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IF IT weren’t for “junk” DNA, women would be in trouble. Some of this
apparently useless DNA may be essential for silencing one of their X
chromosomes, say researchers in Ohio.

In mammals, one X chromosome is enough for both sexes, so females would get a
lethal double dose if one X weren’t shut down (91av, 1 April,
p 4). This is done by wrapping up one X very tightly, with the help of RNA made
by a gene called Xist. Two years ago, Mary Lyon of the MRC’s Mammalian
Genetics Unit in Oxfordshire suggested that junk DNA might be just the handhold
XIST needs to package up the DNA.

Nearly half the human genome is made up of “jumping genes”, or
retrotransposons, bits of “selfish” DNA thought to do little except make more
copies of themselves. But now, with data from the Human Genome Project, Evan
Eichler and his colleagues at Case Western Reserve University in Cleveland have
discovered that the X has twice as much of a class of selfish DNA called L1 as
any other chromosome.

“If parasitic DNA has accumulated by a truly random process, you wouldn’t
expect the X to have more or less,” says Eichler. Because other types of selfish
DNA are evenly distributed, he says, L1 elements seem to have an important
function on the X. Eichler’s team also found that the regions of the X that are
most effectively silenced had the highest density of L1 elements.

“It’s not proof yet,” says Stanley Gartler, a geneticist at the University of
Washington who has studied X inactivation for decades, “but they have taken
Lyon’s idea quite far.”