The force the spindle exerts on a single moving chromosome in anaphase was measured with a flexible glass needle calibrated in dynes per micron of tip deflection. The needle was used to produce a force on the chromosome, which opposed that produced by the spindle and was measurable from needle tip deflection. The measurements were made in intact grasshopper spermatocytes after proving that the presence of materials such as the cell surface did not interfere. The results from 12 experiments in seven cells are as follows: Chromosome velocity was not affected until the opposing force reached approximately 10(-5) dyn, and then fell rapidly with increasing force. The opposing force that caused chromosome velocity to fall to zero--the force that matched the maximum force the spindle could produce--was of order 7 X 10(-5) dyn. This directly measured maximum force potential is nearly 10,000 times greater than the calculated value of 10(-8) dyn for normal chromosome movement, in which only viscous resistance to movement must be overcome. The spindle's unexpectedly large force potential prompts a fresh look at molecular models for the mitotic motor, at velocity-limiting governors, and at the possibility that force may sometimes affect microtubule length and stability.

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