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