Cell Division

All living cells have the property of multiplication. Such a property of multiplication is known as cell division.

Cell cycle

The entire set of changes or events taking place during the division of a mother cell into daughter cells is known as the cell cycle. The cell cycle is completed in the following phases:

1. Interphase
   • G1 phase
   • S phase
   • G2 phase
2. M phase (mitotic phase, karyokinesis)
3. Cytokinesis

G1 phase (Gap one phase / growth phase)

• Active synthesis of RNAs, carbohydrates and lipids occurs.
• The cell grows in size and vacuoles disappear.
• Protein synthesis takes place.

S phase (synthetic phase)

• Replication of the DNA molecule.
• Synthesis of histones.
• Actual duplication of chromosomal material.

G2 phase (second growth phase)

• Synthesis of spindle proteins.
• Synthesis of three types of RNA molecules by DNA.
• Duplication of mitochondria and plastids.

M phase

This is the division of the nucleus. 'M' stands for the mitotic phase in mitosis and the meiotic phase in meiosis. It has four stages:

a) Prophase b) Metaphase c) Anaphase d) Telophase

Cytokinesis

This is the last stage of the cell cycle. In this stage the division of the cytoplasm takes place and finally the mother cell is divided into daughter cells.

Types of cell division

1. Amitosis (amitotic cell division)
2. Mitosis
3. Meiosis

Amitosis (direct cell division)

This is a direct cell division. It occurs among unicellular organisms like some protozoans, bacteria and yeast. At the end of amitosis, two daughter cells are formed which are not necessarily identical to each other or to the mother cell.

Mitosis

Mitosis is known as equational or indirect cell division. It occurs among the vegetative or somatic cells. The number of chromosomes remains constant. Mitosis takes place in haploid or diploid cells. It is completed in the following phases:

i) Interphase

It is the resting phase in which no visible changes occur in the nucleus, but the cell is metabolically active. Its features are:

• The cell has a large nucleus with an intact nuclear membrane and a distinct nucleolus.
• The DNA chromosome is duplicated.
• RNA and protein are also synthesized.
• The size of the nucleus and the cell enlarge.

ii) Karyokinesis

This is the division of the nucleus. It is highly complicated and is completed in the following stages:

a) Prophase

• Chromatin fibres begin to condense and become chromosomes.
• The shortening and thickening of chromosomes begins.
• Each chromosome appears to consist of two chromatids due to longitudinal splitting.
• The nuclear membrane and nucleolus begin to disorganize and disappear by the end of this phase.

b) Metaphase

• The nuclear membrane and nucleolus disappear.
• The spindle fibres appear at two opposite poles and get attached to the centromere of the chromosomes.
• All the chromosomes come to lie at the equator of the cell in a single line.
• The chromosomes are distinctly visible and can easily be counted and their shape and size determined.

c) Anaphase

• The centromeres of the chromosomes split longitudinally so that the two sister chromatids separate from each other.
• Each chromatid now has its own centromere and is known as a daughter chromosome.
• Each chromosome appears 'V', 'X', 'T' or 'I' shaped.
• At the end, two groups with an equal number of chromosomes are formed at the two opposite poles.

d) Telophase

• All the daughter chromosomes reach their respective poles and get organized into a nucleus.
• Each daughter nucleus has an equal number of chromosomes as the mother cell.
• The spindle fibres may partially or completely degenerate.