Helpful junk

摘要

The brain is the most complicated ob-ject known. How it gets that complicated is, however, almost completely unknown. But part of the answer may turn out to be junk-at least that is the conclusion of a study led by Fred Gage of the Salk Institute in La Jolla, California, which has just been published in Nature. One of the puzzling features of the human genome is that although genes are numerous they actually form less than 5% of the DNA in a cell nucleus. The rest was thus, rather cavalierly, dubbed "junk DNA" by those who discovered it. Gradually, a role for some of this junk has emerged. In particular, parts of it regulate the activity of genes, and thus which proteins are produced and in what quantities. That has implications for what a cell does-or, to put it another way, what type of cell it is. One of the most puzzling sorts of junk, though, is something known as a LINE-1 retrotransposon. This is junk that won't stay in one place.