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The mystery of the human genome's dark matter

David Cox
Features correspondent
Getty Images Copy of a DNA electrophoresis gel (Credit: Getty Images)Getty Images
The majority of our DNA forms what is known as 'dark matter' – mysterious genetic sequences that has played a key role in the evolution of our species (Credit: Getty Images)

Twenty years ago, an enormous scientific effort revealed that the human genome contains 20,000 protein-coding genes, but they for just 2% of our DNA. The rest of was written off as junk – but we are now realising it has a crucial role to play.

When the 13-year-long effort to sequence the entire "book of life" encoded within the human genome was declared "complete" in April 2003, there were high expectations. It was hoped that the Human Genome Project, at a cost of around $3bn (£2.5bn), would yield treatments for chronic illnesses, and shed light on everything that is genetically determined about our lives.

But even as the press conferences were being held to herald the triumph of this new era of biological understanding, this instruction manual for human life had already thrown up an unexpected surprise.

At the time, the prevailing belief was that the vast majority of the human genome would consist of instructions for making proteins, the building blocks of all living organisms that perform a bewildering range of roles within and between our cells. With over 200 different types of cells in the human body, it seemed to make sense that each would need its own genes to carry out its necessary functions. The appearance of unique sets of proteins were thought to have been vital in the evolution of our species and our cognitive powers. (We are, after all, the only species capable of sequencing our own genome.)

Of humans and worms

Humans may have as few as 19,000 protein-making genes in total, while in comparison worms have around 20,000 and a fruit fly has approximately 13,000.

Instead, it transpired that less than 2% of the three billion letters of the human genome are dedicated to proteins. Only around 20,000 distinct protein-coding genes were found to exist in the long lines of molecules known as base pairs that make up our DNA sequences. Geneticists were astonished to find that humans have similar numbers of protein-making genes to some of the simplest creatures on the planet. Suddenly the scientific world was faced with an uncomfortable truth: perhaps much of our understanding of what makes us human was actually wrong?

"I just the incredible shock," says Samir Ounzain, a molecular biologist and chief executive of a company called Haya Therapeutics, which is attempting to use our understanding of human genetics to develop new treatments for cardiovascular disease, cancer and other chronic illnesses. "That was the moment where people started wondering, 'maybe we have the wrong conceptualisation of biology">window._taboola = window._taboola || []; _taboola.push({ mode: 'alternating-thumbnails-a', container: 'taboola-below-article', placement: 'Below Article', target_type: 'mix' });