Steric hindrance and its effect on chromosome movement inVicia faba somatic cells

Summary The arrangement of chromosome arms in metaphases and anaphases has been studied inVicia faba root meristem cells. During metaphase, the long chromosome arms are aligned parallel to the spindle axis. As a consequence, at the onset of anaphase, one chromatid can move straight ahead to the spindle pole whereas the other has to invert its orientation. Specially in narrow cells it has been observed frequently that some chromatids move in a reverse orientation to the pole, i.e., they move telomere-first instead of centromere-first. This behaviour results in a chromatid which protrudes beyond the main group of late anaphase or telophase chromatids. It is dicussed that the most likely explanation for the phenomenon is that in narrow cells chromatid behaviour is influenced by steric hindrance by the tightly packed surrounding chromatids and microtubules. When there is insufficient room, some chromatids are unable to make the required U-turn. Under such conditions the kinetochore of a non-inverted chromatid pulls the chromatid in a reverse orientation to the pole. An alternative explanation, i.e., protruding chromatids being the result of a neocentric activity at the telomere end of a reverse-directed chromatid or the lateral associations of spindle microtubules, failed to find support by electron microscopical studies.