, are in the archive. Turning genes on and off implies, firstly, a shuffling of proteins in the corresponding sections of DNA: some proteins open them to other proteins for reading genetic information. Secondly, DNA itself contains many different regulatory sequences that determine the activity of genes. Regulatory sequences are sometimes quite far from those they regulate, and in order to manifest themselves, they must come closer to them. Inside the nucleus, DNA strands wander, loop and straighten, large and small blocks of chromosomal DNA are dragged by proteins from one zone of the nucleus to another. And so, in order to understand the molecular genetic details of cellular life, it is necessary to know not only the genomic text, but also a three-dimensional map of chromosomes inside the nucleus – this map will show which parts of which chromosomes are very close together, which are not, which, on the contrary, are far from each other, etc.
There are appropriate research methods, one of which, the so-called Hi-C method, we have mentioned several times. But in order to study the spatial organization of chromosomes in a cell, these chromosomes must be intact. DNA, for all its strength, breaks over time, disintegrates into pieces that are on average a million times smaller than whole, undamaged chromosomal DNA (chromosomes can be very different, but we are talking about average values now). When they talk about reading ancient DNA, they mean reading pieces like these: a mass of short read sequences is obtained, which are then combined into a genome using powerful computing algorithms. For three-dimensional mapping of chromosomes, the fragments into which ancient DNA disintegrates are simply useless.
A mammoth leg recovered from Siberian permafrost. (Photo: Love Dalén / Stockholm University)
However, in the skin of one of the mammoths, it was possible to find whole chromosomes, in which it was visible which areas were archived by proteins, which were open for reading, and how different zones of the chromosomes were located relative to each other. The division of the genome into active and inactive zones resembled the same division in the Asian elephant (the number of chromosomes in the woolly mammoth and modern elephants is the same – 28 pairs). There were also differences: in particular, the gene responsible for the appearance of fur was clearly more active in the mammoth (which was to be expected). Similar experiments were conducted with samples from another mammoth that lived about 39 thousand years ago and was preserved in a similar way – they also contained whole chromosomes.
The researchers explain the preservation of the chromosomes by the fact that the area around the mammoth carcass was not only very cold, but also very dry. Dehydration with cooling brought the contents of the cell nuclei into a glassy state – in this form, the chromosomes not only retained their spatial configuration, they were inaccessible to microbes that could destroy them with their enzymes. Similar conditions were tried to be reproduced in the laboratory with a bull’s liver, and everything turned out as with the mammoth – in the sense that the chromosomes in the liver cells froze in the form in which they were during life. Perhaps these two mammoths are not the only ones who were lucky with the storage conditions, and in the future, we will find fossil chromosomes of other ancient creatures.
Source: www.nkj.ru
