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The Value of Junk

Hidden genomic treasures.

Donít insult what you donít understand. That is the lesson of “junk” DNA, the hasty label attached to the long stretches of genomes that do not encode proteins. Such DNA sequences constitute the bulk of most genomes—nearly 97% of the human genome, for instance—and their origin and function are currently hot topics among biologists. Perhaps the strongest argument that noncoding DNA is not in fact junk is its sheer abundance. Why would organisms waste energy passing deadwood down the generations? On the other hand, closely related species often differ dramatically with respect to the amount of DNA they carry; if noncoding DNA is important, why can some species get by with so much less of it?

Noncoding DNA crops up throughout the genome. Most is located in intergenic regions, but some segments, the introns, sit within the genes themselves, interrupting the protein-coding regions with tracts of apparent genetic nonsense. These introns are transcribed into RNA and only later spliced out of mature messenger RNA so that protein translation can occur unimpeded. Originally dismissed as code parasites, some of these spliced-out RNA fragments have recently been shown to regulate expression of other genes by a process called RNA interference (see “News Analysis: New Platforms for Therapeutics,” May/June).

Even intergenic regions, long considered wastelands, seem to contain many portions that are transcribed into RNA though not translated into protein. These newly discovered “rna genes” may be as numerous as protein-coding genes, and many seem to play important roles in regulating the expression of their conventional counterparts. Other noncoding RNA appears to play an epigenetic role, controlling the binding of a bewildering array of proteins that regulate everything from transcriptional activation to the packaging of DNA into chromosomes.