Actin filaments are destroyed by severing activity. A long actin filament (red dots), previously shown during its elongation phase in Fig. 2 A, is fragmented into many short pieces. At t = 135 s, the filament was 25.4 µm in length. During the next 20–30 s multiple severing events, some of which are marked by arrows, generate numerous small fragments from the mother filament. Most fragments quickly disappear from the field of view. The apparent severing frequency was seven breaks over 25.4 µm of original filament length during a 23-s period, or 0.012 breaks/µm/s. Three other actin filaments in this field of view (white, blue, and yellow dots) were also eliminated or shortened by fragmentation (arrows). See also Video 5 (available). Time points indicate elapsed time from start of video sequence. Bar, 2 µm.