Dick Ahlstrom, Science Editor, sets the scene for tonight's lecture by Dr James Watson, co-discoverer of the DNA double helix

One of the co-discoverers of the DNA double helix has arrived in Dublin to deliver a historic address on the profound implications of the finding.

Tonight's talk by James Watson at Trinity College Dublin, which can be watched by anyone with suitable Internet connections, marks the 50th anniversary of this ground-breaking work.

Scientists in the late 1940s and early 1950s struggled to understand the shape of DNA, the molecule that carries our genetic blueprint. On February 28th, 1953, James Watson and Francis Crick solved the puzzle, proposing that the chain-like DNA took the shape of a double helix.

They published their finding in the journal Nature several weeks later on April 25th to the astonishment of the world's biological researchers, who were impressed by its elegant simplicity. In 1962 they shared a Nobel Prize for the discovery with collaborator, Maurice Wilkins.

Few scientific discoveries have had the impact of the double helix. With an understanding of DNA's shape came immediate insight into how huge DNA molecules could so faithfully and accurately be copied every time a cell divides. It was also clear that the genetic information was encoded in the sequence of the chemical links which make up the chains of the DNA.

Our new-found grasp of what Crick described as "the secret of life" has led to advances in a diverse range of fields from general biology to medicine to agriculture. It created a wholly new science, biotechnology, now being applied to life-saving drug development, food production, medical diagnosis and forensic science.

The new genetic technologies allowed researchers to develop the rapid test now being used to give an accurate diagnosis within hours of the presence of the SARS virus. The same technologies are being applied towards finding a vaccine and drugs to combat this dangerous new infection.

James Watson has been a seminal figure in the development of genetics and its applications. He continued his studies into DNA at Harvard, becoming director of one of the world's leading laboratories, Cold Spring Harbor Laboratory in New York.

He was the first director of the National Centre for Human Genome Research at the National Institutes of Health, US, and helped initiate what became one of the largest international scientific research collaborations, the cataloguing of the entire human genetic blueprint.

This worldwide effort detailed all of the billions of steps in DNA's double helix code. Information from the human genome project has transformed research efforts to find the genetic causes of disease, develop new more effective drugs and understand the molecular basis of the most important killer diseases, including cancer and cardiovascular disease.

The Smurfit Institute of Genetics, Trinity College Dublin and Science Foundation Ireland invited Prof Watson to speak in Dublin to mark the 50th anniversary of the double helix. He arrived here last night straight from celebrations at Cambridge University where he and Crick worked to crack the shape of DNA. His visit has been organised in association with The Irish Times and the Irish Science Teachers' Association, with the generous sponsorship of Science Foundation Ireland and BioResearch Ireland.

Tonight at Trinity College, Prof Watson will talk about the discovery, but more importantly about its implications. As our understanding of DNA grows, so, too, does our ability to manipulate it, to use it for the greater good of humanity. Yet this technology potentially offers access to dangerous territory, for example, the direct manipulation of the living human genome, designer babies and cloning.

Always a challenging and often a controversial speaker, Prof Watson will ask his audience to consider the ethical issues raised by the technology.

He will draw on many of the themes described below in the coda to his new book, DNA, The Secret of Life.

Admission to Prof Watson's talk is by invitation only, given the severe shortage of space. The lecture will be telecast to the Burke Theatre, TCD.

All of this island's universities have arranged connections via the worldwide web with the sponsorship of BioResearch Ireland. They are arranging debates and presentations linked to tonight's lecture. It is also possible for those with suitable Internet connections to watch Prof Watson's presentation, which starts at 6.30 p.m. tonight.

Instructions about the software and computer systems needed to access the talk are available on the college's website, www.tcd.ie, where you click on "DNA 50-year celebrations".