'Junk DNA' and the mystery of mankind's missing genes

The lexicon of science is riddled with catchy yet misleading terms. The god particle is nothing of the sort. Genes cannot really be selfish, and when astronomers talk about metals, they usually mean something else entirely. Now, we must add junk DNA to the list of scientific misnomers.

Last week, the results of the multinational Encode Project were published across 30 papers in the journals Nature, Science, Genome Biology and Genome Research. The five-year collaboration involved some 450 scientists working in 32 institutions and took up 300 years of computer time. The goal was to analyse the vast bulk of human DNA that does not constitute a gene ie, does not directly code for the creation of particular proteins and is seemingly surplus to requirements.

The conclusion? That this DNA is not junk at all, but absolutely vital for the functioning of our cells. It turns out that as much as a fifth of the 98 per cent of our DNA that falls into this category is instead made up, among other things, of switches bits of DNA that turn some genes on and others off. It is now believed that, in order to get to grips with genetic illnesses such as hereditary heart disease, some forms of diabetes and Crohns Disease, we need to understand these regulatory elements as much as the genes themselves.

It has been clear for a long time that there is a lot more to DNA than just genes. Indeed, one of the great scientific surprises in recent decades has been the discovery that the human genome is surprisingly bereft of actual genes. When the first draft of it was published in the summer of 2001, it did not describe the 100,000 or more genes that most biologists assumed we had, but fewer than 20,000 making Homo sapiens not much more well-endowed genetically than a fruit fly or even a lump of yeast. As an editorial in Nature put it, Unless the human genome contains a lot of genes that are opaque to our computers, it is clear we do not gain our undoubted complexity over worms and plants by using many more genes.

Partly as a result, the idea that scanning a persons genome can tell us pretty much everything about them their likely intelligence, the chance of criminal tendencies, their probable age and cause of death is now seen as a simplistic fantasy. Indeed, the more we learn about our genome, the more complex the story becomes. We have genes that tell our bodies to make proteins, genes that affect other genes, genes that are influenced by the environment, segments of DNA that switch certain genes on and off, as well as our RNA, the still-not-fully understood messenger molecule that conveys information from our DNA to protein factories in the cells.

Despite the fanfare with which the Encode findings were greeted last week, biologists have known for years that junk DNA, a term coined in 1972 by the Japanese-American geneticist Susumu Ohno, performs a host of functions, among them gene regulation. Indeed, it was always obvious that much of our DNA must be tasked with the activation or suppression of other parts of itself: genes that make bone tissue are present in all cells but are only switched on in bone cells; heart muscle genes are present but inactive in your teeth and liver and everywhere else.

Furthermore, as Ohno pointed out, a great deal of the genome consists of pseudogenes non-functioning copies of active genes that form the raw material of evolution. Without this spare genetic material, natural selection would have nothing to act upon. We have also known for some time that the dark part of our genome contains what are known as human endogenous retroviruses: bits of the genetic code from viruses that are a legacy of our long battle with these microbes. In millennia to come, it is likely that bits of the genome for HIV will become similarly incorporated into our DNA, as a legacy of the Aids epidemic.

The more we learn, the more the recipe book of life turns out to resemble less a single tome than a well-organised library, complete with a sophisticated index and with the ability to lend and borrow books. Some of the volumes are crucial a mix-up in the code could kill or cripple us while others moulder in the stacks. There is probably a lot of built-in redundancy, which is not surprising considering that the genomes of any species are the result of three billion years of evolution. Perhaps the most amazing thing is that we can make any sense of it at all.

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'Junk DNA' and the mystery of mankind's missing genes