Introduction

The process of cell replication and cell division is essential for the maintenance and continuity of life of an organism. Cell division is a process of growth and reproduction at a cellular level. It is the process by which new life comes into existence from a pre-existing cell. The process of cell division not only gives rise to a new life but is also responsible for transferring the traits of the parent cell into the daughter cells. 

The process of cell division involves two essential steps, division of the cytoplasm and division of the nucleus. The division of the nucleus is of two types; Meiosis and Mitosis. Out topic of discussion in this article is mitosis. 

Mitosis is a type of nuclear division that is a part of the normal cell cycle of most of the cells. It is a process in which a nucleus divides into two daughter nuclei having the same number of chromosomes as the parent nucleus.  We will study various steps of mitosis, its regulation as well as its importance in living organisms. 

The Cell Cycle

Before jumping to studying the various steps of mitosis, it is important to understand the pre-mitotic events that prepare a cell for mitosis. Recall that mitosis is a part of the normal cell cycle. 

Cell cycle is divided into four phases;

1. G₁ phase
2. S phase
3. G₂ phase
4. M phase

G₁ Phase

It is the first phase of the cell cycle. Once a daughter cell is formed from the parent cell, the G₁ phase of the cell cycle starts. During this interphase, a cell grows in size, becomes structurally and functionally active, and matures into a parent cell. At some time during this interphase, the cell receives signals that it should go division again. These signals lead the cell to enter the next phase of the cell cycle, the S phase.

S Phase

During S phase, also called the synthetic phase, the cell undergoes DNA replication. As a result, chromosomes in cells consist of two identical chromatids i.e. the nuclear material in the cell is doubled. 

G₂ Phase 

During the G₂ phase, the cell prepares itself for cell division. The duplicated DNA is checked for errors, and proteins necessary for cell division are made at an exponential rate. The cell receives signals to undergo cell division and enters the M phase.

M Phase

It is also called the mitotic phase. It is a phase of the cell cycle during which nuclear division takes place. The division of nuclear material is followed by cytokinesis, the division of cytoplasm, and the parent cell gives rise to two daughter cells. 

The phase between two cell divisions are collectively called interphase. 

G₀ Phase

The decision of whether the cell will undergo cell division or not takes place at the end of the G₁ phase. If the cell does not receive signals for cell division, it enter the G₀ phase, a senescence phase in which no division occurs. 

Some cells during their lifetime receive signals for division and come out of the G₀ phase and enter the phase. Others stay in the G₀ phase throughout their life. 

Phases of Mitosis

Mitosis occurs as a part of the mitotic phase of the cell cycle. Cell enters the mitotic phase with replicated DNA and is ready to undergo division of nucleus into two daughter nuclei. This process takes place in a series of four steps. Although the process of mitosis is continuous, it is usually studied by dividing it into four phases for convenience. The four phases or steps of mitosis are as follows;

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

It is more likely a preparatory phase to begin the division of chromosomes. During interphase, the nuclear material in cells is in the form of loosely packed chromatin to promote gene transcription. At the beginning of prophase, the chromatin rearranges itself into tightly packed chromosomes and the gene transcription stops. The chromosomes become visible in the nucleus and can be seen with a light microscope as tall and thin thread-like structures. The nucleolus also disappears in the prophase. The nuclear envelope also begins disappears toward the end of prophase as the chromosomes become more prominent. 

Centrosomes, structures responsible for the formation of mitotic spindle proteins have already been duplicated during the interphase. During the prophase of mitosis, these centromeres make mitotic spindle apparatus by polymerizing the tubulin protein. The motor proteins present in the cytoplasm then push these centrosomes to opposite polls of the cells along with the intervening mitotic apparatus. 

Thus, at the end of prophase, one can see the nucleus replaced by chromosomes and early mitotic apparatus spanning between the two centrosomes. 

Pre-prophase

It is seen only in plant cells. Plant cells have a nucleus at one side of the cell. During the pre-prophase, the nucleus is brought to the center of the cell so that mitosis can start. A transverse sheet of cytoplasm is also formed in the center of the cell along the plane that will divide it into two daughter cells in the future. 

Pro-metaphase

It overlaps with the prophase of mitosis. The disintegration of the nuclear envelope and the migration of centrosomes towards the opposite poles complete during prometaphase. Spindle fibers formed by microtubules originate from the centrosomes and attach to the kinetochore proteins at the centromere of chromosomes. The metaphase begins after the microtubules have attached to the kinetochores of chromosomes. 

Metaphase

This phase begins with the spindles already attached to the centromeres of chromosomes. The mitotic spindles start pulling the chromosomes towards the opposite poles of the cell during the metaphase. As a result, tension is developed that causes the chromosomes to arrange along the metaphase plate or equatorial plate. It is an imaginary line that is located perpendicularly, in the center of two centrosomes. Approximately, it is located in the center of the cell. 

The arrangement of chromosomes in the center of the cells is necessary for their division into two halves. This is ensured by the metaphase checkpoint. If all the chromosomes are arranged in the center of the cell, it passed the checkpoint and enters the anaphase. 

Anaphase

During this phase, the tension in the mitotic spindles become much greater. As a result, the cohesive forces that hold the two sister chromatids of a duplicated chromosome break and the sister chromatids are separated. 

Once the chromatids have become separated, the kinetochore microtubules pull them towards the opposite poles. They reach the opposite poles by the end of anaphase. 

During the late anaphase, the mitotic spindles start lengthening. causing the cell to elongate. Also, the segregated chromosomes become highly condensed.

By the end of anaphase, the total chromosomes have been divided into two halves and are located at the opposite poles of the cell. 

Telophase

This is the final phase of mitosis. During telophase, the spindle microtubules keep growing and lengthening, resulting in even more elongation of the cell. The nuclear envelopes are formed around each set of segregated chromosomes using the residues of the parent nucleus. 

Once the nuclear envelopes have been formed, two daughter nuclei can be seen at the two poles of the cell. The mitotic spindles start disintegration and one centrosome comes close to each nucleus. 

This ends the process of mitosis in which a parent nucleus has been divided into two daughter nuclei, each having an identical number of chromosomes. 

Cytokinesis

It is the process by which the cell cytoplasm is divided into two halves, giving rise to two daughter cells. Cytokinesis is a part of the M phase and takes place immediately after the division of the nucleus i.e. mitosis. 

The process of cytokinesis is different in animal and plant cells. 

Cytokinesis in Animal Cells

In animal cells, cytokinesis takes place by a cleavage furrow that develops in the center of the cell at the same place where the metaphase plate was formed during mitosis. The furrow is formed on the opposite poles of the cell and start growing inwards, pinching the cell into two halves. Thus, two daughter cells are formed.

Cytokinesis in Plant Cells

Plants cells are surrounded by a thick cell wall. A simple furrow cannot divide the cell into two halves as in the case of animal cells. 

In plant cells, vesicles derived from the Golgi apparatus start aligning in the center of the cell and forming a cell plate. A new cell wall is formed along this cell plate dividing the cell into two daughter cells. 

Importance 

The process of mitosis is very important for the normal growth and development of cells. It also plays an essential role in the development and normal functioning of various body parts of an organism. The following are some of the points that highlight the significance of mitosis in the human body. 

Growth and Development

Mitosis is the process responsible for the growth and development of humans. A baby is formed in the uterus starting from a single cell, the zygote. This zygote undergoes multiple cycles of mitosis to form a newborn baby. 

Mitosis is also responsible for the growth and development after birth. The different organs of the body increase in size by the addition of new cells after mitosis. 

Cell Replacement

Some organs of the body undergo a continuous cycle of cell replacement in which the older cells are sloughed off and replaced by new cells. This happens in skin and GIT where the cells are removed due to friction. The new cells are continuously formed by the process of mitosis in these organs. 

The cells present in the blood are also continuously made in the bone marrow through the process of mitosis and are released into the blood.

Regeneration

It is the process by which the lost body part is regenerated. The process of regeneration is also dependent on mitosis. The new cells of the lost part are made the process of mitosis. 

An example in the human body is the regeneration of the liver after liver transplant. The complete liver is regenerated in both the donor and recipient by the process of mitosis. 

Asexual Reproduction

It is a mode of reproduction seen in lower organisms such as hydra. In these organisms, the parent gives rise to identical offspring through the pro0cess of mitosis. It is also seen in plants during the asexual phase of their life. 

Summary

Mitosis is a part of cell division during which the nucleus of a cell is divided into two daughter nuclei having an identical number of chromosomes.

The cell cycle between the two subsequent cell divisions is called interphase, and consists of the following phase;

• G₁ phase, during which a cell grows and decides whether to divide or not
• S phase, during which the DNA of a cell is replicated resulting in duplication of chromosomes
• G₂ phase, during which the proteins necessary for mitosis are formed by the cell

The cells that do not receive a signal for cell division enter the quiescent G₀ phase. 

The process of mitosis consists of the following phases;

• Prophase, during which the nuclear envelope degenerates, chromosomes become prominent, and the mitotic apparatus starts forming.
• Metaphase: In this phase, mitotic spindles are formed, attaching to the chromatids, pulling them and aligning along the mitotic plate.
• Anaphase; During this phase, chromosomes are segregated, and sister chromatids are pulled towards the opposite poles. Cell also begins to elongate.
• Telophase, it results in the formation of the nuclear envelope around the sister chromatids and two nuclei are formed

Mitosis is followed by cytokinesis that divides the cell into two daughter cells. 

• Animal cell is divided by cleavage furrow
• Plant cell is divided by formation of fusion plate via Golgi-derived vesicles

The process of mitosis have the following significance in the life of organisms;

• It is involved in both intrauterine and extrauterine growth and development
• It is involved in the replacement of the cells that are continuously sloughed off
• The process of regeneration is dependent on mitosis
• Asexual reproduction takes place via mitosis

Frequently Asked Questions

What is mitosis, and what are its different stages?

Mitosis is a type of cell replication in which a cell divides to produce two identical daughter cells. Mitosis occurs in four stages; prophase, metaphase, anaphase and telophase.

What is the importance of mitosis?

Mitosis replaces the dead somatic cells in the body and ensures that newly formed daughter cells are identical to the parent cells having the same genetic set-up. Moreover, growth and development also occur through the process of mitosis.

How is chromosome number maintained in mitosis?

In mitosis, genetic material replicates and chromosomes arrange themselves in the equator of the cell. Later on, an equal number of chromosomes is pulled towards opposite poles ensuring that chromosome number remains the same in the daughter cells.

How asexual reproduction occurs by mitosis?

Asexual reproduction occurs through mitosis in some organisms, such as hydra. In this process, the parent organism grows up, replicates its DNA and divides to produce two identical offspring.

References

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