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Hoarding Junk DNA

2 October 2003 (All day)
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We've got something in common. When it comes to DNA, wallabies and humans have some remarkable similarities.

Important genes have resisted change for millions of years. After all, why mess with a good thing? Surprisingly, DNA sequences that don't include genes are even more stable, according to a new study of mammalian genomes. The findings are a strong signal, the authors say, that this "junk" DNA has some important functions.

Natural selection tends to ensure that vital DNA sequences do not accumulate mutations that could interfere with their function. As a result, the greater the similarity between DNA sequences in species with common ancestry, the more important scientists generally assume these sequences are. Comparing human DNA with that of close and distant relatives consequently sheds light on which sequences are essential.

Earlier comparison of human and mouse DNA suggested that at least 5% of the human genome was highly conserved. But surprisingly, roughly 63% of these sequences were not genes. Delving further into humanity's family tree, a team led by geneticists Emmanouil Dermitzakis and Stylianos Antonarakis at the University of Geneva compared our DNA with that of 13 other mammal species, including the green monkey, white-toothed shrew, nine-banded armadillo, and brush-tailed porcupine. Researchers analyzed 191 non-gene-coding sequences from DNA regions matching up with human chromosome 21, a standard model for genomic exploration. Antonarakis and Dermitzakis previously found that these specific sequences were highly conserved in mice and humans.

In findings published online 3 October by Science, the scientists report that the sequences are 80% to 90% the same on average across most of the species, significantly more conserved than protein-coding genes. This "suggests that they are at least as important as genes or even more in mammals," Antonarakis says.

Researchers speculate that these conserved sequences either help regulate genes or package DNA. Developmental biologist Rakesh Mishra of the Centre for Cellular and Molecular Biology in Hyderabad, India, says he finds it exciting that these regions are getting identified across so many species, as it emphasizes the need for further experiments to determine their function. "This kind of work should be taken to organisms beyond mammals--other vertebrates, and perhaps invertebrates," he adds.

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