Over the years, there's been plenty of back and forth over how much of our DNA is important - for decades much of it was thought of as “junk DNA”, but geneticists have gradually come to believe that some of these seemingly pointless segments of DNA may be crucial to regulating the rest of the genome.
Importantly, researchers from the Encyclopedia of DNA Elements (ENCODE) in 2012 stated that about 80 percent of human DNA has some kind of “biochemical function” as Sci-News reports.
The study was controversial, because many researchers argued that the definition of "biochemical function" was too broad, and that just because activity occurs on the DNA, it doesn’t necessarily mean that it has a function. Of course, how do you test the impact that each segment of DNA has on the body?
The new study, led by Gerton Lunter from the University of Oxford's Wellcome Trust Centre for Human Genetics in the UK, builds on this research. In order to work out which DNA segments are actually funtionally useful, they looked at how it's changed over the past 100 million years of mammalian evolution - and found that only 8.2 percent of our DNA was important enough to stay the same.
They came to this conclusion by comparing the DNA of various mammals, such as mice, guinea pigs, rabbits and horses, and looked at which chunks of DNA were conserved across different species. The idea is that if a big segment of DNA has been conserved over 100 million years of evolution, despite countless natural mutations, then it must have a pretty important function.
“Throughout the evolution of these species from their common ancestors, mutations arise in the DNA and natural selection counteracts these changes to keep useful DNA sequences intact,” Lunter told Sci-News.
To find which areas stayed the same, the scientists actually looked for the pattern of insertions and deletions of chunks of DNA in the sequence. These should generally occur pretty randomly along a genome - unless natural selection had acted to keep a stretch of important DNA as it is, in which case there would be wider gaps between these insertions and deletions.
“We found that 8.2 percent of our human genome is functional," Lutner told Sci-News. "We cannot tell where every bit of the 8.2 percent of functional DNA is in our genomes, but our approach is largely free from assumptions or hypotheses. For example, it is not dependent on what we know about the genome or what particular experiments are used to identify biological function."
Chris Rands from the University of Oxford, who was the first author of the paper published in PLOS Genetics, added that not all of the 8.2 percent is equally important. “A little over 1 percent of human DNA accounts for the proteins that carry out almost all of the critical biological processes in the body," he told Sci-News.
The other 7 percent is most likely involved in switching on and off the genes that encode those proteins, like regulatory bodies.
And the other 91.8 percent? Generally, it’s pretty lazy.
“The rest of our genome is leftover evolutionary material, parts of the genome that have undergone losses or gains in the DNA code,” said Lutner.