Summary:

• All living organisms require cell division for growth, repair, and replication. A parent cell divides into two or more daughter cells during this process.
• In a standard eukaryotic cell cycle, the interphase and M phases are the two main phases. G1, G2, S, and G0 Phases make up the Interphase. This is the stage at which a cell is resting.  Mitosis, or the M process, is the time of actual nuclear and cell division during which the duplicated chromosomes are split evenly between two daughter cells.
• Meiosis is a form of cell division in which the parent cell divides into four daughter cells, each with half the number of chromosomes. Mitosis, on the other hand, is a form of cell division that produces two daughter cells, each with the very same number and type of chromosomes as the original present in the parent cell.

Cell Division

Introduction:

Ever wondered how cells arise from cells that exist before them? As all living things are born from pre-existing living organisms, then this must also be true on a cellular level. Rudolf Virchow, a famous German biologist, concocted a very popular statement, “Omnis Cellula e Cellula’. This translates to the meaning that every cell arises from a pre-existing cell.

But how does this happen? Certainly, there must be various biological mechanisms that take place inside the cell that initiate a step-wise, well-regulated, series of stages. These must eventually result in the formation of a brand-new cell, ready to perform its functions. The process that makes all this possible is known as Cell Division. To be more precise, the process where a parent cell divides into two or more daughter cells is known as cell division.

Normally, cell division happens as part of a broader cell cycle. In simpler terms, it could be said that cell division is how cells reproduce. This is understandably very important for many reasons. Cell division is necessary for all living organisms because it is required for development, repair, and replication. Older cells are used to make new ones. It helps in increasing the number of cells available for growth and development. Mitosis, which is a part of the process of cell division, aids in the development of new parts and the replacement of damaged ones in the case of plant cells. This means that injured cells are replaced. Additionally, the process of cell division aids in the reproduction and development of living organisms. It is also in charge of an individual’s form, size, and appropriate growth and development. The total number of chromosomes is maintained because of cell division in living organisms. Moreover, this helps in the repairing of the cells and maintains the health of the cells.

The majority of the body’s tissues expand by multiplying their cells, but this process is tightly regulated to keep a balance between different tissues. Most cell division is used for the renewal of body tissues in adults, rather than development, with several different types of cells being replaced regularly.

To facilitate its offspring cells to work properly and live, a cell must correctly and fully replicate the genetic information encoded in its DNA until it can divide. Because of the length of DNA molecules, this is a difficult problem to solve. Each human chromosome is made up of a lengthy double spiral, or helix, with each strand containing over a hundred million nucleotides.

The Cycle of cell division

The cell division is a part of a bigger cell cycle. The cell cycle is a sequence of events that occur in a cell that results in DNA replication, and cytoplasm, and organelle division, leading to the formation of two new daughter cells. A cell goes through several phases to divide and create new cells. The cell cycle is a sequence of events that occur in a cell leading to its growth and development and eventual division. The cell’s genome (the complete set of genetic instructions) is duplicated, and the cell organelles are produced before the cytoplasm is divided. Human cells go through the normal eukaryotic cell cycle and complete one cycle of growth and development in about twenty-four hours (24 hours). The interphase and the M phases are the two major phases of a typical eukaryotic cell cycle.

• The Interphase: Interphase, also known as the cell cycle’s resting phase, is the time when the cell braces itself for division by going through both cell growth and DNA replication. It takes up about 95 percent of the total cycle time. There are three stages to the interphase:
• G1 phase or Gap 1: The G1 phase is the period between mitosis and the start of replication of the cell’s genetic information. The cell is active in metabolism and continues to develop without copying its DNA throughout this process.
• S Phase: DNA replication occurs during this step of the cell cycle. The cell replicates each of the 46 chromosomes that are present in 23 pairs. The cell creates a full copy of DNA in its nucleus during the S phase.  It also copies the centrosome, which is a structure that organizes the microtubules. These microtubules are a microscopic structure in a tubular form, found in the cytoplasm of cells in large numbers, which can often accumulate to form more complex structures. During the M process, centrosomes assist in the separation of DNA.
• G2 Phase or Gap 2: The RNA, proteins and other macromolecules necessary for duplication of cell organelles, the formation of spindle fibers, and the growth of cell are released during the G2 phase (Gap 2) as the cell prepares to enter the mitotic phase of the cell cycle.
• G0 Phase: While certain cell types are actively dividing, others are dormant. These cells leave G1 and enter G0, also known as the resting state. A cell in G0 is doing its job but not consciously planning to split into daughter cells.  Some cells remain in G0 indefinitely, while others can resume division if the correct signals are received.
• M Phase: The cell divides its copied DNA and its cytoplasm to form two new cells during the mitotic (M) process. Mitosis and cytokinesis are two separate divisions involving processes that occur during the M phase. Mitosis, also known as the M phase, is the time when the chromosomes that have been duplicated are split evenly between two cells during nuclear division and cell division.

Mitosis:

The cycle of cell fusion and separation, nuclear division, and physical separation of the two daughter cells are all visible when a microscope is used to observe these processes. The cell’s nuclear DNA compresses into recognizable chromosomes during mitosis, and the mitotic spindle, a unique structure consisting of microtubules, pulls them apart.

The splitting of the nucleus during the M phase of the cell cycle is known as karyokinesis. In the M process, it is the first step. This procedure is independent of cytokinesis. It evenly separates the genetic material. Two daughter nuclei are formed from the daughter chromosome. DNA replication is the process of copying DNA, which is started by DNA polymerases, which are specific enzymes. These read the sequences of nucleotides that are joined together to form DNA chains as they move along the molecule. As a result, each strand of the DNA double helix serves as a blueprint for a new expanding chain’s nucleotide structure.

The splitting of the cytoplasm during the M process of the cell cycle is referred to as cytokinesis. In the M process, it is the second step. Karyokinesis is needed for this process to take place. Hence, it can be said that cytokinesis depends upon the process of karyokinesis.  The cytoplasm, as well as other cell organelles, differentiates between the two daughter cells during this period of mitosis.

Most cells in the body are diploid, which means that during mitosis, the cell doubles the number of chromosomes to 4N before dividing, and the resulting daughter cells are 2N. A cell that approaches the M phase has 4N DNA and divides into two cells, each with the same 2N complement of DNA.

In one round of the cell cycle, two daughter cells must advance to the next stage. This is dependent on the type of cell. Many types of cells divide quickly, and the daughter cells can experience another process of cell division right away. Many cell types in an early embryo, as well as cells in a tumor, divide rapidly. Cells of other kinds split slowly or don’t do it. These cells can leave the G1 phase and enter the G0 phase, which is a resting state. A cell in the G0 process is not consciously preparing to divide; it is simply doing its task. It may, for example, send and receive signals as a neuron does or preserve carbohydrates as a liver cell. Some cells remain in G0 indefinitely, while others can re-start.

Mitosis is needed for life because it generates new cells for growth and the replacement of damaged cells. Depending on the type of cell and the species of organism, mitosis can take less time such as several minutes, or it can be of longer periods, like a few hours. An interesting fact about this process is that the times of day, temperature, and chemicals all affect this process. Some molecules generated by neighboring cells inhibit other types of cells from dividing, or sometimes they just cannot divide.  As a consequence, certain tissues in the adult body have a significantly diminished ability to support and regenerate damaged or diseased cells. Heart muscle cells, central nervous system nerve cells, and mammalian lens cells are examples of such tissues. These cells’ repair and maintenance are restricted to removing intracellular factors instead of whole cells.

The M phase or mitosis can be divided into four major stages; these are Prophase, Metaphase, Anaphase, and Telophase.

• Prophase: Mitosis starts with the enlargement and tangling of the chromosomes during prophase. The nucleolus, which is a round structure, gets smaller and eventually vanishes. The beginning of the organization of a collection of fibres to shape a spindle and the dissolution of the nuclear membrane signal the end of prophase. Microtubules, long proteins that start forming at opposite poles of the cell, make up the mitotic spindle. The spindle is in charge of dividing the sister chromatids into two cells. Prophase is followed by metaphase.

Prometaphase is the mechanism by which a parent cell’s reproduced genetic material is separated into two identical daughter cells in the nucleus. In prometaphase, the nuclear envelope which is the physical membrane that encloses the nucleus disintegrates.

• Metaphase: Metaphase is the second step of mitosis, the mechanism by which a parent cell’s nucleus divides into two identical daughter cells, separating duplicated genetic material carried in the nucleus. Around the centromere, protein formations are called kinetochores from before metaphase. Kinetochore microtubules are long protein filaments that stretch from the cell’s poles and bind to the kinetochores. During metaphase, the sister chromatids are pulled back and forth by the kinetochore microtubules until they converge along the cell’s equator, known as the equatorial plane. During metaphase, the cell’s chromosomes arrange themselves in the centre of the cell.
• Anaphase: Each chromatid pair splits into two equivalent chromosomes in anaphase, and the spindle fibres pull them to opposing ends of the cell. Sister chromatids, which are duplicated paired chromosomes, divide and begin travelling toward opposite poles of the cell during anaphase.

The centromere is a chromosome structure that binds the two chromatids together which the daughter strands of a replicated chromosome are basically.  The centromere is the attachment point for the kinetochore. The kinetochore is a structure to which the mitotic spindle’s microtubules are attached. During cell division, the spindle is the framework that pulls the chromatids to opposite poles of the cell.

• Telophase: The process of mitosis, which divides the duplicated genetic information held in the nucleus of a parent cell into two identical daughter cells, is called telophase. When the replicated, paired chromosomes have been removed and dragged to opposite poles of the cell, telophase starts. A nuclear membrane forms around each set of chromosomes in telophase. The chromosomes start to untangle, making them less compact and dispersed. Alongside telophase, the cell goes through cytokinesis, which splits the parental cell’s cytoplasm into two daughter cells.

Meiosis:

Meiosis is a form of cell division that creates four gamete cells by halving the number of chromosomes in the parent cell. For sexual reproduction, this process is necessary to produce egg and sperm cells. When sperm and egg combine to form a single cell during the process of reproduction, the number of chromosomes in the offspring is preserved. Meiosis is analogous to, but distinct from, mitosis, a cell division mechanism (mitosis) in which a parent cell divides into two identical daughter cells. Following one cycle of DNA replication in cells of the male or female sex organs, meiosis starts. Meiosis I and meiosis II are the two divisions of the operation, and both have several phases. Meiosis I is a form of cell division that occurs exclusively in germ cells, while meiosis II is similar to mitosis. Prophase I kicks off Meiosis I, the first meiotic division. Chromatin, a complex of DNA and protein, condenses to form chromosomes during prophase I. Sister chromatids are pairs of replicated chromosomes that remain linked at a central spot called the centromere. On each side or pole, of the cell, a big structure called the meiotic spindle is formed from long proteins called microtubules. The pairs of homologous chromosomes form tetrads between prophase I and metaphase I. Any pair of chromatid arms inside the tetrad will overlap and fuse in a process known as crossing-over or recombination.

Recombination is a mechanism in which portions of DNA are broken, recombined, and rejoined to create new gene combinations. The homologous pairs of chromosomes coincide on either side of the equatorial plate in metaphase I. The spindle fibres then compress in anaphase I, pulling the homologous pairs, each with two chromatids, apart and toward each pole of the cell.

The chromosomes are encased in nuclei during telophase I. The cell now goes through cytokinesis, which splits the original cell’s cytoplasm into two daughter cells. Each daughter cell is haploid which means that it has only one set of chromosomes or half of the original cell’s total number of chromosomes. For human beings, this number is 23. Each of the haploid cells formed in meiosis I undergo a mitotic-like division in meiosis II.

Possible errors:

Even though as mentioned before, this cell division process is very strictly regulated to make sure no mistakes are made. However, sometimes this very well organized and administered mechanism is also prone to errors. The separation does not always go as expected. Mistakes in mitosis or cytokinesis have the potential to result in cells with an irregular chromosome number. Cells can have more or fewer chromosomes than the organism’s average. Apoptosis is the process by which cells with irregular chromosome numbers are excluded from the population.  It is also called programmed cell death.

Reference list

1. Admin (2017) ‘Cell Cycle – Definition And Phases of Cell Cycle’, BYJU’S, 31 August. Available at: https://byjus.com/biology/cell-cycle-and-division/ (Accessed: 14 April 2021).
2. Encyclopedia Britannica (2021) Cell – Cell division and growth. Available at: https://www.britannica.com/science/cell-biology/Cell-division-and-growth#ref37459 (Accessed: 14 April 2021).
3. Julianne Zedalis, J.E. (2018) Biology for AP® Courses. Houston, Texas: OpenStax. Available at: https://openstax.org/books/biology-ap-courses/pages/10-2-the-cell-cycle (Accessed: 14 April 2021).
4. meiosis (2019). Available at: https://www.nature.com/scitable/definition/meiosis-88/ (Accessed: 14 April 2021).