Chromosome 21 (human)
There are at least fourteen genes on chromosome 21 , which we already know that their genetic modification can lead to a number of monogenic diseases (caused by a single gene). These include Alzheimer's disease , amyotrophic lateral sclerosis , special forms of epilepsy and autoimmune diseases ( AIRE ), homocysteinuria , muscle diseases ( Bethlem myopathy and Ullrich myopathy ) and an increased susceptibility to leukemia .
With significant participation by German geneticists at three sequencing centers (at the Society for Biotechnological Research Braunschweig, at the Institute for Molecular Biotechnology Jena and at the Max Planck Institute for Molecular Genetics Berlin), an international team has the more than 33 million building blocks ( bases ) of the chromosome 21 sequenced to the extent of 99.7 percent with 99.99 percent certainty . This makes chromosome 21, after chromosome 22, the second chromosome that has been completely sequenced. As we now know, chromosome 21 contains a total of 225 genes and 59 pseudogenes . The 225 genes include 127 that were known before the chromosome was sequenced. The other 98 genes are unknown genes determined by predictions based on computer-based DNA sequence analysis . In doing so, the scientists take advantage of the fact that certain sequence sections appear in the genome that mark the beginning or end of a supposed gene. Compared to the similarly sized chromosome 22, which carries 545 genes, chromosome 21 is very poor in genes.
Among the predicted genes, there are some good candidates to be identified as the cause of certain genetic diseases. These diseases have long been seen in connection with chromosome 21. They include various forms of deafness , tumor diseases and manic depression . In addition, were tumor suppressor genes found that the suppression of tumors cause (for example, gene Ets2 ). The two genes DSCR1 and DYRK1A located on chromosome 21 also counteract pathological tissue growth.
The analysis of the genes of this chromosome is of great medical importance. People who have three copies of the entire chromosome 21 or three copies of parts of a 21st chromosome in all or part of their body cells have a form of Down's syndrome ( trisomy 21). A critical region has now been identified on chromosome 21 that is involved in all typical aspects of Down syndrome (DSCR, Down syndrome critical region , position 21q22). The genes contained in this region are now associated with the respective phenotype and enable a future causal treatment of the symptoms associated with this defect.
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