The answer is prophase. In prophase the mitotic spindle forms and the chromosomes condense. This is in preparation for metaphase where spindle fibers are then formed and attached to the kinetochores of the chromosome’s centromere awaiting to pull the chromosomes sister chromatids (in mitosis) of sister chromosome (in meiosis) to opposite poles. The spindle fibers are composed of microtubules hence interference of microtubule assembly by the colchicine result to non-formation of the spindle. The cell mitosis cannot, therefore, progress past metaphase stage.
