Mitosis - Equational Division: Stages, Overview & Importance

What is Mitosis?

Mitosis is the basis of cell division in organisms with more than one cell. Mitosis is the process of cell division by which one cell, called the mother cell, becomes two daughter cells. It plays a very essential role in developing, growing, and repairing tissues in multi-celled organisms.

Definition and Overview

In mitosis, one cell doubles and then splits to form two daughter cells which are genetically identical and contain the exact number of chromosomes as the parent cell. The whole process is subdivided into prophase, metaphase, anaphase, and telophase, after which cytokinesis ensues.

Role in Growth and Repair

Mitosis allows for growth, development, and tissue repair. It enables an organism to replace damaged or dead cells and increase its population of cells while maintaining genetic stability. Due to this controlled process of cell division, the health and function of an organism are sustained.

The Cell Cycle

A cell reproduces by carrying out an orderly sequence of events in which it duplicates its contents and then divides in two. This cycle of duplication and division, known as the cell cycle. The cell cycle is divided into four major phases.

G1 Phase

The cell increases in size, doubles its organelles and accumulates materials that will be used for DNA synthesis. Cells are constantly performing their usual functions.

S Phase

Before a cell divides, it must replicate its DNA. At the beginning of the S stage, each chromosome is composed of one DNA double helix. Following DNA replication, each chromosome is composed of two identical DNA double helix molecules and each double helix is called a chromatid.

G2 Phase

At this stage, the proteins that will be helpful during the cell division are synthesised. Once a cell has successfully progressed through G2, it is ready to enter the M phase.

M Phase (Mitosis)

M phase (Mitosis), where the cell divides duplicated chromosomes into two sets, yielding two daughter cells. The M phase is further divided into four sub-phases: prophase, metaphase, anaphase, and telophase upon completion of which is cytokinesis. This phase is essential for the daughter cells to get a real and full set of chromosomes so that genetic regularity is kept in cells.

Phases of Mitosis

The main objective of mitosis is to accommodate growth, tissue repair, and asexual reproduction in organisms. Unlike meiosis, which results in four genetically varied daughter cells with half the chromosome number of the parent cell, mitosis is the process vital to the structure and function maintenance of the living organism.

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 fibres with the help of kinetochore.

Metaphase

• Chromosomes become fully condensed and distinct and move towards the equatorial plane of spindles or metaphase plates.

• Chromosomes are arranged with their arms directed towards the pole and centromere towards the equator.

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.

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

• Cytokinesis, the process by which the cytoplasm is cleaved in two. It usually begins in anaphase but is not completed until the two daughter nuclei have formed in telophase.

• In animal cells, cytokinesis occurs through the formation of the cleavage furrow.

• The cleavage furrow represents the indentation of the membrane between the two daughter nuclei.

• In plant cells, a rigid cell wall prevents the furrow formation. Therefore, a new cell wall is formed between the daughter nuclei to complete the cytokinesis in plant cells.

Regulation of Mitosis

Several checkpoints and regulatory proteins largely explain how the mitotic process is controlled. These ensure that the process of cellular division during mitosis is carried out accurately.

Checkpoints (G1/S, G2/M, Spindle)

Key checkpoints include the G1/S checkpoint, the G2/M checkpoint and the spindle assembly checkpoint, in which the preparation for cellular division is measured and ensured at a proceeding stage.

At G1 to S phase, the control system confirms that the environment is favorable for proliferation before committing to DNA replication. At G2 to M phase, the control system confirms that the DNA is undamaged and fully replicated. Finally, during mitosis, the control machinery ensures that the duplicated chromosomes are properly attached to the mitotic spindle.

Role of Cyclins and CDKs

Molecular regulators, such as the cyclins and the cyclin-dependent kinases (CDKs), are central to the checkpoints and the regulation and fidelity of those checkpoints.

Control of Genetic Stability

Cyclins and CDKs regulate genomic stability and prevent mutations so that each one of the daughter cells will have the correct number of chromosomes. Therefore, the integrity of the genetic information of the organism is maintained.

Significance of Mitosis

The significance of mitosis include:

Growth and Development

Mitosis results in the development of multicellular organisms, as they begin their life in the form of a unicellular zygote. It undergoes divisions to form a multicellular embryo that gives rise to a whole organism

Tissue Repair and Regeneration

In multicellular organisms, mitosis helps in growth and repair. Whereas, in unicellular organisms, mitosis results in cell division. Mitosis is also essential to maintain the nucleocytoplasmic ratio.

Role in Cancer Research

Cancer is a group of diseases in which cells in the body divide and do not stop dividing maliciously. Thus, an understanding of mitosis contributes to the development of targeted therapies, such as chemotherapy and radiation, which control cell division in a cancer cell.

Importance in Stem Cell Therapy

Recent progress in research and development has opened possibilities for finding more research on mitotic inhibitors used in the discovery of precision medicine approaches. It is also important to regenerative medicine and in general to stem cell research, fostering hope for repairing damaged tissues and treating degenerative diseases.

Mitosis NEET MCQs (With Answers & Explanations)

Important topics for NEET exam are:

• Phases of Mitosis

• Checkpoints

Practice Questions for NEET

Q1. What is the chromosome structure during the Metaphase stage of Cell Division?

1. It is composed of two sister chromatids kept together by the centromere.

2. It is made up of several sister chromatids that are connected to spindle fibres at the centrosome.

3. There are no sister chromatids.

4. It is made up of non-sister chromatids that are linked to a cell pole.

Correct answer: 1) It is composed of two sister chromatids kept together by the centromere.

Explanation:

During the Metaphase of mitosis, which is a critical phase in eukaryotic cell division, the chromosomes exhibit a high level of organization.

1. High Density and Microscopic Visibility:
The chromosomes are maximally condensed and coiled, allowing for clear visualization under a microscope. This state is essential for safeguarding the DNA and facilitating precise separation during the process.

2. Duplicated Chromosomes - Sister Chromatids:
Each chromosome is composed of two sister chromatids that are identical in genetic material. This duplication is a consequence of DNA replication that occurs in the preceding S phase of the cell cycle.

3. Metaphase Plate Alignment:
Chromosomes are meticulously aligned at the equatorial plane, known as the metaphase plate. This meticulous arrangement guarantees that each resulting daughter cell will obtain an equal complement of chromosomes post-division.

4. Kinetochore Formation and Function:
Kinetochores, which are protein complexes, form at the centromeres of chromosomes. These structures serve as the attachment points for spindle fibres, which are microtubules involved in chromosome movement.

5. Spindle Fiber Connections:
The spindle fibres, extending from opposite cell poles, connect to the kinetochores of the sister chromatids, creating the tension necessary to maintain their alignment and ensure their separation in the subsequent phase, Anaphase.

6. Bipolar Orientation:
The sister chromatids are arranged in such a way that one faces one cell pole, and the other faces the opposite pole. This orientation is vital for their separation and proper distribution to daughter cells during Anaphase.

Hence, the correct answer is the option 1) It is composed of two sister chromatids kept together by the centromere.

Q2. Mitosis is characterised by

1. Reduction division

2. Both reduction and equational division

3. Equal division

4. Pairing of homologous chromosomes

Correct answer: 3) Equal division

Explanation:

The process of cell division known as mitosis produces two identical daughter cells when the chromosomes duplicate and are distributed equally. Each daughter cell has the same number of chromosomes as the parent cell, making them diploid. Consequently, the process of mitosis is referred to as equal division.

Hence, the correct answer is the option 3) Equal division.

Q3. During the metaphase stage of mitosis , spindle fibres attach to chromosomes at

1. Kinetochore

2. Both centromere and kinetochore

3. Centromere

4. Centromere, kinetochore and area adjoining Centromere

Correct answer: 1) Kinetochore

Explanation:

Chromosomes - During different stages of cell division, cells show structured chromosomes in places of the nucleus. Every chromosome essentially has a primary constriction or the centromere on the sides of which disc-shaped structures called kinetochores are present. A kinetochore is a complex protein structure where microtubes attach themselves to the chromosomes.

Hence, the correct answer is option 1) Kinetochore.

Also Read:

• MCQs on Mitosis

• Stages of Meiosis

• Bacterial Cell Division

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