Junk DNA
Within a chromosome or a genome, the junk DNA is those portions of the DNA for which no function has been identified or intuited.
In the genomes of most plants and animals, an overwhelming percentage of the DNA serves no known biological role. The portions of a chromosome which are genes are identifiable as open reading frames even when biologists lack information about the proteins these genes presumably encode. Genome scientists find it reasonable to assume that these regions are important, even if they do not know exactly how. There are also "noncoding" DNA sequences that are known to be important. These include origins of replication, which define the starting points of DNA replication, and regulatory sequences such as promoters, which figure in turning genes on and off.
"Junk" is somewhat a misnomer, because molecular biology remains a young science. Segments of DNA may function in additional ways that have not yet been discovered, which might suggest uses for much or all of the junk. Scientists generally keep this likelihood in mind even as they persist in using the word "junk," which for better or worse has stuck.
One hypothesis about the junk is that these chromosomal regions are trash heaps of defunct genes, sometimes known as pseudogenes, which have been cast aside and fragmented during evolution. Evidence for a related hypothesis suggests that the junk represents the accumulated DNA of failed viruses. Yet another hypothesis is that the junk DNA provides a reservoire of sequence from which potentially advantageous new genes can emerge.
About 97% of the human genome has been designated "junk." The onion genome is 12 times the size of the human one, presumably because it contains even more junk. In contrast, the pufferfish genome is only about one tenth the size of the human, yet seems to have about the same number of genes. Therefore it seems that the ratio of functional and junk DNA differs widely per species.
- See also: intron--centromere--telomere--Alu repeat--repetitive sequence--satellite DNA--molecular evolution--selfish DNA
References
- Gibbs W.W. (2003) "The unseen genome: gems among the junk", Scientific American, 289(5): 46-53. (A review, written for non-specialists, of recent discoveries of function within junk DNA.)