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Cell Cycle And Cell Division


In order to keep alive, every cell has to obtain organic and inorganic materials from its immediate environment, metabolise the same, and dispose the metabolic waste products into the environment. These processes not only generate the flow of bioenergy but also result in the biosynthesis of macromolecular components, which lead to the steady growth of cells.

The metabolic reactions are controlled by the genetic material present in the nucleus. As the cell grows, the normal equilibrium between cell volume and surface area on one hand and between the volume of cytoplasm and size of nucleus on the other hand gets distributed.

After attaining the optimum growth, it becomes obligatory for the cell to divide and restore equilibrium if it has to survive.

Cell Cycle

The cell cycle may be defined as an orderly set of stages which takes place between the time a eukaryotic cell divides and the time the resulting daughter cells also divide or it is the life of the cell from the time it is formed from a dividing parent cell until its own division into two (see Figure 4.5).

When a cell is going to divide, it grows larger, the number of organelles doubles, and the amount of DNA doubles as DNA replication occurs.

The cell cycle involves two stages, namely, interphase and mitotic phase (M phase).

  • G0 phase is distinguished in cells which do not enter the S phase. They stop at about the middle of G1 phase and then differentiate to take up a specific structure and function.



Cell Division


Cell division is one of the most fundamental characteristics of life. Cell division is a process by which cells reproduce their own kind. In organisms growth, reproduction and repair take place through cell divisions. There are two types of cell divisions as follows:

  1. Mitosis—cell division leading to growth and development
  2. Meiosis—cell division leading to production of gametes (sex cells)

Mitosis: Indirect Nuclear Division It occurs in somatic cells. Mitosis is the cell division in which two identical daughter cells are produced by the division of one parent cell. The unique feature of mitosis is that the same normal chromosome number is maintained at each division of the cell.

  • Interphase is the resting phase and no change in chromosome is visible externally, but actually it is quite active in synthesising the chromosome substance DNA. This is the stage just before the division of the cell. It prepares for this change and has doubled the quantity of DNA. The term mitosis was introduced by Fleming.
  • Phases of mitosis (see Figure 4.6)—The four phases of mitosis are as follows:
  1. Prophase
  2. Metaphase
  3. Anaphase
  4. Telophase


Differences in Mitosis of Animal and Plant Cells


  • In plant cells, there is no centrosome (centrioles) and the asters (star-like figures at opposite poles) are not formed but spindle formation still occurs, formed by cytoplasmic strands (microtubules).
  • In telophase of plant cells, the cytoplasm does not constrict (furrow is not formed). Instead, a cell plate or a new cell wall is laid down in the cytoplasm at the equatorial plane of the spindle.
  • It grows from centre to periphery, thus dividing the original cell into two daughter cells.

Significance of Mitosis

  • Genetic stability: Maintains same chromosome number in daughter cells
  • Growth due to formation of new cells in the tissues
  • Replacement of cells and tissues in the body involves mitosis
  • Repair of damaged and wounded tissues by renewal of the lost cells

Cancer happens when something goes wrong in the production of new cells. Instead of an exact copy being made, the cell misbehaves, and grows and divides in an uncontrolled way. This can lead to the
growth of a lump called a tumour. The cancer cell is not able to switch itself off and it goes on producing abnormal cells that continue to divide indefinitely.


Meiosis Meiosis or reduction division occurs in germ cells during the formation of gametes–sperms and eggs in animals and during spore formation in plants. Since the amount of DNA and the number of chromosomes are reduced to half the original set in comparison to the parent cell, meiosis is often referred to as the reduction division. Meiosis was first observed by Strasburger (1888). Farmer and Moore coined the term meiosis.

Meiosis involves Meiosis I and meiosis II.

Meiosis I is a reduction division. This results in the formation of two daughter cells in which the chromosome number is reduced to half, i.e. from 2n (diploid) number to 1n (haploid) number. Thus, it is reductional in nature. It involves 4 stages namely prophase I, metaphase I, anaphase I and telophase I.

Meiosis II
(2nd meiotic division or homeotypic division or equational division) includes prophase II, metaphase II, anaphase II and telophase II (see Figure 4.7).

Significance of Meiosis 
Meiosis brings about reduction in chromosome number of gametes and maintenance of a constant chromosome number in the species. It also brings about recombination of genes by chiasmata formation and crossing over.


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