Colchicine

In 1937 O.G. Eigsti first succeeded in inducing tetraploidy in cells of onion root tips by the
chemical colchicine.
This is an alkaloid obtained from the seed and corm of liliaceous plant Colchicum autumnale. In high concentrations the drug is highly toxic to cells. If used in concentrations as low as 0.01% to 0.5% it inhibits the formation of spindle fibers. The molecular basis of colchicine action is now known. In the electron microscope spindle fibres of eukaryotes are found to be made up of numerous hollow, cylindrical structures known as microtubules, about 240 Å in diameter, and several microns in length. The hollow core of
the microtubule is surrounded by a shell made up of globular protein called tubulin. Colchicine
binds specifically to microtubules; one mole of colchicine binding to one mole of tubulin. The binding leads to breakdown of microtubules . Consequently spindle fibres do not form.Due to absence of spindle fibres, chromosomes which have already duplicated remain in metaphase for a much longer time than they do in untreated cells, a condition described as the c-metaphase. Their centromeres divide, and as there is no anaphasic movement (for lack of spindle fibres), the nuclear membrane is formed around them and the cell enters interphase.
Such a nucleus resulting from a c-mitosis contains double the number of chromosomes which would have otherwise been distributed to two daughter nuclei. In plants colchicine is applied to growing tips, meristematic cells, seeds or axillary buds in aqueous solution or mixed with lanolin.