Describe the various sequences of events that occur during mitosis

Mitosis is a type of cell division that occurs in somatic cells of eukaryotic organisms. It is known as equational division because the number of chromosomes remains unchanged from the parent cell to the daughter cells. This process is very important for the growth of the body, replacement of old or damaged cells and asexual reproduction in many organisms. In humans and most animals, mitosis helps in increasing cell numbers without changing the genetic material. Each daughter cell formed after mitosis has the same number and type of chromosomes as the mother cell.

The whole mitotic process happens in a stepwise manner and is completed in two main stages:
  1. Karyokinesis (division of the nucleus)
  2. Cytokinesis (division of the cytoplasm)

1. Karyokinesis (Division of the Nucleus)

Karyokinesis is the first stage of mitosis in which the nucleus of the cell divides properly to ensure that each daughter cell receives an exact copy of the genetic material. The word karyokinesis is made up of "karyo" meaning nucleus and "kinesis" meaning movement or division.

In many earlier books and older classification systems, karyokinesis was described in four phases: prophase, metaphase, anaphase and telophase. But according to modern cell biology and updated textbooks, it is now divided into five phases, with prometaphase added as a distinct stage between prophase and metaphase.

Now let us understand all five phases one by one in the correct sequence:

i. Prophase:

  • This is the beginning stage of karyokinesis. In this phase, the chromatin present in the nucleus starts condensing and forms visible thread-like chromosomes. Each chromosome consists of two sister chromatids attached at a centromere. The nucleolus begins to disappear and the centrosome starts moving towards opposite poles of the cell. Spindle fibres start forming from the centrosomes. The spindle apparatus is very important for the later stages of mitosis.

ii. Prometaphase:

  • Prometaphase is a newly accepted separate phase that comes after prophase. In this phase, the nuclear envelope breaks down completely, which allows spindle fibres to contact the chromosomes. Special disc-shaped structures called kinetochores form on each chromatid at the centromere. Spindle fibres attach to these kinetochores and help in the movement of chromosomes. The chromosomes now start moving towards the center of the cell.

iii. Metaphase:

  • Metaphase is the stage where the chromosomes become most clearly visible under a microscope. The chromosomes align themselves along the metaphase plate, an imaginary line at the centre of the cell. This alignment ensures that the separation of chromatids in the next phase will be equal. At this point, the spindle assembly checkpoint also occurs to make sure that all chromosomes are properly attached to the spindle fibres.

iv. Anaphase:

  • Anaphase begins when the centromeres split, and the two sister chromatids of each chromosome are pulled apart. Now, each chromatid becomes an independent daughter chromosome. These daughter chromosomes are pulled by the spindle fibres towards opposite poles of the cell. Anaphase ensures equal distribution of chromosomes between the two forming daughter cells.

v. Telophase

  • This is the final phase of karyokinesis. In telophase, the chromosomes that have reached the poles begin to uncoil and become less visible. A new nuclear envelope starts forming around each set of chromosomes. The nucleolus also reappears in each daughter nucleus. The spindle fibres disappear and now the cell has two separate nuclei, each with the same genetic material.

2. Cytokinesis (Division of Cytoplasm)

After karyokinesis is completed, the cell enters cytokinesis. In this phase, the cytoplasm and the remaining cell contents are divided between the two daughter cells. The method of cytokinesis differs slightly in plant and animal cells:
  • In animal cells, a cleavage furrow forms at the centre of the cell. This furrow deepens and finally pinches the cell into two separate daughter cells.
  • In plant cells, due to the presence of a rigid cell wall, a cleavage furrow cannot form. Instead, a cell plate forms in the centre of the cell from Golgi vesicles. This cell plate gradually becomes the new cell wall between the daughter cells.
The whole mitotic process happens in a stepwise manner and is completed in two main stages: Karyokinesis (division of the nucleus) Cytokinesis (division of the cytoplasm) 1. Karyokinesis (Division of the Nucleus) Karyokinesis is the first stage of mitosis in which the nucleus of the cell divides properly to ensure that each daughter cell receives an








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