SELFISH, parasitic bits of DNA that proliferate throughout our genome might play a key role in determining our individuality.
Mobile genetic elements, or 鈥渏umping genes鈥�, are genetic parasites whose sole purpose seems to be to copy themselves. Amazingly, jumping genes make up around 95 per cent of our genome, although the vast majority are relics no longer capable of copying themselves. One class, called L1 retrotransposons, make up about 20 per cent of mammalian DNA.
Fred Gage鈥檚 team at the Salk Institute for Biological Studies, California, created their own version of a human L1 element and let it loose in cell culture. They found that the engineered L1 was far more active in neural cells than in other cells. Previous studies have shown that L1s are active in testes and ovaries, 鈥渂ut nobody has ever demonstrated mobility convincingly in cells other than germ-line cells,鈥� says Gage.
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And when the engineered L1 element was introduced into mice, on one occasion it copied itself into a key gene in neural precursor cells, altering the gene鈥檚 activity and thus changing the types of cells the precursors developed into (Nature, DOI: 10.1038/nature03663). On this basis, Gage speculates that jumping genes might be at least partly responsible for the differences between the brains of individuals.
But other researchers are sceptical. 鈥淢y guess is that it accounts for one-thousandth of 1 per cent,鈥� says John Brookfield, a geneticist at the University of Nottingham, UK.
