EVERY time a cell divides, the chromosomes must be distributed accurat
ely to the daughter cells. Errors in distribution arise if chromosomes
are improperly attached to the mitotic spindle. Improper attachment i
s detected by a cell-cycle checkpoint in many cells(1,2) and the compl
etion of cell division is delayed, allowing time for error correction.
How is an improperly attached chromosome detected? An absence of tens
ion from mitotic forces is one possibility(3). Here we test this possi
bility directly by applying tension to an improperly attached chromoso
me with a micromanipulation needle, In the absence of tension, the ent
ry into anaphase and the completion of mitosis was delayed by 5-6 hour
s. When the misattached chromosome was placed under tension, however,
the cell entered anaphase in 56 minutes, on average. Tension from mito
tic forces or from a micromanipulator's needle evidently signals to th
e checkpoint that all is in order and that cell division can proceed.