CELL DIVISION

           Growth and development are characteristic features of cells. During this period, the cells increase both in numbers and size. Most of the living organisms are made up of number of cells therefore, if a single cell has to become multicellular, they must undergo repeated cycles of growth and division. Such cycle is known as cell cycle. This helps the organism not only to increase the cell number and size but also transfer genetic information from parents to off springs (sexual reproduction).

Hence, cell division is important in a living organism for

Growth and development

Reproduction and

Tissue renewal (in mature organisms)

The cell cycle is essentially divided into two periods – (a) Interphase and (b) Division phase 

(a)Interphase

1. It is known as non-division phase or resting phase.

2. It lasts for more that 95% of the duration of cycle.      

3. Although it is called as resting phase, but the cell is biochemically very active. During this phase, the cell prepares itself for the ensuring cell division and synthesizes various biomolecules including two similar copies of DNA.

4. It‘s divided into G₁ phase (“First –Gap”) , the  S phase (Synthesis) and G₂ phase (Second Gap)

 (b)Division phase

1. It is also known as M. phase or Mitosis phase.

2. It lasts for only 5% of the duration of cycle.

3. The division phase consists of two phases –M-phase (or Mitosis phase) with division of nucleus or karyokinesis and division of cytoplasm or cytokinesis. M-phase is further subdivided into Prophase (“meta” –between), Ana-phase (“ana-back) and Telophase (“end”). Howard and Pele divided cell cycle into these four phases.

MITOSIS

(In Greek, “mitos” means thread). It’s a mechanism in which a cell distributes its different components in equivalent amounts that have already been duplicated during interphase. Therefore, it’s also called as educational division.

Prophase

The chromatin threads (each chromosome is made up of two chromatin threads) become more tightly coiled or condense into discrete chromosomes, which can be observed with a light microscope.

Meaphase  (It’s the longest phase of mitosis)

The chromosomes are thickest and shortest because of further condensation.

The chromosomes become attached to the microtubules of the mitotic spindle at kinetochore, aligned at the equatorial plate. Exactly in the middle of the spindle.

Anaphase (Shortest stage)

The centromere* splits causing the sister chromatids to separate. The two free chromatids now begin to move towards the opposite poles.

Telophase

The  polar migration of daughter chromosomes is completed i.e., daughter chromosomes reach the end of the poles.

Daughter chromosomes also start to uncoil and become less condensed. As a result they again form chromatin threads.

Nuclear envelope, nucleoli, golgi complex and ER reappear. In this way two daughter nuclei form within the cell.

The nuclear division is followed by division of cytoplasm, which will result in the formation of two daughter cells.

Cytokinesis: In animal cells, cytoplasm divided by forming a cleavage furrow (because of absence of cell wall). Whereas in a plant cell it starts with the formation of a cell plate at the centre, which later extends into both the sides towards the periphery.