14 DAY TRIAL // We know that DNA is the hereditary molecule from which the genes are made of. But not all DNA represents genes. The non-gene coding and DNA patches with unobvious role have been regarded by scientists for long as junk and useless, a sort of evolutionary ballast.
But a new research showed that this tiny, jumping DNA fragments are in fact crucial players in the evolution of the mammals. In the case of the human species, junk DNA forms about 95 % of the genome (total DNA amount), and is distributed in over 10,000 short fragments. Some of these fragments, called transposons, can copy themselves and attach into new locations, up to millions of times. "One of the big questions is: Where does novel functional DNA come about in the genome?" said co-researcher Gill Bejerano of Stanford University, evolutionary developmental biologist. "We think we've hit on a force here that was underappreciated before."
Bejerano's team investigated junk DNA, which appeared extremely similar, in 6 mammalian species-human, chimp, rhesus macaque, dog, mouse and rat. This degree of similarity pointed that it should have some role, otherwise random accumulations would have gathered in tens of millions of years of divergent evolution. The mammalian sequences were compared to chicken genome, to select off all that appeared before mammals evolved.
5 % of the uniquely segments in the mammalian junk DNA were similar in composition to transposons. "The source of the remaining 95 % is unclear", said Bejerano. The conserved transposon elements are located in gene deserts (DNA zones extremely poor on genes), at a large distance from active genes. "The actual functions of the conserved elements remain untested, but the proximity suggests that evolution may have harnessed the bits of junk DNA to control the activities of the nearby genes," said Bejerano.
"In this way, genes might become active in new places or at new times, contributing to the differences between species. Researchers had turned up a few examples of conserved transposon elements before, but they were isolated cases," said Bejerano. "This is a genome-wide phenomenon that appears to play a significant role." But the 95 % of the junk DNA still poses a puzzle for the scientists: which are its origins?