Chromosome 21 is the smallest of the 23 pairs of human chromosomes, with an estimated 225 protein-coding genes, and only the second to be completely deciphered. A consortium of international scientists, led by German and Japanese researchers from the Institute for Molecular Biotechnology in Germany, and the RIKEN Genomic Sciences Center in Japan, mapped the sequence which is published in this week's edition of the science journal Nature.

``The complete sequence analysis of human chromosome 21 will have profound implications for understanding the pathogenesis of diseases and the development of new therapeutic approaches,'' the scientists said in the report. ``The challenge now is to unravel the function of all the genes on chromosome 21,'' the scientists added.

The genetic map will help scientists develop more precise diagnostic tests and new treatments for diseases linked to the chromosome, including a particular type of epilepsy, auto-immune diseases and an increased susceptibility to leukemia. Experts in India, however, add that logically this breakthrough would have to be revalidated across races globally.

Current therapies for the treatment of Alzheimer's include Novartis' Exelon (which has recently been granted marketing clearance by the US FDA) and Pfizer's Aricept. Exelon's US approval is based on data from more than 3,900 patients worldwide, the largest phase three clinical program ever conducted with a medication for dementia of the Alzheimer's disease type.

The small number of genes on the chromosome and the 545 active genes in chromosome 22, which was sequenced last year, have also led the scientists to revise the estimated total number of human genes to 40,000 from 70,000 to 100,000 genes. Up to a third of chromosome 21 contained no genes. Both chromosomes are small, but together they make up 2 to 3 per cent of the human genome, the basic blueprint of life.

Each chromosome is made up of a molecule of DNA in the shape of a double helix which is composed of four chemical bases represented by the letters A (adenine), T (thymine), G (guanine) and C (cytosine).

The arrangement, or sequence, of the letters determines the cell's genetic code.

The scientists mapped out the 33.5 million base pairs of chromosome 21. The human genome contains three billion base pairs of DNA.

The achievement is particularly significant for the study of Down's Syndrome, which affects one in 700 live births. The complex disorder is caused by an extra copy of the chromosome.

Down's Syndrome, loosely defined, refers to certain chromosomal disorders characterised by abnormalities which could include mental retardation and retarded growth. Characteristically, such patients have a short nose, flat face, small low-set ears and stubby fingers.

``These are the tools to design strategies to identify the culprits, the genes, which are responsible for the pathogenesis of the disease,'' Marie-Laure Yaspo, of the Max Planck Institute fur Molecular Genetics in Berlin who led one of the research groups, said in a telephone interview.``I would call it a tool box, where we have everything we would need to design novel strategies. We have to design novel strategies because the problem of Down's Syndrome is not a mutation. It is too much of something - that is one more chromosome,'' she added.

In addition to Down's Syndrome, chromosome 21 is also linked to the early onset Alzheimer's disease, certain types of leukemia, a form of manic depression and congenital heart disease. Nobuyoski Shimizu and scientists at Keio University in Japan and Helmut Blocker and researchers at the Corporation for Biotechnological Research in Brunswick in Germany as well as Swiss, French, American and British scientists contributed to the research.