Sunday, June 17, 2007


A new understanding of how DNA shapes our health and makes us human has emerged from the most exhaustive analysis yet of the workings of the human genome. The first "parts list" of genetic elements that are biologically active in the body has revealed that DNA functions in a much more complex fashion than was once assumed, offering insights into the inherited roots of diseases such as diabetes and cancer.

The work of the Encode Consortium - the acronym stands for Encyclopedia of DNA Elements - also sheds important light on the genetic differences that separate humans from chimpanzees and other species. While the human genome is made up of approximately three billion DNA "letters", only about 3 per cent of these are known to contribute to 22,000 or so genes - DNA "sentences" containing instructions for making proteins that control the body's chemical reactions. Most of the remaining 97 per cent has traditionally been thought of as "junk DNA", which appeared to be an evolutionary relic that performed no tasks of significance. The new research shows that much of this junk DNA is not redundant but is chemically active in ways that influence how genes are switched on and off. Mutations in these regulatory genetic regions are thus likely to explain some of our varying susceptibility to disease - some have already been linked to type 2 diabetes and prostate and colon tumours.

While the bulk of our genes are shared with other organisms, much more of our [so-called] junk DNA is peculiar to our species: 99 per cent of human and chimpanzee genes are identical compared with only 96 per cent of all DNA. As there is more variation in the junk, this could influence traits such as intelligence and language.

Ewan Birney, of the European Bioinformatics Institute, near Cambridge, who led the analysis, said: "Our data certainly agree with the idea that many of the differences between mammals lie in this junk DNA. We now have a much better idea of what most of our DNA might actually be doing. That is also going to help us to characterise what is going on in disease."

Francis Collins, director of the US National Human Genome Research Institute, which funded the project, said: "This impressive effort has uncovered many exciting surprises and blazed the way for future efforts to explore the functional landscape of the entire human genome."

The consortium, which pub-lishes its results today in Nature and Genome Research, set out to examine what every bit of DNA does by looking in detail at 30 million letters or base pairs - 1 per cent of the genome. About 3 per cent of the DNA - the genes - was found to be transcribed into the signalling molecule RNA and then to make proteins. Another 6 per cent hitherto regarded as junk, however, was unexpectedly found to be written into RNA without producing proteins. It is this part of the genome that appears to play a critical regulatory role, controlling when genes are active or silent.

Some of this active DNA outside genes, however, appears to make RNA without affecting the functions of cells - it is chemically alive but neutral. While scientists do not yet know what proportion is neutral, or why, one theory is that it provides a stock of genetic material from which potentially useful mutations can arise to drive evolution. "It may be a kind of warehouse for natural selection," Dr Birney said. "Evolution seems to have kept this around for a reason, to somehow set itself up for the future. It is a bit like Pop Idol- if you throw the net widely, you can pick up talent when it appears." The Encode team is working to scale up the project to cover the entire human genome.


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