Shapes of plus ends of MTs elongating in vitro. (A–C) Slices through tomograms of three examples of the axonemal doublet MTs that served as seeds to nucleate the polymerization of purified porcine brain tubulin. (D–F) Tomographic slices and models of three representative MTs elongating from axonemal doublet MTs. Many of the PFs curve outward from the MT axis. Red crosses mark the origins of the coordinate systems used. Bars, 50 nm. (G) LOESS-smoothed traces of PFs from 60 growing MTs. N, numbers traced. (H and I) Distributions of lengths and angles for the PFs in G. (J) Means and SEM of angles between adjacent segments along PFs plotted as a function of distance from the PF tips.