More than two decades after the draft of the human genome set a scientific milestone, scientists have finally completed the work. The first complete, gapless sequence of the human genome has been published, paving the way for new insights into health and what makes our species unique.
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Dr Karen Miga, a scientist at the University of California, Santa Cruz, who led the international consortium behind the project, said: "These parts of the human genome that we haven't been able to study for 20-plus years are important to our understanding of how the genome works, genetic diseases and human diversity and evolution."
Until now, about 8% of the human genome was missing, including large sections of highly repetitive sequences, sometimes described as “junk DNA.” In reality, however, these repetitive sections were omitted due to technical difficulties in sequencing them, rather than out of pure lack of interest.
Sequencing a genome is a bit like slicing a book into text fragments and then trying to reconstruct the book by putting them back together. Text fragments containing many common or repeated words and phrases would be harder to put back together than those containing unique pieces of text. New “long-read” sequencing techniques that decode large chunks of DNA at once—enough to capture many repeats—have helped overcome this hurdle.
Scientists were able to further simplify the puzzle by using an unusual type of cell that contains only DNA inherited from the father (most cells in the body contain two genomes, one from each parent). Together, these two advances allowed them to decode the more than 3 billion letters that make up the human genome.
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“In the future, when someone submits their genome sequence, we will be able to identify all the variations in their DNA and use that information to better guide their health care,” said Dr. Adam Filippi, of the National Human Genome Research Institute in Maryland and chairman of the consortium. “Completing the human genome sequence was really like putting on a new pair of glasses. Now that we can see everything clearly, we are one step closer to understanding what it all means.”
One area of interest is that parts of the genome with many repeat stretches include those where most of the human genetic variation is located. Variability in these regions may also provide crucial clues to how our ancestors underwent rapid evolutionary changes that led to more complex cognition.
The work is also likely to lead to a better understanding of enigmatic components of the genome known as centromeres. These are dense bundles of DNA that hold chromosomes together and play a role in cell division, but until now were considered unmapped because they contain thousands of segments of DNA sequence that are repeated over and over again.
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The science behind the sequencing effort and some initial analysis of the new genome regions are described in six papers published in the journal Science.
"By opening up these new parts of the genome, we believe there will be genetic variation that contributes to many different traits and disease risk," said Rajiv McCoy, of Johns Hopkins University and a participant in the Telomere to Telomere (T2T) consortium.
