What is Cell Division?

Cell division involves the formation of new daughter cells from the parent cells. It is a part of the cell cycle that takes place in both prokaryotic and eukaryotic organisms. Cell division is required for three main reasons:

• It is required for the growth and functioning of unicellular organisms.
• It is required for the reproduction and, therefore, survival of an organism.
• It is essential for passing genetic information from one generation to another.

Types of Cell Division

There are mainly two types of cell division:

1. Mitosis
2. Meiosis

Mitosis

It is the type of cell division where the parent cell nucleus is divided into two daughter cells with the same genetic material. This type of cell division occurs in both asexually and sexually reproducing animals. It is also known as equational cell division because the number of chromosomes in the parent and daughter cells are the same. 

Interphase 

This stage is the preparatory phase for mitosis. The interphase stage is further divided into three phases that are G₁, S, and G₂.

1. G₁: In this phase, the cell is metabolically active. The cell synthesizes all the proteins required for DNA (deoxyribonucleic acid) replication. The cell also duplicates its organelles and cytosolic components.
2. S: The replication of genetic material takes place during this phase. Each chromosome replicates to form two sister chromatids held at the center, a region called the centromere.
3. G₂: In this phase, the cell continues to grow and produce new proteins. Enzymes and ATP (adenosine triphosphate) synthesis take place in high amounts. 

Stages of Mitosis 

Mitosis could be divided into four different stages. These stages are prophase, metaphase, anaphase, and telophase. The events that take place in various stages of mitosis are as follows:

Prophase

It is the first stage after the interphase. The following events take place during the prophase:

• Condensation of DNA to form compact mitotic chromosomes.
• Initiation of the mitotic spindle formation that helps in organizing the chromosomes during mitosis.
• The disintegration of the nuclear envelope shows that the nucleus is ready to break down.
• The nucleolus disappears.

Pro-metaphase

• During pro-metaphase, the microtubules attach to the kinetochore.

Metaphase

• In this stage, the chromosomes are arranged at the center of the cell and thus are visible.
• The microtubules begin to pull the chromosomes with equal force.

Anaphase

• The splitting of sister chromatids at two opposite spindle poles of the cell occurs during this stage.
• The protein which holds the sister chromatids is broken down.

Telophase

• It is the final stage of mitosis.
• Chromosomes separate and reach the opposite poles of the cell.
• Disassembling of the mitotic spindle occurs.
• A nuclear envelope begins to form.
• The nucleolus reappears.

Cytokinesis 

After the nucleus division, the cytoplasm division and organelles distribution in the two daughter cells occur. This process is known as cytokinesis. Other names for cytokinesis include cell cleavage or cytoplasmic division. Cytokinesis starts in the late stages of telophase. If, due to any reason, cytokinesis does not take place after mitosis, a multinucleated cell is formed.

Function of Mitosis

• It helps in the replacement of damaged tissue.
• It contributes to the growth and development of an organism.
• Mitosis is a kind of cell division used by unicellular organisms for asexual reproduction.

Significance of Mitosis

• Mitosis is responsible for the equal distribution of genetic material to the daughter cells or new cells.
• The development of zygotes into adults takes place through mitosis.
• Mitosis assists in maintaining the purity of the genome in new cells.
• It is required by a cell to maintain the proper size.

Using Caenorhabditis elegans as a model organism to study cell division

The embryo of C. elegans is used as a powerful model organism to study eukaryotic cell division because C. elegans have all the regulatory genes required for the cell division that is conserved among all metazoans, including humans. Other phenomena observed in cell division like centrioles duplication, assembly of kinetochores, formation of mitotic spindle could be studied in detail using C. elegans.

Meiosis

This type of cell division takes place only in sexually reproducing organisms. It brings genetic variation between the individuals of a population. This genetic variation helps the organism to adapt according to changing environmental conditions. Meiosis forms four daughter cells, each having half the original amount of the parent cell’s genetic information. It takes place in germ cells (sperms and eggs) and involves two rounds of division known as meiosis I and meiosis II.

Meiosis also starts with interphase followed by two rounds of divisions.

Meiosis I

This division is called a reductional division as chromosome number is reduced to half in daughter cells. This division involves the four stages: prophase I, metaphase I, anaphase I, and telophase I.

Prophase I

• This stage is further divided into five sub-stages: leptotene, zygotene, pachytene, diplotene, and diakinesis.
• Centrosome assembles into spindle fibers.
• During leptotene, duplicated chromosomes contain two sister chromatids condense from diffuse chromatin into long, thin strands.
• During zygotene, the homologous chromosomes come closer to each other.  These close chromosomes form a synapse, and paired chromosomes are called bivalent or tetrad chromosomes.
• During pachytene, a crossing-over occurs involving the exchange of genetic material between non-sister chromatids of homologous chromosomes. Chiasmata are the point in chromosomes where crossing over takes place. Therefore, this phase is very important in developing genetic variations that are a raw material source for evolution.
• During diplotene, the homologous chromosomes get separated from each other and dissociate from synaptonemal complexes.
• During diakinesis, chromosomes get condensed. The nuclear envelope and nucleolus disappear, and the spindle fibers start to form.

Metaphase I

• Paired homologous chromosomes start to align at the equator of the chromosomes.
• The protein complex cohesion holds sister chromatids together.

Anaphase I

• Homologous chromosomes move to the opposite poles of the spindle due to spindle contraction that splits the bivalents.

Telophase I

• Chromosomes start to get decondensed, resulting in the reformation of the nuclear membrane.

Result of Meiosis I

Meiosis I result in the production of two haploid cells that have half of the chromosomes of the parent cell.

Meiosis II

This phase of cell division is known as equational division because chromosome numbers remain the same in this division. The second phase of meiosis is divided into four sub-phases: prophase II, metaphase II, anaphase II, and telophase II.

Prophase II

This phase starts with the disappearance of the nuclear envelope, followed by the shortening and thickening of chromatids. Further, centrosomes assemble spindle fibers for the second meiotic division.

Metaphase II 

Spindle fibers from opposing centromeres attach to chromosomes. The new equatorial plate rotates by 90º as compared to the metaphase I plate. The microtubules are attached to the kinetochores. 

Anaphase II

During anaphase II, the contraction of spindle fiber separates the sister chromatids, followed by the movement of these chromatids to the opposite poles.

Telophase II

During this phase, the nucleolus reappears, a nuclear envelope is formed, and the chromosomes decondense.

Result of the Meiosis II

Meiosis II results in the formation of four daughter cells with the same chromosome number as the haploid cells formed at the end of meiosis I.

Cytokinesis

The division of cytoplasm takes place after the formation of four haploid cells after meiosis II.

Regulation of The Cell Cycle

It is very important to regulate each step of the cell division to ensure that daughter cells produce the same chromosome number as the parent cell. The dividing cells undergo certain internal control mechanisms. A checkpoint is one of several points at which the progression of a dividing cell to the next stage can be paused until conditions are optimum to divide.  These checkpoints occur near the end of G₁, at the G₂/M transition, and during metaphase. Cyclin-dependent kinases are the families of protein kinases involved in regulating the cell cycle. The virus for coronavirus disease of 2019 (COVID-19) also hijacks the cell cycle of humans to cause infection in them.

Cancer

Cancer is an abnormal condition in the body of an organism when uncontrolled cell division takes place. It is referred to as unchecked cell growth. This abnormal cell division happens due to mutation in the genes that regulate cell division. Stem cells are the source of cancer as they have a long life compared to the other cells, and therefore, they are the site of mutation that leads to cancer.

Context and Applications

• For Bachelors of Science students: Cell division is the fundamental concept of biology and should be studied by graduate students to understand various other biological phenomena and use google scholar to study various research papers.
• For Master of Science students: Various phenomena present in cell division like kinetochore formation, spindle formation, cancer cell development, and centriole duplication should be studied in detail. The students are also advised to use google scholar to study novel research associated with cell division.