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A very small percentage of an organism's genome consists of actual protein-encoding genes--genes make up less than 2% of the human genome. The rest is often dismissed as "junk DNA," and many researchers feel that studying it is a tragic waste of time and money. However, there is much more to DNA than the blueprints for protein synthesis; a large portion of noncoding DNA is used in the regulation of transcription or as introns or telomeres. As it turns out, a much more significant portion consists of identical units (sometimes close to 200 base pairs in length, sometimes much longer) that are repeated in tandem up to 10 million times in a single haploid genome; these repeating monomers are what scientists call satellite DNA.

Most satellite DNA is located in the centromeric regions of chromosomes. Satellites rarely undergo transcription, and their function remains unknown. However, thanks to the efforts of Celera Genomics and the Human Genome Project in revealing the sequence of the human genome, scientists are beginning to discover some importance to the satellites. Some theorize that they play a crucial role in the inactivation of X chromosomes. Perhaps most importantly, however, this "barren" area may serve as a vast source of raw material for evolution.

The most significant satellite in the human genome is the Alu sequence, which is found in the CG-dense regions (where gene density is highest) of chromosomes. Although its exact function still remains unknown, most scientists agree that it is very important to the proper functioning of a human cell.

Nature, February 15, 2001.
Science, February 16, 2001.

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