生物谷报道：大家都知道人类，大鼠等动植物基因组中90%以上是内含子序列，也即是不表达基因的，甚至可以称为是垃圾序列，至少是很少表达基因的（虽然现在证明内含也可能具有一定的功能）。但是小鼠的基因组却是十分特别，几乎纯净的，都表达相应的基因，这一结果令人不得其解，是什么原因让小鼠的基因组保存如此完美？在长期进化过程中，基因没有退化或改变，没有产生垃圾序列？最新Nature上一项相关研究对此作了一定的解释，值得读读。
Mice seemed normal even when 3% of their genome had been deleted.
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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.
Knowles speculates that transposable elements could control embryonic differentiation, activating or reprogramming parental chromosomes. "I think they contain controlling sequences," she says.
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
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
Peaston A. E., et al. Developmental Cell, 7. 597 - 606 (2004). | Article | PubMed |
Nóbrega M. A., et al. Nature, 431. 988 - 993 (2004). | Article |
