Class IX, BIOLOGY, "Mitosis"

Mitosis

It is that cell division in which the number of chromosomes in both daughter nuclei remains same as in parent nucleus.


Events of Mitosis

Mitosis has the following phases:

1. Prophase
2. Metaphase
3. Anaphase
4. Telephase
5. Cytokinesis

Prophase


1. In this phase, coiling of chromosomes starts and their length decreases but diameter increases. It means that chromosomes become shorter and thicker, this process is called condensation.

2. Microtubules arrange to form a structure called spindle.

3. In animal cell, there are also present centrioles on both poles of spindle. From each centriole, small microtubules or fiber arise forming a star shaped aster.

4. Spindle fibers, centrioles and aster collectively form mitotic apparatus. In plants, this apparatus is made up of only spindle fibers as asters are absent in these cells.

5. Nuclear membrane is broken down. Nucleolus disappears and chromosomes scatter over the spindle fiber.

6. Each chromosome consists of two similar threads like structure called chromatids, these chromatids are united to each other by means of centromere.

Metaphase

1. The chromosomes arrange themselves on equator of the spindle to form an equatorial plate. In this condition, chromosomes become more visible.

2. The chromosomes are attached at their centromere to one spindle fiber from each pole.

Anaphase

1. First of all spindle fibers shrink and become short.

2. The centromere of each chromosome then divides and the two chromatids of each chromosome start separating. At this stage these are not called chromatids because these are no in united condition these are called chromosomes.

3. These chromosomes start moving slowly towards the opposite poles. In this way, one set of chromosomes moves towrds one pole while other towards the other pole.

Telophase

1. The chromosomes reach their poles.

2. The chromosomes uncoil and become less visible.

3. Nuclear membrane reforms and nucleolus appears too. Therefore two nuclei are formed. Each daughter nucleus has the same number of chromosomes as the parent cell.

Cytokinesis

The division of cytoplasm is called cytokinesis. It begins at the last stages of nuclear division. In plant cell, cytoplasm divides by formation of cell plate which is also called phragmoplast. It gradually extends outward and finally two daughter cells are separated.

In animals, cytoplasm divides by furrowing. During this, there occurs inward pinching of cell membrane resulting into two daughter cells. In mitosis, two daughter cells are formed from one parent cell which are identical to their parent cell.

Significance of Mitosis

1. Mitosis occurs in all types of somatic cells.

2. Daughter cells formed as a result of mitosis have same number of chromosomes as that of parent cell. In this way, all cells of body of an organism have same number of chromosomes.

3. Zygote divides by mitosis to form embryo and after hatching or birth, mitosis continues up to maturity of an individual.

4. Mitosis also results in growth and repairing of damaged or worn out tissues.

5. Healing of wounds is also due to mitosis