Describe the different phases of mitosis with the help of a diagram.

MITOSIS:

 Mitosis is the process by which a nucleus divides into two identical daughter cells. Mitosis takes place in all parts of plants and animals. The time taken for mitosis of one nucleus changes with the species of organisms and prevailing temperature. Drosophila, fruit fly, takes 7 minutes while man, 100 minutes. 

Phases of Mitosis:

 Mitosis is a continuous process, but four main phases are recognized, each of which merges into the other. 

(1) Prophase:

 (I) Chromosomes become visible as two lengthwise halves or chromatids. The chromatids are attached to each other at the centromere. The chromosomes are visible because the chromatin material has shortened and thickened by condensation. 

(ii) The centrosome divides to form two centrioles. The centrioles move towards the opposite pole of the cell and form spindle fibres. Centrioles are absent in higher plants. 

(iii) The nuclear membrane disappears. 

(iv) Nucleoli (singular nucleolus) disappear. 

(v) From each centriole, fibres originate forming a star shaped aster. Spindle fibres, centrioles and aster are collectively called mitotic apparatus. 

(2) Metaphase: 

Chromosomes arrange themselves on the cell equator position or midline of the spindle fibre. Each chromosome is attached to a separate spindle fibre by its centromere. 

(3) Anaphase: 

(i) The centromere divides and the two chromatids are drawn apart and become chromosomes. 

(ii) The chromosomes move towards the opposite poles drawn by the spindle fibres. 

(4) Telophase: 

(i) Chromosomes reform by a.n uncoiling process, become thinner, and finally disappear. 

(ii) Nucleoli reform. 

(iii) Nuclear membranes reappear While the nucleus is dividing by mitosis, the cell cytoplasm and plant cell wall are also dividing.

    Animal cells and amoebae divide by a 'furrowing' of the plasma membrane cytoplasm along the cell equator position. Plant cells develop a cell plate along the cell equator position which grows inwards to meet the opposite cell plate. Cellulose is then deposited on either side of the cell plate, the original cell now has formed two daughter cells.