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Mice do fine without 'junk DNA'

October 20, 2004 By Roxanne Khamsi This article courtesy of Nature News.

Deleting non-coding regions from the genome has no apparent effect.

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Mice born without large portions of their 'junk DNA' seem to survive normally. The result contradicts the beliefs of many scientists who have sought to uncover the function of these parts of the genome.

More than 90% the genome of organisms such as mice and humans does not appear to code for any proteins. And yet this DNA shows striking similarities between species. If they had no function, over time mutations would scramble the sequences. Why have these bits of the genome remained so highly conserved?

One study, published this month in Developmental Cell1, reports that parts of the non-coding DNA may be involved in embryonic development. Barbara Knowles and her colleagues from the Jackson Laboratory in Bar Harbor, Maine, found that non-coding regions known as transposable elements, which can regulate genes, are highly active in mouse embryos.

Survival in the laboratory for a generation or two is not the same as successful competition in the wild for millions of years.
David Haussler
University of California, Santa Cruz
Knowles speculates that transposable elements could control embryonic differentiation, activating or reprogramming parental chromosomes. "I think they contain controlling sequences," she says.

Take out the trash

But transposable elements are only a small part of the non-coding regions. And now Edward Rubin's team at the Lawrence Berkeley National Laboratory in California has shown that deleting large sections of non-coding DNA from mice appears not to affect their development, longevity or reproduction.

The team created mice with more than a million base pairs of non-coding DNA missing - equivalent to about 1% of their genome. The animals' organs looked perfectly normal. And of more than 100 tests done on the mice tissues to assess gene activity, only two showed changes. The results are reported in this week's Nature2.

The group has now created mice missing three million base pairs. "We can see no effect in them," Rubin says.

Tough test

Knowles cautions that the study doesn't prove that non-coding DNA has no function. "Those mice were alive, that's what we know about them," she says. "We don't know if they have abnormalities that we don't test for."

David Haussler of the University of California, Santa Cruz, who has investigated why genetic regions are conserved, says that Rubin's study gives no hint that the deleted DNA has a function. But he also believes that non-coding regions may have an effect too subtle to be picked up in the tests to far.

"Survival in the laboratory for a generation or two is not the same as successful competition in the wild for millions of years," he argues. "Darwinian selection is a tougher test."

References

  1. Peaston A. E., et al. Developmental Cell, 7. 597 - 606 (2004).
  2. Nóbrega M. A., et al. Nature, 431. 988 - 993 (2004).

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