Question

Spindle formation can be disrupted by exposing cell to the microtubule poison such as
A. High concentration of oxygen
B. Vitamin A
C. Cholesterol
D. Colchicine

Answer 1

Hint: Centrosome and centrosphere are visible only in non-dividing cells. During cell division, they transform to the mitotic apparatus; the centrosome gives rise to spindle fibers and the centrosphere forms astral rays. At the beginning of cell division, centrioles undergo duplication and the resulting daughter centrioles migrate to opposite poles.

Complete Answer:
- Mitosis in which the mitotic apparatus consists of the mitotic spindle and two astropheres is called astral or amphiastral mitosis. It is common among plants and animals. In higher plants some invertebrates (eg, Diptera) centrioles are absent.
- So, in them astral spindles and astrospheres will be absent, and the mitotic apparatus consists of only mitotic spindles. Mitosis in which asters are altogether absent is called anastral mitosis. Mitotic apparatus is significant in that it provides the principal mechanism for the movement and the segregation of chromosomes.
- During anaphase of mitosis, chromosome separation takes place and the cell gets half the number of chromosomes. But if we introduce colchicine, it affects the microtubule formation and thereby spindle formation. That cell gets a pause at the metaphase position and results in polyploidy due to a greater number of chromosomes.

So, here the correct answer is D) Colchicine.

Additional information:
- In multicellular organisms, mitosis is a means of growth, development, repair, regeneration and revitalization.
- In unicellular organisms, mitosis serves as a mechanism of asexual reproduction and genetic transmission.
- Mitosis is necessary for maintaining the accuracy in the orderly development of organisms, and also for preserving the genetic constancy of a species.

Note: Mitosis involves two closely related processes, namely nuclear duplication and cytoplasmic cleavage known as karyokinesis and cytokinesis respectively. These two are completed in a definite stage of cell cycle, called mitotic phase (M phase). The interval between the end of one mitotic cycle and beginning of next is the interphase. So, the cell cycle consists of two distinct phases, namely interphase and mitotic phase.