Mitosis Definition and Stages Of Mitosis - Practice Questions & MCQ

Mitosis

• Mitosis is also called the equational division (because the daughter cells have an equal number of chromosomes as that of parents) and somatic cell division (because it occurs in somatic cells).
• Mitosis was first observed by Strasburger in 1875. 
• The term mitosis was given by Fleming in 1882.
• The following two theories have been proposed to substantiate the occurrence of the mitosis:

a) Kern plasm theory: It was proposed by Hertwig. 

1. According to this theory mitosis occurs due to disturbance in Karyoplasmic Index.
2. Karyoplasmic Index of small cell is high as they have less cytoplasm. Nucleus efficiently controls the activity of cytoplasm in small cells.
3. In a large cell nucleus fail to control the activity of cytoplasm. 
4. To attain the control of nucleus on metabolism a large cell divides into two cells.

b) Surface-volume Ratio:

1. When a cell grows in size its volume increases more than its surface. 
2. So a stage will reach when the surface area becomes insufficient to draw the material. 
3. At such a critical stage, division of cell started.

• It takes 30 minutes to 3 hours for mitosis to complete. 
• The time is species-specific and also dependent upon the type of tissues and temperature.
• Mitosis is completed in two steps: Karyokinesis and Cytokinesis
• The karyokinesis is further divided into four substages:

1. Prophase
2. Metaphase
3. Anaphase
4. Telophase
    

Different Stages of Mitosis

The karyokinesis of mitosis can be studied under the following heads:

1. Prophase: 

• During this stage the chromatin condenses and the chromosomes are visible.
• The nucleolus disappears and the nuclear envelope fragments.
• With the disappearance of the nuclear membrane, the centrioles migrate to the opposite end and the spindle formation occurs.
• Till prophase, the chromosomes have no particular orientation because the spindle has not yet formed.
• During late prophase or prometaphase, kinetochores appear on each side of the centromere. 
• The sister chromatids are attached to the so called kinetochore spindle fibers with the help of kinetochore.

2. Metaphase:

• Chromosomes become fully condensed and distinct.
• Chromosomes move towards equatorial plane of spindles or metaphasic plate.
• Chromosomes are arranged with their arms directed towards pole and centromere towards equator.

3. Anaphase: 

• The centromere splits and the two chromatids of the duplicated chromosomes separate from each other.
• Each chromatid now becomes a daughter chromosome.
• Daughter chromosomes, each with a centromere and single chromatid appear to move toward opposite poles.
• Anaphase is the shortest phase of mitosis.

4. Telophase:

• During this phase, the spindle disappears and new nuclear envelopes develop around the daughter chromosomes.
• Each daughter nucleus contains the same number and kinds of chromosomes as the original parent cell.
• The chromosomes become diffuse chromatin and nucleolus reappears.
• It is followed by the division of the cytoplasm

Cytokinesis in Animal Cells:

• In animal cells, cytokinesis occurs through the formation of the cleavage furrow.
• Cleavage furrow represents the indentation of the membrane between the two daughter nuclei.
• A band of actin filament called the contractile ring starts to form the circular constriction between the two daughter nuclei.
• Due to the action of the contractile ring, furrow deepens continuously and ultimately a cell divides into two daughter cells.
• Cytokinesis in the animal cell occurs centripetally, that is, from the periphery to centre.

Cytokinesis in Plant Cell:

• In plant cells, the presence of a rigid cell wall prevents the furrow formation.
• Therefore, in plant cells, a new cell wall is formed between the daughter nuclei to complete the cytokinesis.
• Many Golgi vesicles and spindle microtubules arrange themselves on the equator to form phragmoplast.
• The membrane of Golgi vesicles fuses to form a plate-like structure called a cell plate.
• Golgi vesicles secrete calcium and magnesium pectate. 
• The further cell plate is modified into middle lamella. 
• In plants, cytokinesis occurs in centrifugal order (cell plate formation is from centre to periphery).