The spontaneous nucleation of individual microtubules from tubulin dimers is observed directly under conditions of marginal nucleation. The results are compared with measurements of bulk turbidity following a deep quench [W. A. Voter and H. P. Erickson, J. Biol. Chem. 259, 10 430 (1984)]. The size of the critical microtubule nucleus is 12±2 tubulin dimers in the two regimes, but the assembly pathway is different.

• Received 17 October 1994

DOI:https://doi.org/10.1103/PhysRevE.51.5058