Define Cell Division And Structure Of Chromosomes

1. Cell Division is a method by which new cells are originated from pre-existing cells. These new cells need to be produced for growth, development, replacement, repair and reproduction. All organisms contain an even number (2n) of chromosomes in their somatic or body cells. The cell division leading to growth, development, repair and replacement is called Mitosis. The cell division leading to production of gametes (sex cells) is called meiosis.

Cell Cycle : The sequence of events including growth and division, that a cell undergoes from the time of its formation upto its division into daughter cells is called cell cycle. Every cells are capable of division passes through cell cycle which consists of two phases:

1. Interphase (Resting or non-dividing phase; I phase]

2. Mitotic phase or Mitosis (Dividing phase; M phase]

1. Interphase (Resting phase): It is the non-dividing preparatory phase during which a cell grows to its maximum size (by increase in volume of nucleus and nucleolus) and the synthesis of DNA (chromosome material) takes place. when DNA duplication occured that contents the chromatin material also duplicates

Interphase is divided into three sub-stages.

(a) G₁-phase : Volume of cytoplasm increases and protein synthesis takes place.

(b) S-phase : Synthesis of DNA takes place.

(c) G₂- phase : Necessary spindle proteins are synthesized.

The cell is quite active metabolically during this phase and several processes, including DNA synthesis, occur at this time, thus no alternative is left for the cell except to divide, by the process of mitosis.

2. Mitotic phase or Mitosis [Dividing phase] : It is the dividing phase during which nucleus as well as cytoplasm divide into two daughter cells. It is studied in two parts:

(i) Karyokinesis

(ii) Cytokinesis

(1) Karyokinesis (Division of nucleus): In this many events take place in a sequence which are studied in four stages.

1. Prophase : It begins with the appearance of chromosomes which have become short, thick and clearly visible inside the nucleus.(Each chromosomes get duplicated by splitting longitudinally to form two chromatids that are attached to each and every other at a small part called centromere. The centrosome (in animal cell) splits into two along with duplication of centrioles which move towards opposite poles of the cell. Each centriole is surrounded by asters (centrioles along with astral rays). A number of spindle fibres appear between two daughter centrioles.

In plant cell due to the absence of centrosome (centriole), the formation of spindle fibres take place from cytoplasmic strands (microtubules) of the cell.

The nuclear membrane and nucleolus disappear. The chromosomes start moving towards equator of the cell.

2. Metaphase : The chromosomes get arranged at the equatorial plate of the cell. Each chromosomes get attached itself to a spindle fibre by its centromere.

3. Anaphase : Centromere divides and two sister chromatids of each chromosome separate and are drawn apart towards opposite poles to form two groups of chromosomes.

4. Telophase : Each Chromatid or daughter chromosome turns into a network of chromatin threads. Nuclear membrane and nucleolus reappear.

(ii ) Cytokinesis (Division of cytoplasm) : A constriction (furrow) appears in the cell membrane at the middle of the animal cell, which deepens and finally splits the cytoplasm into two, thus producing two new cells. In plant cell instead of a furrow, a cell plate is laid down in the cytoplasm at the equatorial plate. It grows from center to periphery, thus dividing the original cell into two daughter cells.

Note: In Mitosis two identical daughter cells are produced by division of one parent cell. The main aspect of this division is that the same normal chromosome number is maintained at each division of the cell. Hence, it is also called as Equational cell division.

Identification points of Mitosis :

1. Prophase - Chromosomes with two chromatids, disappearance of nuclear membrane as well as nucleolus and formation of the spindle.

2. Metaphase - Chromosomes arranged at equatorial plate.

3. Anaphase - Movement of daughter chromosomes towards pole.

4. Telophase - Uncoiling of chromosomes, disappearance of spindle, reappearance of nucleolus and nuclear membrane.

Differences between mitosis in animal and plant cell:

Animal cell

1. Spindle formed is amphiastral.

2. Cytokinesis by constriction (furrow) method. 

3. Cytokinesis proceeds from outside to centre. 

4. Centrosome present so 2 asters are formed. 

Plant cell

1. Spindle formed is anastral.

2. Cytokinesis by formation of cell plate.

3. Cytokinesis proceeds from centre towards periphery.

4. Centrosome absent so no asters are formed.