Every time a cell divides, the chromosomes must be distributed accurately to the daughter cells. Errors in distribution arise if chromosomes are improperly attached to the mitotic spindle. Improper attachment is detected by a cell-cycle checkpoint in many cells and the completion of cell division is delayed, allowing time for error correction. How is an improperly attached chromosome detected? An absence of tension from mitotic forces is one possibility. Here we test this possibility directly by applying tension to an improperly attached chromosome with a micromanipulation needle. In the absence of tension, the entry into anaphase and the completion of mitosis was delayed by 5-6 hours. When the misattached chromosome was placed under tension, however, the cell entered anaphase in 56 minutes, on average. Tension from mitotic forces or from a micromanipulator's needle evidently signals to the checkpoint that all is in order and that cell division can proceed.