For the first time, scientists have observed how a fragment of a damaged chromosome independently formed a centromere. After this process, the fragment could duplicate itself like a complete chromosome. We get our genes, among other things, with the help of the so-called centromeres. These are specialized sections of chromosomes that hold together the existing pair of identical strands (sister chromatids) after the doubling of the chromosome strand in the run-up to cell division. During cell division, they ensure that each newly formed cell receives one of the sister chromatids.
Gary Karpen and his colleagues from the Salk Institute in LaJolla are working on the localization of centromeres in the chromosomes of fruit flies. In the course of these studies, they exposed flies to damaging gamma rays, which broke off fragments at the ends of some chromosomes. When the fruit fly cells divided, a startling discovery was made: some of the fragments were passed on for dozens of generations (January issue of Nature Genetics).
The scientists suspected that a new centromere had formed in the fragments. After doubling, the fragments are able to separate and be passed on. Karpen and his group linked the "new" chromosomes to a fluorescent antibody that binds to specific proteins found only in centromeres, proving their suspicion.
Karpen believes that the gamma rays altered the DNA of the fragments in such a way that they were able to form centromeres. He suspects that the radiation initially caused an inversion of the DNA, in which the end of the affected chromosome came close to the centromere-forming regions. Then the centromere either added a protein to the end of the chromosome or changed the structure.
This experiment showed that a gene sequence that normally has nothing to do with the formation of a centromere can be made to take over this function. Especially for applications in gene therapy, it could be of great interest to learn how any DNA can be converted into a centromere.