Colchicine addition to cells 1. When is colchicine added to dividing cells for karyotype studies? I know that colchicine inhibits polymerization of microtubules. 2 So, why should it stop the cell cycle at metaphase when microtubules have already assembled and the next step(anaphase) requires depolymerization rather than polymerization ?

Colchicine inhibits the formation of the microtubules by binding to tubilin and rendering it unavailable for the polymerization. Thats why the cells get arrested in the metaphase and can not go on further in the cell cycle and divide. For chromosome studies this is very useful since this is the phase where the chromatin is most condensed and can be viewed best. It inhibits the positive side producing the microtubules to prevent it from moving to the negative side, in cell division.