M P HASE This is the most dramatic period of the cell cycle, involving a major reorganisation of virtually all components of the cell. Since the number of chromosomes in the parent and progeny cells is the same, it is also called as equational division . Though for convenience mitosis has been divided into four stages of nuclear division (karyokinesis), it is very essential to understand that cell division is a progressive process and very clear-cut lines cannot be drawn between various stages. Karyokinesis involves following four stages: .
. Prophase Prophase which is the first stage of karyokinesis of mitosis follows the S and G phases of interphase. In the S and G phases the new DNA molecules formed are not distinct but intertwined. Prophase is marked by the initiation of condensation of chromosomal material.
The chromosomal material becomes untangled during the process of chromatin condensation (Figure . a). The centrosome, which had undergone duplication during S phase of interphase, now begins to move towards opposite poles of the cell. The completion of prophase can thus be marked by the following characteristic events: Chromosomal material condenses to form compact mitotic chromosomes.
Chromosomes are seen to be composed of two chromatids attached together at the centromere. Centrosome which had undergone duplication during interphase, begins to move towards opposite poles of the cell. Each centrosome radiates out microtubules called asters. The two asters together with spindle fibres forms mitotic apparatus.
You have studied mitosis in onion root tip cells. It has chromosomes in each cell. Can you tell how many chromosomes will the cell have at G phase, after S phase, and after M phase? Also, what will be the DNA content of the cells at G , after S and at G , if the content after M phase is 2C?
Cells at the end of prophase, when viewed under the microscope, do not show golgi complexes, endoplasmic reticulum, nucleolus and the nuclear envelope. . . Metaphase The complete disintegration of the nuclear envelope marks the start of the second phase of mitosis, hence the