Book of life: Chapter one

Chromosome 22, decoded by an international team of scientists, is one of the 24 distinct bundles of DNA within our cells that contain the genetic information to make a human being.

It has taken the large team most of this decade to decipher all 33 million letters of the genetic code on chromosome 22. The work has revealed hundreds of new genes and over 30 medical disorders are now known to be influenced by the chromosome, including heart defects, cancers and schizophrenia.

The first step in its exploitation will be in using the information to detect diseases in people. As more is understood about the meaning of the code, new drugs and therapies will emerge, and eventually treatments will be tailored to each individual's precise genetic make-up.

But chromosome 22 is merely the first historic step for the £1.9bn Human Genome Project (HGP), which seeks to unravel the entire hereditary instructions of humans. The project will produce a rough draft of the complete genome next spring, with a final version in three to five years.

"This is the first time that we've had a complete chapter in the human instruction book, and that's pretty amazing," said Dr Francis Collins, from the US National Institutes of Health and who chairs the HGP.

"I think this is probably the most important scientific effort that mankind has ever mounted," he said. "That includes splitting the atom and going to the Moon."

Life code

The human genetic code, or genome, consists of three billion pairs of chemicals known as bases.

Given the letters A, T, C and G, they form the central part of the DNA molecules that inhabit nearly all our cells. DNA in those cells is coiled and folded into structures called chromosomes.

What HGP scientists have done is sequence the precise order of the bases in the DNA that makes up chromosome 22, the second smallest of all the chromosomes.

The researchers say this includes at least 545 working genes - the distinct regions of sequence that code for the proteins that build and maintain our bodies.

Computer modelling suggests there may be as many as 1,000 genes on chromosome 22, but even this is but a small fraction of the estimated 60,000 to 100,000 genes in our cells.

Human disorders

"One down, the others to go," said Dr Ian Dunham, a biochemist at the Sanger Centre in Cambridge, UK, and lead author on the scientific paper in the journal Nature that announced the genetics landmark. "It's a great relief to have it finished."

Mutations to genes along chromosome 22 contribute to heart defects, immune system disorders, cancers, and mental retardation. A gene linked to schizophrenia is also thought to reside somewhere on chromosome 22.

"You will see buried in all sorts of papers that people have used the sequence already," said Dr Peter Little of Imperial College in London. He uses the information to study a protein on the surface of nerve cells.

The Human Genome Project is competing with Celera Genomics, run by Dr Craig Venter, to be first to publish a rough draft of the human genetic code. Scientists say that the completed genome will be vast mine of data, equivalent to roughly half a million pages of information, that will take many years to sift.

The chromosome 22 sequence is principally the work of the UK Sanger Centre, Keio University in Japan, and US laboratories at the University of Oklahoma in Norman, and Washington University in St Louis, Missouri.