( ing celldivision. However that unders...)Needling Chromosomes Reveals Cell Division Secret

ing cell division. However, that understanding was based on biochemical experiments that revealed when the "glue" protein called cohesin that holds chromatids was degraded during cell division. Also, microscopic studies had shown that there appeared to be two separate chromatids during an early stage of cell division, so it was believed that they had detached from one another at that time. "What hadn’t been done was to attempt to separate chromatids to directly determine whether they, in fact, are held together or not," said Nicklas. "So, Leocadia set out to use micromanipulation to distinguish between visible separateness and physical separateness." To study chromatid adhesion, Paliulis mastered the high art of manipulating two infinitesimal glass needles to impale each of two sister chromatids in cultured grasshopper cells at the appropriate time in cell division. Then, she would ever-so-gently apply force to pull them apart. Upon release, if they remained apart it revealed they were separated; but if they snapped back together the researchers would know the chromatids were still attached. Paliulis was a Duke graduate student when she performed the experiments, but is now a postdoctoral fellow at the University of North Carolina at Chapel Hill Paliulis's skill in the task was extraordinary, said Nicklas. "First of all the needles are invisible in the cell, so you have to continually move them back and forth to detect their position by how they disturb structures around them. Also, the micromanipulation apparatus is arranged such that your view is up through the bottom of the cell, and the needles are coming down through a layer of oil covering the cell to preserve it. "So, you also have to constantly adjust the focus to determine where the needle is coming down into the cell. This is difficult with one needle, but with two it's a terrific challenge; and you really need an almost tactile sense of where the needles are." Nicklas said that even the smallest misstep c
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