From end to end
Scientists have completely deciphered the X chromosome
Maria Krivochenko, Naked Science
The first complete assembly of the human chromosome from beginning to end was announced by a group of scientists from the National Institute for Human Genome Research in the USA. She was able to close all the existing gaps and repaired the gap in the centromere, a repeating section of DNA in the center of the chromosome. The results of the work are published in an article in the journal Nature (Miga et al., Telomere-to-telomere assembly of a complete human X chromosome).
"This achievement begins a new era in genomics research," he said. Eric Green, MD and Director of the Institute. "The ability to create truly complete sequences of chromosomes and genomes is a technical achievement that will help us gain a complete understanding of the functions of the genome and use the information it carries in medicine."
The authors chose the X chromosome for their work because it is associated with many serious diseases, including hemophilia, chronic granulomatous disease and Duchenne muscular dystrophy. They used the cells of a vesicular drift – a defective embryo that develops in an egg without chromosomes, fertilized by two spermatozoa. Such samples have two identical DNA sequences. This allows you to get more data than studying one male X chromosome, and avoid inaccuracies that may arise when analyzing two female ones.
Decoding has become possible thanks to new sequencing technologies based on the use of nanopores. They allow ultra-long sequences to be read and leave DNA molecules largely intact. After analyzing the human chromosome, the group assembled the resulting chains into one. As a result, they achieved a reading accuracy of 99.995%.
The modern reference model of the human genome is the most complete ever created for vertebrates, nevertheless there are still too many gaps in it. The first genome survey was completed back in 2000, but over the next 20 years of constant research, not a single chromosome could be deciphered to the end. Gaps remain, which most often contain repeating DNA segments. They are difficult to count, but they may be related to human health and diseases.
The genome is so long that sequencing machines cannot read it in its entirety. Instead, the researchers cut it into smaller pieces and then analyze each one to get short sequences that connect together. It is not always possible to do this correctly, and some of the information is lost during the assembly process.
Lead author Adam Phillippi compared this problem to assembling a puzzle. "You work with small fragments and try to understand where they came from. Some of them do not contain any clues at all, for example, fragments with a blue sky," he said. "The same is true with sequencing the human genome: until now, the pieces were too small and it was not possible to put the most difficult pieces of the puzzle together."
The group believes that the study will seriously advance the study of the human genome. She is going to create a complete sequence of it by the end of 2020, if there are no problems with decoding the first and ninth chromosomes, in which the repeating DNA segments are much larger than in the X chromosome.
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