December 22, 2014

Jr Inter Botany SAQ Important Questions - Cells

Q: Which division is necessary to maintain constant chromosome number in all body cells of φ multicellular organisms and why?
A: Mitotic cell division is necessary to maintain the chromosome number in all the cells of an organism. The mother cell in mitotic cell division produces 2 daughter cells which are equal to their mother cell both in quantity and quality of chromosomes.

Every sexually reproducing diploid organism starts its life with a single cell called Zygote. It under goes numerous mitotic divisions and forms an organism which consists the same chromosome number in all its cells.

The spore also produces a gametophyte by mitotic divisions due to which multicellular the chromosome number is maintained constant in all the cells.

Q: Though redundantly described as a resting phase, interphase does not really involve rest. Comment.

A: Interphase represents the phase between two successive M phases. Interphase lasts more than 95% of duration of cycle. Thus it is the largest stage. It is a preparatory stage or resting phase. It is also called phase of non-apparent division. Though it is called resting phase, it is actually the most active phase as it involves the following activities.

It has 3 phases.
1) G1 (G = Gap): Cytokinesis is followed by G1
The cell is active and shows normal metabolism. Organelle duplication occurs. The amount of DNA in the cell is 2C.

2) S (S = Synthesis):
It is between G1 and G2. It is the largest phase of Interphase. DNA synthesis (replication) occurs in the nucleus 2C DNA becomes 4C. But there is no increase in chromosome number. Parallel to this in animal cells centriole duplicales in cytoplasm.
3) G2 (G = Gap)
Cytoplasm increases. The amount of DNA is 4C. Protein synthesis, ATP synthesis occurs. The cells are ready to participate in Prophase.

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