Today’s Top Science Headline –“Puzzle Posed By ‘Dark Matter’ in DNA Finally Solved”




A puzzle posed by segments of 'dark matter' in genomes — long, winding strands of DNA with no obvious functions — has teased scientists for more than a decade. Now, a team has finally solved the riddle.

The conundrum has centered on DNA sequences that do not encode proteins, reports today's Nature, and yet remain identical across a broad range of animals. By deleting some of these ‘ultraconserved elements’, researchers have found that these sequences guide brain development by fine-tuning the expression of protein-coding genes.

The results1, published on 18 January in Cell, might help researchers to better understand neurological diseases such as Alzheimer’s. They also validate the hypotheses of scientists who have speculated that all ultraconserved elements are vital to life — despite the fact that researchers knew very little about their functions.

“People told us we should have waited to publish until we knew what they did. Now I’m like, dude, it took 14 years to figure this out,” says Gill Bejerano, a genomicist at Stanford University in California, who described ultraconserved elements in 20042.

Bejerano and his colleagues originally noticed ultraconserved elements when they compared the human genome to those of mice, rats and chickens, and found 481 stretches of DNA that were incredibly similar across the species. That was surprising, because DNA mutates from generation to generation — and these animal lineages have been evolving independently for up to 200 million years.

Genes that encode proteins tend to have relatively few mutations because if those changes disrupt the corresponding protein and the animal dies before reproducing, the mutated gene isn't passed down to offspring. On the basis of this logic, some genomicists suspected that natural selection had similarly weeded out mutations in ultraconserved regions. Even though the sequences do not encode proteins, they thought, their functions must be so vital that they cannot tolerate imperfection.

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