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Only eight percent of DNA is 'functional'

4 August 2014
Appeared in BioNews 765

Researchers at the University of Oxford have estimated that only 8.2 percent of human DNA has been left unchanged as the species has evolved and is therefore likely to be functional.

The team worked with the hypothesis that over millions of years of evolution, natural selection would keep the most useful sections of DNA relatively unchanged across a whole host of related species because they were the genes that were advantageous.

Looking for insertions and deletions in DNA sequences, the team analysed the genomes of a number of mammals including mice and humans to see which sections of DNA contained changes few and far between and had been 'preserved'. It was these areas of the genome that the Oxford University team classified as likely possessing some 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', Dr Gerton Lunter from the Wellcome Trust Centre for Human Genetics at Oxford University and joint senior author of the paper said.

'We found that 8.2 percent of our human genome is functional', said Dr Lunter. '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'.

The figure differs significantly from that given by some of the researchers involved in ENCODE, who stated that 80 percent of the genome has some biochemical function, leading to a debate over the definition of the word 'functional' (reported in BioNews 672).

'This is in large part a matter of different definitions of what is "functional" DNA', explained joint lead author Professor Chris Ponting of the MRC Functional Genomics Unit. 'We don't think our figure is actually too different from what you would get looking at ENCODE's bank of data using the same definition for functional DNA'.

'But this isn't just an academic argument about the nebulous word "function". These definitions matter. When sequencing the genomes of patients, if our DNA was largely functional, we'd need to pay attention to every mutation', he continued.

'In contrast, with only eight percent being functional, we have to work out the eight percent of the mutations detected that might be important. From a medical point of view, this is essential to interpreting the role of human genetic variation in disease'.

The remainder of the human genome is made up of leftover evolutionary material and is known as 'junk' DNA. Dr Lunter said ‘We haven't been designed. We've evolved and that's a messy process. This other DNA really is just filler. It's not garbage. It might come in useful one day. But it's not a burden'.

Comparing the genomes across different species, the researchers also observed that humans share 2.2 percent of their function DNA with mice. Professor Pointing said: 'The fact that we only have 2.2 percent of DNA in common with mice does not show that we are so different. We are not so special. Our fundamental biology is very similar'.

'Biologically, humans are pretty ordinary in the scheme of things, I'm afraid'.

8.2% of the Human Genome Is Constrained: Variation in Rates of Turnover across Functional Element Classes in the Human Lineage
PLOS Genetics |  24 July 2014
8.2 percent of our DNA is 'functional'
Eurekalert (press release) |  24 July 2014
90 per cent of human DNA does nothing at all, scientists find
Daily Telegraph |  25 July 2014
Less than 10% of human DNA has functional role, claim scientists
The Guardian |  25 July 2014
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