RAPID BACKWARD MOVEMENT OF ANAPHASE CHROMOSOMES WHOSE KINETOCHORE FIBERS WERE CUT BY ULTRAVIOLET MICROBEAM IRRADIATION

Kinetochore spindle fibers in meiosis I and II grasshopper spermatocyt es were cut with a heterochromatic ultraviolet (UV) microbeam convergi ng on the specimen to form a slit-shaped microspot 1.5 x 8 mum or 3 x 8 mum. A total exposure of 3 x 10(-8) joules per mum2 was administered within 0.8-2.4 s, which was sufficient for severing. The cells were o bserved with a high extinction polarizing microscope or phase contrast optics and a record made by time-lapse video microscopy, continuously before, during and after the irradiation. When kinetochore fibers wer e irradiated in anaphase with UV, an area of reduced birefringence (AR B) was produced at the exposed site. The newly created + ends of the m icrotubules rapidly disassembled poleward, at a constant speed of 17 m um/min. The - ends at the edge of ARB also depolymerized at a slower r ate. When a kinetochore fiber was cut with UV in early anaphase at whi ch time its associated chromosome had not disjoined from the partner c hromosome, the chromosome of the irradiated kinetochore fiber moved ra pidly back to its partner. The speed during this movement was faster t han the normal poleward chromosome movement in anaphase by an order of magnitude or more. When a kinetochore and its associated kinetochore fiber were included in the irradiation area, the effects were more pro nounced than the effects of irradiation of a kinetochore fiber alone: the direction of the line connecting the irradiated half-bivalent with the partner half-bivalent deviated so much from the longitudinal axis of the original spindle with time that the division assumed a tripola r figure.