Answer:
Colchicine
Explanation:
Colchicine is an alkaloid derived from the autumn crocus (Colchicum autumnale).
The effect of colchicine, which inhibits microtubule polymerization and thus assembly of the mitotic spindle, demonstrates the presence of another checkpoint in the cell cycle. When colchicine is added to cultured cells, the cells enter mitosis and arrest with condensed chromosomes. With increasing time, a large fraction of the cells in a culture become arrested, thus permitting determination of the size, shape, and number of mitotic chromosomes — that is, the karyotype — in multiple cells. A checkpoint control somehow senses when the mitotic spindle has not assembled properly and prevents activation of the APC polyubiquitination system that normally leads to degradation of the anaphase inhibitor, required for onset of anaphase, and later to the degradation of mitotic cyclins, required for the exit from mitosis. As a result, MPF activity remains high, chromosomes remain condensed, and the nuclear envelope does not re-form.
