Ncert Exercises & Solutions

Mitosis results in producing two cells which are similar to each other. What would be the consequence if each of the following irregularities occur during mitosis?

1. Nuclear membrane fails to disintegrate

2. Duplication of DNA does not occur

3. Centromeres do not divide

4. Cytokinesis does not occur

(a) If nuclear membrane fails to disintegrate, the spindle fibres would not be able to reach chromosomes and they would not move towards opposite poles of the cell. In certain protozoans, such as Amoeba, the spindle is formed within the nucleus and this is called intranuclear mitosis or pre-mitosis.

(b) If DNA duplication does not occur than cell might not be able to surposs S-phase of cell-cycle as no chromosome formation will take place, and will not be able to enter M-(mitotic phase) or if in case it enters mitosis the cycle will cease.

(c) If the centromeres do not divide, one of the daughter cell will receive a complete pair of chromosomes and other cell would not get any of them. This may result in trisomy.

(d) If cytokinesis does not occur, then multinucleate condition called coenocyte, syncytium is produced, as in Rhizopus and Vaucheria, etc.

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