[Islam-Online- Science, Ideas & Technology]

PARIS (AFP) - An international team of researchers has for the first time mapped the genetic structure of chromosome 21, the smallest human chromosome, the British scientific journal Nature reported. The research, to be published in full in the May 18 issue of Nature, was carried out by 62 researchers from Britain, France, Japan, Switzerland and the United States.

While Chromosome 21 is the smallest of the human chromosomes, further analysis of its genes could permit a deeper understanding of Down syndrome and its complications, as well as a range of other linked genetic disorders such as Alzheimer's disease and certain forms of cancer, researchers said.

The director of the Chromosome 21 Mapping and Sequencing Consortium said in Geneva that its mapping would help in developing earlier diagnosis and treatments, but warned years of work lay ahead. "After completing the sequencing of this chromosome we can push harder on discovering the function of each of its 225 genes, but this could take a dozen years," said Stylianos Antonorakis of the University of Geneva.

The discovery of as few as 225 active genes in the chromosome's 33.8 million base pairs of DNA has also lead researchers to re-evaluate the total number of genes in the human body. Scientists found 545 genes in the 33.4 million base pairs of chromosome 22, the first human chromosome to be sequenced and published late last year in the British journal. Now scientists believe the total number of human genes may be closer to 40,000 rather than previous estimates of between 70,000 and 140,000.

The paucity of genes may also help explain why people with a third, extra copy, or trisomy, of chromosome 21 survive to adulthood despite a spectrum of physiological disorders known as Down syndrome. Scientists around the world are racing to map the DNA sequences of each of the 23 pairs of human chromosomes, as well as the genes of which they are composed.