A cell divides by either of the two processes, namely mitosis, and meiosis. Of these two, mitosis is quite common as all the cell types except germinal cells undergo division by this method. The other type of meiosis method is limited to female and male reproductive cells like ova and spermatozoa, respectively.
The 4 Stages of mitosis
Cell division is the breakdown of the parent (original) cell into two or more daughter cells.
In eukaryotes (with a defined nucleus), the cell divides by either mitosis or meiosis.
Mitosis is the process by which a cell divides into two daughter cells such that each cell has a chromosome number equal to that of the parent cell.
Each daughter cell is genetically identical to the parent cell.
Meiosis is the process by which a parent cell divides into four daughter cells such that the chromosome number in the daughter cell is half of the parent cell.
The primary cell division happens in 2 steps-
- Karyokinesis- Division of the nucleus
- Cytokinesis- Division of cytoplasm
Karyokinesis occurs in 4 stages as
- Thin chromatin fibers start condensing, becoming shorter and thicker and becoming chromosomes.
- Each chromosome has two chromatids joined by the centromere.
- The nuclear envelope and the nucleoli start to disintegrate
- Mitotic spindle fibers start forming. This is made up of proteins and microtubules. The spindles form at opposite poles.
- Due to the lengthening of the microtubules, the two pairs of centrioles separate and move towards opposite poles.
- Microtubule fibers that stretch from the pole to the equator of the cell are called polar fibers.
- The spindle fibers attach to the kinetochores at the centromeres. Kinetochores are disc-shaped structures present on both sides of a centromere.
The chromosomes move towards the equator of the cell.
- The nuclear envelope disappears completely.
- The spindle fibers reach maturity.
- The chromosomes are arranged along the equator in a line. This is called the metaphase plate.
- The spindle fibers from opposite poles keep chromosomes in position at the equator.
- These polar spindle fibers exert opposing pushing forces at the centromeres and the chromosome is held in place.
- The Polar spindle fibers shorten and the centromere splits and the chromosome splits into two sister chromatids (which each becomes a daughter chromosome in the daughter cell).
- The Polar spindle fibers shorten further and the chromatids migrate towards opposite poles of the cell, with the centromere traveling first and the arms of the chromatids trailing behind.
- The cell itself gets elongated because the spindle fibers that are not connected to the chromosomes lengthen; thus, the cell becomes longer in shape.
- Thus the two cell poles move further apart in anaphase.
- Cytokinesis starts at the end of anaphase.
At the end of anaphase, both cell poles have a complete set of chromosomes.
- The spindle fibers continue to lengthen, thus elongating the cell.
- The nuclear envelope starts forming around the cluster of chromosomes at both poles of the cell.
- The nuclear envelope forms from the remaining parts of the parent nuclear membrane and the endomembrane system.
- The reappearance of the nucleoli.
- Thus the nuclei start forming at opposite poles.
- Chromosomes uncoil and become thinner and longer to form chromatin fibers. The nuclear envelope reforms.
- Thus the genetic material of the parent nucleus is divided into 2 nuclei that are genetically equal and identical.
The stage of Cytokinesis
- Cytokinesis is the process of division of the cytoplasm of the cell.
- After Cytokinesis ends, two genetically identical daughter cells are produced.
- These daughter cells have the exact number of chromosomes that were contained in the parent cell.
Cytokinesis in the animal cell (without a cell wall)
- A ring of protein filaments forms around the equator of the cell just under the plasma membrane.
- This ring is called the contractile ring. The contractile ring starts shrinking, thus creating a furrow at the equator.
- This is called the cleavage furrow. Thus the cytoplasm gets pinched inwards till, finally, the ring completely separates the parent cell into 2 daughter cells.
Cytokinesis in the plant cell (with cell wall)
- Membrane-covered vesicles from Golgi bodies migrate towards the equator of the cell at the metaphase plate and then fuse to form a cell plate.
- This cell plate grows outwards and finally fuses with the cell membrane forming two daughter cells, with each having its cell wall and membrane.
- A cell wall forms between the two daughter cells.
Thus cytokinesis is responsible for separating the cytoplasm and the cell organelles between the two daughter cells.
What is the purpose of mitosis?
By mitosis, the following things happen.
- Formation of new cells
- Decrease in telomer length of chromosomes.
Mitosis is a way of sustenance of life for an organism as it helps in the replacement of worn-out cells with new cells.
Any damaged or unwanted cells are removed from the body by Autophagy.
This way, the body’s organs can remain healthy and maintain robust physiology.
Also, the length of telomeres reduces with each passing cell cycle. After a cell undergoes approximately 28 cycles or divisions, it would not be able to continue further.
Thus, this also indicates the life span of the organism.
What must happen before a cell can begin mitosis?
Before mitosis begins the cell has undergone S phase and G phases as it is a part of cell cycle.
The ‘S’ phases leads to fomation of duplicate chromosomes (2x+2x) required for chromosomal division.
While the ‘G’ phases provide cell growth and size required for the process.
Also, the cell cycle proteins help to check for any errors in chromosome and prevent the cycle from going into mitosis.
This is a protective mechanism that prevent abnormal cell formation.
What type of cells do not undergo mitosis?
Haploid cells and cells without a proper nucleus like the red blood cells, spermatzoa and ova do not undergo mitosis.
What is the end result of mitosis and cytokinesis?
Formation of two identical daughter cells with diploid chromosomes.
What would happen without mitosis?
There would be no new healthy cells to replace old and wornout cells.
This can lead to damage of organ, inhibition of physiology and eventual death.