The Gordian node of the X chromosome
Scientists have solved the main "female" mystery of the human genome
Geneticists have found out how the "extra" female X chromosome folds into the so-called Barr body and remains inactive in the body of healthy women and female mammals, according to an article published in the journal Proceedings of the National Academy of Sciences (Darrow et al., Deletion of DXZ4 on the human inactive X chromosome alters higher-order genome architecture).
Humans and all other mammalian species have a X-system for determining sex – males and males have one "female" X chromosome and a "male" Y chromosome, and females and females have two X chromosomes.
One of the main genetic mysteries of life is how the body "learns" about the presence of an extra copy of the X chromosome and turns off one of them, turning it into a so-called Barr body – a tightly twisted "bundle" of DNA, clearly visible under a microscope.
Miriam Huntley from Harvard University (USA) and her colleagues found out what makes an extra X chromosome curl into a ball and how its shape can be flexibly controlled by studying the structure of the Barr body at the molecular level.
Observing the structure of this tangle of DNA with the help of computer modeling technologies, scientists found out that the genome strands in the Barr body are braided into giant loops containing tens of millions of "letters"-nucleotides. If you look at this tangle in three-dimensional form, the threads in these loops were located very close to each other, despite the fact that in the "two-dimensional" projection they were separated by hundreds of thousands and millions of nucleotides.
These hyper-loops, as scientists call them, were woven into an extremely complex tangle thanks to one simple thing – small fragments of the "meaningless", as previously thought, DXZ4 genetic code, copies of which were inserted into those parts of the X chromosome that, when it was "disconnected", became the tips of DNA loops. Similar loops and DXZ4 sites, according to geneticists, are present not only in the human genome, but also in mice and monkeys.
When scientists removed this section from DNA with the help of the universal genomic editor CRISPR/Cas9, the structure of the tangle was broken, and it actually fell apart, multiplying in volume. This was due to the fact that the protein cohesin, which weaves DNA in the Barr body, did not find DXZ4 tags on the surface of the "off" X chromosome.
Violations in its work, according to scientists, can cause those rare problems that occur in women whose "extra" X chromosome is not completely turned off. These include autism, Rett's disease and a number of other mental development disorders.
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19.07.2016