Chromosome 21 (human)

Chromosome 21 is one of about 34 million blocks of the smallest human chromosomes. It comprises only 1.5 percent of human genetic information.

Features

On it are the already longer 14 known genes, of which we know already that their genetic modification to a series of severe monogenic ( caused by a single gene ) can cause diseases. These include Alzheimer's disease, epilepsy, and specific forms of auto-immune diseases, muscle diseases ( Bethlem myopathy and Ullrich myopathy ) and an increased susceptibility to leukemia.

Sequencing

With significant participation German geneticist at three Sequenzierzentren ( at the Society for Biotechnological Research Braunschweig, the Institute of Molecular Biotechnology in Jena and the Max Planck Institute for Molecular Genetics, Berlin), an international team has more than 33 million blocks ( bases) of chromosome 21 to the extent of 99.7 percent sequenced with 99.99 percent certainty. 21 so that the chromosome is chromosome 22 after the second chromosome which has been completely sequenced. The chromosome 21 contains, as we know now, a total of 225 genes and 59 pseudogenes. Among the 225 genes, 127 are included, which were already known before sequencing of the chromosome. The other 98 genes are unknown genes were determined by predicting on the basis of computer-based sequence analysis. Here, the scientists make advantage of the fact that certain sequence segments appear in the genome that mark the beginning or end of a putative gene. In comparison with the similarly sized 22 chromosome carrying 545 genes, chromosome 21 is thus very poor in genes.

Wherein the predicted genes are some good candidates to be identified as the cause of certain genetic disorders. These diseases have long been seen in connection with the chromosome 21. They include, inter alia, various forms of deafness, tumors and manic depression. In addition, tumor suppressor genes have been found to lead to the suppression of tumors ( for example, gene Ets2 ). The two located on the chromosome 21 genes DSCR1 and DYRK1A counteract pathological tissue growths.

Medical importance

The analysis of the genes of this chromosome has a great medical importance. People who possess three copies of the entire chromosome 21 or three copies of parts of chromosome 21 in all or part of their body cells have a form of Down syndrome ( trisomy 21). On chromosome 21 is now a critical region has been identified which is involved in all aspects typical of Down syndrome ( DSCR, Down syndrome critical region, position 21q22 ). The genes contained in this region are now associated with the respective phenotype and allow a future causal treatment of symptoms associated with such failure.

Swell

• Roger H. Reeves: recounting a genetic story. Nature 405, pp. 283-284 (18 May 2000), DOI: 10.1038/35012790
• M. Hattori et al.: The DNA sequence of human chromosome 21, Nature 405, pp. 311-319 (18 May 2000) DOI: 10.1038/35012518