The tension mounts at centromeric loops

Centromeres are more than just simple stretches of DNA that recruit the kineto-chore proteins required to bind spindle microtubules and segregate chromosomes during mitosis. In fact, the chromatin surrounding centromeres is organized into a spring-like structure that bridges the cen-tromeres of sister chromatids and resists the pull of spindle microtubules. The tension generated when sister kinetochores are correctly attached to microtubules emanating from opposite spindle poles helps silence the mitotic checkpoint and allow sister chro-matid segregation. Lawrimore et al. now describe how the pericentric chromatin spring establishes intracentromere tension (1). Kerry Bloom and colleagues at the University of North Carolina at Chapel Hill previously demonstrated that condensin and cohesin proteins accumulate on the pericentric chromatin of budding yeast chromosomes and organize it into a " bottle brush " configuration, with numerous DNA loops radiating out from a central axis that runs between sister kinetochores in parallel to the spindle axis (2, 3). Now, Bloom says, his team wanted to understand how this structure helps to generate tension between sister centromeres. To learn more about the forces produced at centro-meres, Bloom and colleagues , led by graduate student Josh Lawrimore, fluorescently labeled a segment of pericentric chro-matin and followed its movements in metaphase yeast (1). Surprisingly, Lawrimore et al. found that pericentric chromatin moved in similar ways regardless of whether or not cells were treated with benomyl, an inhibitor of micro-tubule dynamics. During metaphase, therefore , most centromeric chromatin movements are not driven by microtubules. " That means there's a force regime near to the centromere that's largely microtubule independent , " Bloom explains. Instead, the researchers determined, the movements of pericentric chromatin are mainly driven by a combination of Brownian motion and the random, ATP-dependent activities of chaperones and chromatin-remodeling proteins. To characterize the pericentric forces produced by these microtubule-independent movements, Lawrimore et al. used a special plasmid that carries two centromeres separated by a relatively short stretch of DNA. Almost all of this intervening, peri-centric DNA can be fluorescently labeled, allowing the researchers to assess its overall structure when the plas-mid was aligned on the metaphase spindle. The researchers found that, even in the presence of benomyl, the plasmid's pericentric chromatin was stretched out along the spindle axis, so much so that some of its nu-cleosomes would have to be unwrapped and released in order for it to extend so far. Thus, thermal fluctuations and random, ATP-dependent enzyme activities …