One of the greatest discoveries this Nobel prize-winning scientist and new president of the Royal Society ever made was the truth about his own origins

The greatest genetic insight in the life of Sir Paul Nurse came courtesy of a striking and unusual source: the US Department of Homeland Security. Three years ago, the UK biologist, who won the 2001 Nobel prize for physiology for his work on the role of DNA in cell division, was told by the US immigration authorities that his application for a green card had been rejected. The short version of his birth certificate, which he had included in his paperwork, did not give his parents' names. Try again, he was told.

So Nurse, who was then president of Rockefeller University in New York, applied for a full version of his birth certificate from Britain's General Register Office. Their reply transformed his life. "When I opened the new certificate, I found that the name of my mother was in fact my sister's," Nurse told me last week. "My father's name was just a blank. It was an extraordinary moment."

Thus the geneticist, who was recently named Britain's most important scientist and who takes up the presidency of the Royal Society next month, learned the true nature of his genetic identity. The parents who had raised him were really his grandparents while his "sister" Miriam was his birth mother. As for the men and women who he believed were his nieces and nephews, they were actually half-brothers and half-sisters.

Miriam had become pregnant in 1948, at the age of 17, and had been sent away to an aunt's for the last months of her pregnancy. Then her mother took the baby back to her home in Wembley, in north London, where she and her husband raised Paul as their own. His other, older "siblings" had long ago left home, so Nurse grew up as an only child.

Nurse says he feels no anger or resentment at the deception. "Indeed, I have huge respect for my grandparents. They did their best for their daughter and for me. I had a very happy childhood. I just feel sorry for my mother. She ended up never having me as a child. I only learned after her death, she had kept four pictures of her children next to her bed. Three were her legitimate babies. I was the fourth."

Others might have chosen to keep hidden this family secret, but Nurse has made a point of speaking out. He owes it to his mother's memory, he says. "In those days, things were done like that. But it is wrong that people were compelled to take such drastic action and I am very glad it is no longer the case. I just want to put things right for my mother now."

Such remarks say much about the future Royal Society president, a man whose liberalism seems embedded in his DNA. He is compassionate, anti-elitist and highly approachable. Most people who have met the 61-year-old scientist are bowled over by his enthusiasm and engaging charm.

There is an iron fist in the velvet glove, however, a point stressed by Tim Hunt, the UK biologist with whom Nurse shared his Nobel prize. "He was my boss when we worked at the Cancer Research Campaign in the 90s and we got on well," says Hunt. "But he could be pretty brutal with those who crossed him. He would liquidate them – metaphorically. He is not a doormat."

And Nurse will certainly need to be tough in the years ahead. Apart from taking over the presidency of the Royal Society, he will also become chief executive of the UK Centre for Medical Research and Innovation on 1 January 2011. Funded by several major UK science trusts, this huge, state-of-the-art biomedical centre is being built near St Pancras station in London, to co-ordinate British research on human health and to seek new ways to treat common ailments such as cancer and heart disease.

It will be Nurse's job to get the centre up and running by 2015 while also presiding over the Royal Society, the world's oldest learned body, and speaking out on behalf of British science. The combination will make him one of the most powerful scientists in the world, a remarkable transformation for a man who had no steady job and who was employed on only fixed-term research contracts until he was in his thirties.

"He has ended up in charge of just about everything he has been involved with," admits Hunt. "It is not so much that he is a power-crazed egomaniac. It is just that he doesn't like being led by others so he takes over leadership himself. It just seems the easiest way out to him. And to be fair, he is pretty effective as a leader."

Paul Maxime Nurse was born on 25 January 1949. The man he took to be his father worked in the Heinz food processing factory in north London while his "mother" had a job as a part-time cleaner. She was a Baptist and for a while Nurse was a believer. But his faith slowly ebbed away during his school years and he now describes himself as "a sceptical agnostic".

At Harrow county grammar school, Nurse did well, showing a particular interest in natural history and astronomy. A key influence, he recalls, was a lecture – about the moon – by Patrick Moore at the junior astronomical society. Decades later, Nurse repaid the favour by nominating Moore for a Royal Society fellowship.

However, when it came to applying to university, Nurse got a shock. Although he had a good set of A-levels, he was rejected by every university he applied to because students then had to have at least one O-level in a foreign language. "I was hopeless at languages," he admits. "I failed my French O-level six times." This linguistic frailty reveals a rare and slightly reassuring whiff of weakness in an otherwise daunting intellectual pedigree.

After working as a technician in a small laboratory run by Guinness for a year, he was eventually accepted by Birmingham University where he studied biology. Here, like so many contemporaries, he became radicalised. Nurse used to sell the Socialist Worker and was involved in a student occupation of the vice-chancellor's office. He remains unapologetic. "A scientist should be engaged with society," he says.

After Birmingham, Nurse moved on to East Anglia, where he worked on his PhD on fungi, and then took up a series of post-doctoral posts that would last to the mid-1980s, a career that took him to Edinburgh for six years and then down to Sussex before he was given his first full-time job at the Imperial Cancer Research Fund in 1984.

For all this time, Nurse was absorbed by one burning issue: gaining a scientific understanding of the fundamental question of what distinguishes living things from the non-living. Answers, he decided, would come from studying how cells divide. After all, those cells that cannot divide will die out, while those that can will proliferate. And because cell division is such a vital process for every living organism, the same genes might govern cell division in everything from yeast to humans, he reasoned.

So Nurse devised a set of experiments to test the idea. Remarkably, these confirmed his suspicions: not only did the genes control cell division in yeast, they wielded the same control over the life and death of human cells. The genes controlled the essence of life itself. "It was the work of a first-rate scientist who has deep and clear insight into what he was doing," says Hunt.

The discovery propelled Nurse from what he calls the basement of scientific research to the top floor, culminating in the Nobel prize. Shortly after, he was appointed president of Rockefeller University. Since then, Nurse has not looked back.

A stocky, ebullient figure, he has wallowed in the trappings of the good life. He used his Nobel prize money to buy a powerful motorbike – and still rides a Kawasaki 500 – and has a share of a plane which he flies when living at the family home which he has maintained in Oxfordshire.

He is married to Anne, a social scientist, and has two daughters: Sarah, who works for ITV, and Emily, a physicist based at University College London and at Cern, the particle physics laboratory outside Geneva.

Before he takes up his new jobs, Nurse will spend a few weeks finishing off a documentary for the BBC science series Horizon on the subject of trust in science.

"I think it is clear that the rise of the blogosphere and the internet has allowed a very small group of vociferous anti-science doubters to have a disproportionate impact on policy issues," he says.

For his part, Nurse – "as ever the old socialist", as Hunt puts it – is clear about the solution: engagement with the public, a mantra that he is likely to repeat throughout his Royal Society presidency. As he says: "Scientists have to earn their licence to operate and that means getting out there to talk to people and explain what we do."