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

Kinetochore spindle fibers in meiosis I and II grasshopper spermatocytes were cut with a heterochromatic ultraviolet (UV) microbeam converging 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 observed 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 were irradiated in anaphase with UV, an area of reduced birefringence (ARB) was produced at the exposed site. The newly created + ends of the microtubules rapidly disassembled poleward, at a constant speed of 17 mum/min. The - ends at the edge of ARB also depolymerized at a slower rate. When a kinetochore fiber was cut with UV in early anaphase at which time its associated chromosome had not disjoined from the partner chromosome, the chromosome of the irradiated kinetochore fiber moved rapidly back to its partner. The speed during this movement was faster than 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 pronounced 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 tripolar figure.