An international team of researchers has reached a scientific milestone by deciphering the complete genetic sequence of chromosome 22, marking the first time that the whole genetic code of a human chromosome has been revealed.

Writing down all the 34 million "letters" that make up the entire sequence of chromosome 22 is akin to opening "first chapter of the book of life", scientists told a news conference Wednesday in London.

The researchers, from Britain's Sanger Center, Keio University in Japan and the University of Oklahoma and Washington University, in the United States, have identified 679 genes on chromosome 22, of which 55 percent were previously unknown in humans.

This information will provide new insights into the way genes are arranged along a strand of DNA and how they might be controlled, paving the way for huge advances in medical diagnosis and treatment, the researchers said.

There are 23 pairs of chromosomes in the human genome. Among them chromosome 22 is the second smallest.

Each human chromosome consists of a single linear molecule of DNA, which is composed of four chemical bases represented by the letters A(adenine), T(thymine), G (guanine) and C(cytosine).

It is the order, or "sequence", and context of these letters which determines how our bodies are made, and our predisposition to certain diseases.

Previous research has already revealed that chromosome 22 is implicated in the workings of the immune system, congenital heart disease, schizophrenia, mental retardation and several cancers including leukaemia.

The availability of the complete DNA sequence will revolutionize the future of research on these diseases, and the function of other genes on chromosome 22.

The work on chromosome 22, to be published Thursday in the science journal Nature, is the first step in the Human Genome Project, an international collaboration with the aim to decode the 3 billion letters that make up the whole human genome.

One third of the genome has already been completed and scientists estimate that with a first working draft to be ready by spring 2000, the highly accurate, finished form of human genome will be available before 2003. (Xinhua)