This study investigates the dynamic instability of individual microtubules using video light microscopy. Porcine brain tubulin, free of microtubule-associated proteins, was assembled onto axoneme fragments at 37°C, and the dynamic behavior of the plus and minus ends of microtubules was analyzed for tubulin concentrations between 7 and 15.5 μM. The study found that elongation and rapid shortening are distinct phases. Elongation is characterized by second-order association and first-order dissociation reactions. Association rate constants were 8.9 and 4.3 μM⁻¹s⁻¹ for the plus and minus ends, respectively, while dissociation rate constants were 44 and 23 s⁻¹. The rate of rapid shortening was similar at both ends and did not vary with tubulin concentration. Transitions between phases were abrupt and stochastic. The study also found that the frequency of catastrophe decreased at both ends with increasing tubulin concentration, while rescue frequency increased dramatically at the minus end. The results suggest that microtubules assembled from pure tubulin undergo dynamic instability over a twofold range of tubulin concentrations, and that the dynamic instability of the plus and minus ends can be significantly different. The study provides new insights into the molecular events responsible for the dynamic behavior of microtubules. The results are consistent with the existence of a GTP cap during elongation but not with existing GTP cap models. The study also found that the rate of elongation was directly proportional to the free tubulin concentration for both ends. The plus end association rate constant was about twice that of the minus end. The study also found that the rate of rapid shortening was similar for both ends and appeared independent of tubulin concentration. The study also found that pauses occurred during both elongation and rapid shortening phases. The frequency of catastrophe was slightly greater at the plus end, while the frequency of rescue was greater at the minus end. The study also found that the critical concentration for elongation was similar for both ends, around 5 μM. The study also found that the dissociation rates during elongation at the plus and minus ends were significantly faster than previously reported. The study also found that the kinetic events during the rapid shortening phase were best described by a first-order dissociation rate constant. The study also found that the rate of rapid shortening did not decrease at higher tubulin concentrations, indicating no significant tubulin association during the rapid shortening phase. The study also found that the dissociation rate during rapid shortening was much greater than during elongation. The study also found that the frequency of catastrophe was slightly greater at the plus end, while the frequency of rescue was greater at the minus end. The study also found that the frequency of rescue was greater at the minus end. The study also found that the frequency of catastrophe was not steeply dependent on elongation rate. TheThis study investigates the dynamic instability of individual microtubules using video light microscopy. Porcine brain tubulin, free of microtubule-associated proteins, was assembled onto axoneme fragments at 37°C, and the dynamic behavior of the plus and minus ends of microtubules was analyzed for tubulin concentrations between 7 and 15.5 μM. The study found that elongation and rapid shortening are distinct phases. Elongation is characterized by second-order association and first-order dissociation reactions. Association rate constants were 8.9 and 4.3 μM⁻¹s⁻¹ for the plus and minus ends, respectively, while dissociation rate constants were 44 and 23 s⁻¹. The rate of rapid shortening was similar at both ends and did not vary with tubulin concentration. Transitions between phases were abrupt and stochastic. The study also found that the frequency of catastrophe decreased at both ends with increasing tubulin concentration, while rescue frequency increased dramatically at the minus end. The results suggest that microtubules assembled from pure tubulin undergo dynamic instability over a twofold range of tubulin concentrations, and that the dynamic instability of the plus and minus ends can be significantly different. The study provides new insights into the molecular events responsible for the dynamic behavior of microtubules. The results are consistent with the existence of a GTP cap during elongation but not with existing GTP cap models. The study also found that the rate of elongation was directly proportional to the free tubulin concentration for both ends. The plus end association rate constant was about twice that of the minus end. The study also found that the rate of rapid shortening was similar for both ends and appeared independent of tubulin concentration. The study also found that pauses occurred during both elongation and rapid shortening phases. The frequency of catastrophe was slightly greater at the plus end, while the frequency of rescue was greater at the minus end. The study also found that the critical concentration for elongation was similar for both ends, around 5 μM. The study also found that the dissociation rates during elongation at the plus and minus ends were significantly faster than previously reported. The study also found that the kinetic events during the rapid shortening phase were best described by a first-order dissociation rate constant. The study also found that the rate of rapid shortening did not decrease at higher tubulin concentrations, indicating no significant tubulin association during the rapid shortening phase. The study also found that the dissociation rate during rapid shortening was much greater than during elongation. The study also found that the frequency of catastrophe was slightly greater at the plus end, while the frequency of rescue was greater at the minus end. The study also found that the frequency of rescue was greater at the minus end. The study also found that the frequency of catastrophe was not steeply dependent on elongation rate. The