Is Junk DNA Really Junky?
By Sam Kean
The delicious, religious debate over what most of our genome is good for.
Courtesy of Tom Whipps/Nature
In the 12 years since the human genome was sequenced, so many critters have had their DNA deciphered—oysters, bees, eels, camels, clawed frogs, elephant sharks—that it’s hard to suppress a yawn sometimes. But every so often, a genome cuts through the indifference and makes geneticists’ eyes goggle out. Take the humped bladderwort, a humble aquatic plant whoseDNA was sequenced this past May.
The humped bladderwort has yellow, snapdragon-like flowers, and it’s actually carnivorous, capable of trapping and eating not just insects but even tadpoles and tiny fish. But this combination of beauty and death isn’t what makes the bladderwort special. Most organisms have loads of junk DNA—less pejoratively, noncoding DNA—cluttering their cells. The bladderwort doesn’t: 97 percent of its DNA is classic, hardworking, protein-building DNA. And that lean, mean bladderwort DNA challenges some trendy notions about how all DNA works, including (if not especially) in human beings.
First, a primer on junk DNA, one of the most reviled terms in science. Anyone who took Bio 101 remembers (if only vaguely) that DNA gets turned into RNA, which in turn gets turned into proteins. The protein-producing stretches of DNA are called genes, and genes reside on much longer molecules called chromosomes.
A century ago, as biologists came to grips with the vast number of different proteins needed to build and maintain the body, they decided that genes must be packed very tightly together on chromosomes, since tight packing would be more efficient. They couldn’t have been more wrong. In humans, a typical species in this regard, less than 2 percent of our 3 billion letters of DNA actually builds proteins. Chromosomes were more like vast Saharan wastelands, broken up only sporadically by oases of genes.
So what does that extra 98 percent do? Here’s where things get contentious. Some of the excess—the pseudogenes, the transposons, the tedious stretches where Mother Nature held her fingers down on the keyboard (ACACACACACA ... )—does look like garbage. Heck, 8 percent of our genome is nothing but old, broken-down virus DNA, the genetic equivalent of a Pontiac Firebird on cinderblocks. The name junk DNAemerged in the early 1970s as a catchall term for this cruft.
Even at the time, though, some scientists objected to the term as too dismissive. Molecular biologists had already discovered bits of junk that, far from being irrelevant, actually managed genes: They turned genes on or off and regulated when and where genes were active. As more and more examples of this type of control emerged in the 1980s, the term junk DNA seemed less and less appropriate.
Cont. here:
http://www.slate.com/articles/health_and_science/human_genome/2013/10/junk_dna_debate_critics_of_the_encode_project_cite_humped_bladderwort.html
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