The Effects of Tension on the Cohesin Cylinder (2009)

Undergraduate: William Lewis

Faculty Advisor: Elaine Yeh
Department: Biology

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The viability of all eukaryotic organisms requires that the segregation of genetic material be carried out accurately during cellular division. This calls for a balance of forces between paired sister chromatids and the mitotic spindle. Cohesin, a complex of four protein subunits, is primarily responsible for sister chromatid pairing. In budding yeast, Saccharomyces cerevisiae, cohesin is highly enriched around the centromere and collectively forms a cylindrical structure about the mitotic spindle. Given the discovery of this structure and its role in chromosome segregation, it is of great interest to understand how the cylinder responds to changes in tension. Two methods were used to disrupt the balance of forces responsible for maintaining the structure of cohesin in metaphase. The first method involved altering the packaging of DNA, reducing the chromosomes’ ability to resist the pull of the mitotic spindle. Second, the spindle’s ability to pull on metaphase chromosomes was enhanced by over-expressing the motor protein Cin8p. Changes in the cohesin cylinder were monitored by use of fluorescence microscopy. After disrupting the packaging of DNA, the spindle elongated, and the cohesin cylinder became dispersed and speckled. However, over-expression of Cin8p, created multiple phenotypes; all containing intact cohesin cylinders. These results suggest that the cohesin cylinder is capable of withstanding large amounts of force generated by the mitotic spindle, and that its strength is dependent upon the structural integrity of pericentric chromatin.