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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.
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Fygenson et al. (1995) studied this question.
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