London, England (Reuters) - Scientists have completed the finished sequence of the human genome (genetic blueprint of life) which holds the keys to transforming medicine and understanding disease. This comes two years earlier than expected and less than three years after finishing the working draft of the three billion letters that make up human DNA.

"We put out the draft sequence as a way of getting it out to scientists as quickly as we could. It gives them something to work with and get going, but the aim was always to generate a reference sequence for the human genome. It's a bit like moving on from a first attempt demo music tape to a classic CD," said Dr. Jane Rogers, head of sequencing at the Wellcome Trust Sanger Institute. This institute sequenced nearly one-third of the genome.

Since the rough draft was announced in June 2000, researchers from 120 countries have downloaded information which has been freely available online. Scientists in the U.S., France, Germany, Japan, and China have also worked on the publicly-funded Human Genome Project.

The Human Genome Project has already aided scientists in discovering a mutation that causes a deadly type of skin cancer, and accelerated the search for genes involved in leukemia, diabetes, and childhood eczema. The completed sequence will help scientists to identify the 25,000 - 30,000 genes in humans, including those involved in complex diseases like cancer and diabetes.

While Professor Allen Bradley, director of the Wellcome Trust Sanger Institute, cautions that people should not expect immediate major breakthroughs, "But there is no doubt that we have embarked on one of the most exciting chapters of the book of life." He believes that the completion is a vital step on a long road, and that the eventual health benefits could be "phenomenal."

The human genome has already revealed some of its secrets. There are far fewer genes than scientists had expected, and proteins (which build tissues and regulate the body's function) are much more complex than they thought.

"The nematode worm has about 17,000 genes, so we haven't really got very many more than a small worm. But we manage to construct a great variety of proteins from those genes. Understanding how that happens is the next challenge," Rogers explained.

Scientists have already identified more than 1.4 million single nucleotide polymorphisms (SNPS), which are variations in the three billion letters of the human genetic code. SNPS are individual changes in the arrangement of the letters that make people different from each other. They hold the key to susceptibility to illnesses such as cancer, diabetes, heart disease, and individual responses to medications. Scientists hope to identify specific DNA variations that cause the tendency for a certain disease by looking at different subsets of the genome and comparing the results of several people. This holds the potential for new treatments, customized drugs to individual genetic profiles, and earlier diagnosis of disease.