Washington:
Our DNA is actually less human than previously thought, according to a
new study that found 19 new pieces of non-human DNA left by viruses in
our ancestors hundreds of thousands of years ago.
One stretch of newfound DNA, in about 50 of the 2,500 people
studied, contains an intact, full genetic recipe for an entire virus,
said researchers from University of Michigan and Tufts University.
Researchers do not know yet whether it can replicate or reproduce.
But other studies of ancient virus DNA have shown it can affect the
humans who carry it, they said.
In addition to finding these new stretches, researchers also
confirmed 17 other pieces of virus DNA found in human genomes by other
scientists in recent years.
The study looked at the entire span of DNA, or genome, from people
from around the world, including a large number from Africa — where the
ancestors of modern humans originated before migrating around the world.
Researchers used sophisticated techniques to compare key areas of
each person’s genome to the “reference” human genome. The findings add
to what science already knows about human endogenous retroviruses
(HERVs).
HERVs is the name for the ancient infectious viruses that inserted a
DNA-based copy of their own RNA genetic material into our ancestors’
genomes. They are part of the same type of virus that includes the
modern human immunodeficiency virus, which causes AIDS.
Over generations, the virus-generated DNA kept getting copied and
handed down when humans reproduced. That is how it ended up in our DNA
today, researchers said.
About 8% of what we think of as our “human” DNA actually came from viruses.
In some cases, HERV sequences have been adopted by the human body to
serve a useful purpose, such as one that helps pregnant women’s bodies
build a cell layer around a developing foetus to protect it from toxins
in the mother’s blood, they said.
The new HERVs are part of the family called HERV-K. The intact whole
viral genome, or provirus, just found was on the X chromosome; it has
been dubbed Xq21. It is only the second intact provirus found to be
hiding in human DNA, researchers said.
“This one looks like it is capable of making infectious virus, which
would be very exciting if true, as it would allow us to study a viral
epidemic that took place long ago,” said John Coffin from Tufts
University.
“This research provides important information necessary for
understanding how retroviruses and humans have evolved together in
relatively recent times,” Coffin said.
The findings were published in the journal
PNAS.
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