FIGURE 2.

(a) DIC image of an elongating microtubule. The growing plus end is on the right and the seed is on the left. The scale bar corresponds to 5 μm. (b) Same microtubule as in Fig. 2 a imaged 5 min later. The microtubule has grown out of the field of view and the shape has been changed by thermal fluctuations except for the left seed region. A digitized curve, found by automated image tracing (see Fig. 1), is superimposed on the image. The displayed coordinate system is chosen as explained in c. (c) Parameterization of microtubule shape. The path length s is chosen to be zero at the transition between the clamped (shaded) and the free (black plus white) part of the microtubule. The center of the coordinate system is located at s = 0 with the x axis along the direction of clamping. Shape can be parameterized either by y(s) or θ(s). In the experiment, only the length between s = 0 and s = (black) is analyzed. During the course of the experiment, the microtubule is growing and its full length corresponds to s = L.