Mitosis: Definition, Stages & Significance

Have you ever wondered how our injuries will heal after a few days? Are cells playing a major role in the healing of injuries? How does a small baby develop into an adult? What type of cell division is included in this? Yes, Mitosis is the type of cell division that is responsible for the replacement of damaged tissues. The term mitosis was coined by Fleming in \(1882\). Mitosis occurs in somatic cells and is responsible for the growth, development or repair of tissues and wounds in an organism. Hence it is also known as somatic cell division. In this article, we will learn about stages, significance, and much more about mitosis.

What is Cell Division?

According to cell theory, every cell arises from a pre-existing cell. The process by which the formation of two or more daughter cells from a single mother cell occurs is called cell division.

Types of Cell Division

There are three types of cell division. They are:

1. Amitosis- It is also called direct cell division or karyostenosis. In this kind of division, a polyploid nucleus divides into two parts with or without division of cytoplasm. It is mainly seen in prokaryotic cells like bacteria. This is called direct cell division, in which there is first a simple division of the nucleus, without any changes in its structure, followed by division of the protoplasm of the cell.
2. Mitosis- Mitosis is a type of nuclear division that results in two daughter cells of the same ploidy.
3. Meiosis- It is a type of nuclear division that gives rise to four haploid daughter cells commonly referred to as gametes.

Cell Cycle and Continuation of Life

The cell cycle is a genetically controlled series of changes that occur in a newly formed cell by which it duplicates its contents, undergoes growth and division to form two daughter cells.

It has two states or phases:

1. Interphase or I-phase
2. Division phase or M-phase

Interphase: The \({{\rm{G}}_{\rm{1}}}\) Phase, S Phase, and \({{\rm{G}}_{\rm{2}}}\) Phase together form the interphase. The cell spends a period of its growth under interphase. This is the longest phase in the cell cycle.
\(‘{{\rm{G}}_{\rm{1}}}’\) Phase: \({{\rm{G}}_{\rm{1}}}\) is called the first gap or first growth phase.

1. It is the preparatory stage for oncoming divisions.
2. The cell grows in size, besides synthesising RNA and proteins.
3. No change occurs in its DNA content.
4. \({{\rm{G}}_{\rm{1}}}\)  Phase is the most variable with regard to duration. Some cells that no longer divide may enter into a resting phase (\({{\rm{G}}_{\rm{0}}}\) phase).

‘S’ Phase or Synthesis Phase:

1. During this phase, replication (duplication) of DNA and preparation of histone proteins occur.
2. The chromosomes are duplicated.

\({{\rm{G}}_{\rm{2}}}\) Phase:

1. This is a second gap or second growth phase.
2. In this phase, the synthesis of RNA and protein continues which is required for cell growth.
3. Two centrosomes have formed by replication of a single centrosome.
4. As the \({{\rm{G}}_{\rm{2}}}\) phase draws to a close, the cell enters into the M phase.

Once cytokinesis is completed, the cell successfully goes through the cell cycle and produces two cells from a single precursor.

Fig: Cell Cycle

What is Mitosis?

Mitosis is a type of nuclear division that results in two daughter cells, each having a nucleus containing the same number and types of chromosomes found in the mother cell. Mitosis ensures that all the cells of an individual are genetically identical to each other.

Mitosis is also known as equational division because the chromosome number in the parent cell and daughter cells remains the same.

This was first observed in plant cells by Strasburger in \(1870\) and in animal cells by Boveri and Flemming in \(1879\).

Stages of Mitosis

The mitotic phase is studied in two parts, i.e.,

1. Karyokinesis (a division of the nucleus) 
2. Cytokinesis (a division of the cytoplasm). 

Fig: Stages of Mitosis

Karyokinesis: 

Karyokinesis is derived from two words, i.e. Karyon meaning nucleus and kinesis meaning movement. This is basically the division of the nucleus. It is further divided into four stages. They are:

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

Prophase: (Pro – first, phase – period or stage)

Fig: Prophase

1. The chromatin network of the nucleus starts thickening into distinctive chromosomes.
2. In animal cells, the daughter centrioles start to move apart and finally occupy opposite poles of the cell.
3. Each centriole is surrounded by radiating rays called aster rays. In animals and lower plants, fibrils appear like spokes of a wheel around each centriole to form an aster.
4. The nucleolus disappears at the end of the prophase.
5. The nuclear membrane too gradually disappears.

Metaphase: (Meta – adjacent or in a line, phase – period or stage)

Fig: Metaphase

1. Discontinuous fibres radiate out from two poles and get connected to the disc-shaped structure, or Kinetochore, at the surface of the centromere.
2. Chromosomes are lined up in one plane called the metaphase plate at the equator.
3. Chromosomes are completely condensed and clearly visible at this stage.
4. It is the best step to study the karyotype of an organism.
5. Karyotype: A technique that refers to the pictorial representation of the chromosomal complement of an organism, obtained by sequentially pairing up chromosomes in the decreasing order of their height.
6. Colchicine is the inhibitor of the metaphase stage. It does not allow the spindle fibre to polymerise.
7. The phenomenon of bringing the chromosomes on the equatorial plane while remaining attached to the spindle is called congression.

Anaphase (The Shortest Phase): (Ana – up, Phase – Period or Stage)

Fig: Anaphase

1. The centromeres divide as the spindle fibres contract. 
2. Sister chromatids move towards opposite poles, which are now regarded as chromosomes. 
3. The chromosomes are then pulled towards the pole by the fibres attached to the kinetochores of each chromosome. 
4. The centromere of each chromosome leads at the edge while the arms trail behind it.
5. The movement of equal numbers of chromosomes or chromatids to opposite poles is called disjunction, while the failure of them to separate is called non-disjunction, which leads to chromosomal abnormalities.
6. At poles, the number of chromosomes remains the same but the amount of DNA becomes half as compared to the amount present at the start of prophase.

Telophase: (Telo – Terminal or End Phase – Period or Stage)

Fig:  Telophase

1. The end of the polar migration of the daughter chromosomes marks the beginning of the telophase.
2. A new nuclear membrane is formed around each set of chromosomes in the respective poles.
3. Chromosomes overlap one another, forming chromatin. Nucleoplasm surrounds the area of chromatin.
4. There are two nuclei formed at the poles of the spindle. Thus, Telophase is essentially the reverse of prophase.
5. Endoplasmic reticulum and Golgi complex are re-formed.

Cytokinesis: (Cytos – Hollow or Cell, here Cytoplasm, kinesis – Division)

Fig: Cytokinesis

1. During cytokinesis, the cytoplasm divides by a process called cleavage. 
2. With two daughter cells at the poles, it becomes imperative for the cytoplasm also to be partitioned. 
3. In the plant cells, a cell plate formation begins at the centre of the cytoplasm with the gradual deposition of cellulosic material. The cell plate begins in the centre and proceeds towards the cell wall. This type of cytokinesis is known as ‘centrifugal’. 
4. In animal cells, the cell membrane starts constricting from the sides and proceeds inward till the mother cell is divided into two daughter cells. This type of division is known as ‘centripetal’.

Fig: Cytokinesis in Plant and Animal Cell

Cytokinesis of Mitosis in Plants and Animals

Diagrammatic Representation of Mitosis

Fig: Mitosis Stages in Plant Cell and Animal Cell

Comparison Between Mitosis and Meiosis

Significance of Mitosis

1. Mitosis plays an important role in vegetative growth, wound healing, repair and regeneration.
2. In drones (male honey bees), algae and the organism that reproduce by parthenogenesis, gametes are formed by mitosis.
3. Mitosis helps in asexual reproduction in unicellular organisms like Amoeba.
4. The old dead cells of the body are replaced due to mitosis.
5. Maintains the number of chromosomes as those of parents cells.

Summary

Mitosis is a type of nuclear division that results in two daughter cells, each having a nucleus containing the same number and types of chromosomes found in the mother cell. The chromosome number in the parent cell and daughter cell remains the same in this type of cell division. Mitosis occurs mainly in somatic cells but can form gametes in various organisms. The chromosomes and genes in each daughter cell are identical to the parent cell. Mitosis helps in the replacement of damaged tissues.

FAQs

Q.1. Why is mitosis called equational division?
Ans: The cell is also known as equational cell division because the chromosome number in the parent cell and daughter cell remains the same.

Q.2. What are the stages of mitosis?
Ans: The stage of mitosis are as follows:
a. Prophase
b. Metaphase
c. Anaphase
d. Telophase

Q.3. What is the significance of mitosis?
Ans: The following are the significance of mitosis:
a. Growth and development.
b. Repair.
c. Healing and regeneration.
d. Genetic stability.

Q.4. Why is acetocarmine used in mitosis?
Ans: Acetocarmine mainly stains nucleus and chromatin. 

Q.5. Where does mitosis occur?
Ans: Mitosis occurs in somatic cells. Mitosis helps in the replacement of damaged tissues. In plants, mitosis happens most rapidly during periods of growth.

Q.6. What are asters in mitosis?
Ans: An aster is a cellular structure shaped like a star, including a centrosome and its associated microtubules during the early stages of mitosis in an animal cell.

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