Fig. 1. Three states of kinetochore attachment to the spindle. Chromosomes in blue, kinetochore microtubules, spindle outlines and poles in gold. (A) Mitosis in vertebrate cells. Top to bottom: a chromosome with both kinetochores (blue ovals) unattached (u), a chromosome with both kinetochores strongly attached (s), under tension from oppositely directed poleward forces (green arrows), and a misattached chromosome with one kinetochore unattached (u) and one weakly attached (w); tension from antagonistic antipoleward forces (red arrowheads) and poleward forces (green arrow) is present. (B) First division of meiosis in grasshopper spermatocytes. As for A, except the misattached chromosome at the bottom has both kinetochores weakly attached (w) to the same pole and tension is absent – only poleward forces are present.