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How Chromosomes Got Hitched
4 June 2007 (All day)
Dividing chromosomes are molecular Siamese twins: two bodies of genetic material joined along a stretch of DNA called the centromere. Centromeres help direct the separation of the chromosomes to ensure that newly formed cells have the right amount of DNA. But no one knows exactly how they evolved. Now there is a controversial new theory: These key innovations started out at the ends of ancient chromosomes.
Researchers debate whether centromeres evolved once or many times. The sequence of these stretches of DNA varies quite a bit from organism to organism, suggesting independent origins. Yet centromeres all do the same thing: They keep newly formed chromosomes together at first, then help anchor protein strands that pull chromosomes apart as the cell splits in two. Moreover, the proteins involved in these actions are the same in all organisms. This evidence points to a single origin.
Almost 10 years ago, Alfredo Villasante, a molecular biologist at the Autonomous University of Madrid, Spain, discovered a striking fact about the centromeres of fruit flies. Some of the sequence matched repetitive DNA in telomeres, the caps of chromosomes, which often shorten each time a cell divides and play an important role in aging. Knowing that protein strands sometimes attach to telomeres to move chromosomes, he and his colleagues have developed a scenario to explain chromosome and centromere evolution.
Villasante assumes that chromosomes were originally circular, just as they are in bacteria today. But sometimes the circles broke open, at which time mobile elements embedded in the genome hopped to the ragged ends, capping them and preventing circles from reforming. Over time, these mobile elements accumulated and became the telomeres, says Villasante. As the sequences expanded and evolved, regions just inside the capped ends became the grandfathers of the centromere, Villasante and his colleagues propose online this week in the Proceedings of the National Academy of Sciences.
Even during these early days, the DNA in these regions formed RNA, which combined with certain proteins that attracted the protein strands that pull chromosomes apart. At first, a separate set of strands attached to another, more ancient anchor point in the genome as well. With two sets of strands yanking it at different places, the chromosome tended to break up. Some of the fragments reunited, putting the protocentromeres more in the middle of the chromosomes--just as they tend to be in modern chromosomes, says Villasante.
The theory is getting a mixed review. "The authors have made a good case, within the limitations of the data," says John Kelly, a molecular biologist at the London School of Hygiene & Tropical Medicine. However, Steven Henikoff, a biochemist at Fred Hutchinson Cancer Research Center in Seattle, Washington, calls this scenario "highly improbable," pointing out that new centromeres are evolving all the time, and not from telomeres. "There is no reason to suppose any connection between centromeres and telomeres in function, in time of action, or in sequence," he says.