Physics Colloquium - Monday, January 29th, 2007, 4:00 P.M.

E300 Math/Science Center; Refreshments at 3:30 P.M. in Room E200

Dan Needleman
Harvard Medical School

Single Molecule Dynamics in Cell Division

A wide variety of subcellular structures exist in a non-equilibrium steady state with a constant flux of molecules and energy continuously modifying and maintaining their architecture. A prime example of this is the spindle: a remarkable, self-organizing molecular machine that segregates chromosomes during cell division. The spindle is a highly dynamic structure composed of the protein tubulin, which assembles into long polymers called microtubules, and a variety of other proteins that regulate microtubule nucleation, polymerization, depolymerization, and translocation. We investigated the motions of thousands of individual tubulin molecules in spindles, using a combination of single molecule confocal microscopy and automated particle tracking. This study allowed us to perform the first detailed analysis of microtubule polymerization in spindles. Our single molecule data can be quantitatively explained by a first-passage analysis of a very simple model of microtubule dynamics. These results unambiguously rule out a number of proposed mechanisms of microtubule turnover, leading to surprising implications for models of spindle organization.