More than 20 years ago, scientists announced that they had sequenced the entire human genome in all its glory - a major and historic breakthrough that was hailed across the world.
However, 8 per cent of the genome remained unmapped and unexplored due to the limitations of technology. These millions of DNA fragments were dismissed by some as “junk,” serving no clear function.
Now, a large international team of scientists from the National Institutes of Health, in the US, have filled this missing gap and revealed the composition of the final eight per cent of the human genome.
“It’s a big deal,” said Professor Erich Jarvis, a co-author of the study who helped develop a number of new techniques central to unlocking the final pieces. “Every single base pair of a human genome is now complete.”
It was first announced last year that the human genome had been completed, but it is only now, in a paper published in the academic journal Science, that the details and significance of the long-missing chunks of our genome are crystallising into view.
More than just “junk,” within the last eight per cent are mysterious pockets of non-coding DNA that do not make protein, but still play crucial roles in many cellular functions. Scientists believe these segments of the genome may lie at the heart of conditions in which cell division runs amok, such as cancer.
“You would think that, with 92 per cent of the genome completed long ago, another eight percent wouldn’t contribute much,” said Prof Jarvis. “But from that missing eight per cent, we’re now gaining an entirely new understanding of how cells divide, allowing us to study a number of diseases we had not been able to get at before.”
The human genome is made up of billions of ‘letters’, otherwise known as nucleotide bases, that are paired together in different combinations and, together, provide genetic instructions for making the human body.
This coding determines everything about us, from the way we look - eye and hair colour, height, the size of our nose - to our health and predisposition for certain illnesses. Knowing the full breadth of the human genome can therefore open new doors to better understanding the human form.
Dr Adam Phillippy, a co-author of the research, said: “In the future, when someone has their genome sequenced, we will be able to identify all of the variants in their DNA and use that information to better guide their healthcare.
“Truly finishing the human genome sequence was like putting on a new pair of glasses.
“Now that we can clearly see everything, we are one step closer to understanding what it all means.”
The majority of the human genome is rich in DNA, loosely packaged, and busy making molecules that will later be translated into protein. This was sequenced by the Human Genome Project, which delivered its results around the turn of the millennium.
Left untouched, however, was a labyrinth of tightly wound genetic material - a smaller portion of the genome, which does not produce protein.
This missing 8 per cent was the hardest to sequence region, full of repeating letters that were simply impossible to read with the technology of the time.
Then, about 10 years ago, scientists began developing new techniques for producing longer sequence reads that filled in gaps in the genomes of humans and other species.
Other key advances included rapid improvements in Oxford University’s gene sequencing machines that can accurately read a million letters of DNA at a time. The machines ran non-stop for six months - with scores of scientists assembling the pieces and analysing the results.
With updated tools and renewed resolve, Prof Jarvis and other scientists involved in the research were able to help finish what the Human Genome Project started and describe, at long last, a truly complete human genome.
Co-author Professor Evan Eichler, of the University of Washington, said: “We are seeing chapters that were never read before.
“It turned out many of the regions I was interested in were in the gaps.”
Dr Philliphy said: “It is a whole new treasure chest of variants we can study to see if they have functional significance.”
