A cell divides by two processes namely the mitosis and meiosis. Of them mitosis is quite common as most types of cells undergo division by this methods. Where as meiosis is limited to very few cells like the reproductive cells.
Stages of mitosis include
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 each, joined by the centimeter.
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.
Microtubules fibers which 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 at 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.
Chromosomes are kept in position at the equator by the spindle fibers coming from opposite poles. These polar spindle fibers exert opposing pushing forces at the centromeres and the chromosome is held at place.
The Polar spindle fibers shorten and the centromere splits and the chromosome split 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 not connected to the chromosomes lengthen and thus, in turn, 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 poles of the cell have a complete set of chromosomes.
The spindle fibers continue to lengthen, thus elongating the cell.
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.
Thus the genetic material of the parent nucleus is divided into 2 nuclei which are genetically equal and identical.
The stage of Cyotokinesis:
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 separate the parent cell in 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 their 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.