Micromanipulation of Chromosomes Reveals that Cohesion Release during Cell Division Is Gradual and Does Not Require Tension

In mitosis, cohesion appears to be present along the entire length of the chromosome, between centromeres and along chromosome arms. By metaphase, sister chromatids appear as two adjacent but visibly distinct rods. Sister chromatids separate from one another in anaphase by releasing all chromosome cohesion. This is different from meiosis I, in which pairs of sister chromatids separate from one another, moving to each spindle pole by releasing cohesion only between sister chromatid arms. Then, in anaphase II, sister chromatids separate by releasing centromere cohesion. Our objective was to find where cohesion is present or absent on chromosomes in mitosis and meiosis and when and how it is released. We determined cohesion directly by pulling on chromosomes with two micromanipulation needles. Thus, we could distinguish for the first time between apparent doubleness as seen in the microscope and physical separability. We found that apparent doubleness can be deceiving: Visibly distinct sister chromatids often cannot be separated. We also demonstrated that cohesion is released gradually in anaphase, with chromosomes looking as if they were unzipped or pulled apart. This implied that tension from spindle forces was required, but we showed directly that no tension was necessary to pull chromatids apart.