Mitosis: Definition, Cell Division, Phases, Stages, Inheritance, Growth and Regulation of the Cell Cycle

Most eukaryotic cells divide in such a way that the ploidy or number of chromosomes remains conserved.

This happens except in the case of germ cells where the number of chromosomes is reduced by half.

Mitosis is the phase of the cell cycle where the nucleus of a cell divides into two nuclei with an equal amount of genetic material in both daughter nuclei.

It succeeds the G2 phase and is replaced by cytoplasmic division after separation from the nucleus.

Mitosis is the essential process for life as it provides new cells to achieve growth and the replacement of worn out cells. Essentially, mitosis is life.

The time in which mitosis takes place can be minutes or hours, and it will depend on the type of cells and the species of organisms.

Among the factors that regulate mitosis are: temperature, time of day and chemicals.

Cell division

Cell division is a universal process among living organisms, where each cell originates from another cell

The main mechanism by which organisms generate new cells is through cell division. During this process, a single stem cell will divide and produce identical “daughter” cells.

In this way, the stem cell can transmit its genetic material to each of its daughter cells. First, however, cells must duplicate their DNA.

Essentially, the chromosomes are released from the nucleus, attached by microtubules from the spindle to the centrioles via the kinetochore chromosomes, separated on opposite sides of the cell.

It then begins to separate into two individual daughter cells as the chromosomes revert to chromatin, while the nuclear envelope reforms around the chromatin.

DNA replicates semi-conservatively (basically the original DNA strand splits by helicase, DNA polymerase enters and joins complementary nucleotides on either side of the DNA strand).

They are then joined together by ligase, forming two new daughter DNA strands, each of which contains one strand of the original DNA.

That is the purpose of mitosis; grow as an organism and produce new cells to replace old worn out cells.

The mechanisms of cell division vary between prokaryotes and eukaryotes. Prokaryotes are single-celled organisms, like bacteria and archaea.

They have a simple internal structure with free floating DNA.

They use cell division as a method of asexual reproduction, in which the genetic makeup of the parent and the resulting offspring are the same.

A common mechanism of asexual reproduction in prokaryotes is binary fission. During this process, the parent cell duplicates its DNA and increases the volume of its cellular contents.

Eventually, a crack emerges in the center of the cell, leading to the formation of two identical daughter cells.

Eukaryotic cells, on the other hand, have an organized central compartment, called the nucleus, and other structures, such as mitochondria and chloroplasts.

Most eukaryotic cells divide and produce identical copies of themselves by increasing their cell volume and duplicating their DNA through a series of defined phases known as the cell cycle.

Since their DNA is contained within the nucleus, they also undergo nuclear division.

Mitosis is the division of a eukaryotic nucleus, although it is sometimes interpreted as a complete cell cycle that is used for cell duplication.

Cell division is necessary for the growth and repair of damaged tissues.

Eukaryotic cells can also undergo a specialized form of cell division called meiosis, which is necessary to produce reproductive cells such as sperm, egg, and spore cells.

The cell cycle is a series of defined and synchronized events that allow a cell to grow and divide.

  • G1 phase (first gap phase): During this phase, cells that are destined for mitosis grow and carry out various metabolic activities.
  • S phase (synthesis phase): During this phase, the cell duplicates its DNA. Eukaryotic DNA wraps around spherical histone proteins to create a rod-shaped structure called a chromosome. During the S phase, each chromosome generates its copy, or sister chromatid.
  • G2 phase (second gap phase): During this phase, the cell continues to grow and generate the proteins necessary for mitosis. (Phases G1, S, and G2 are collectively called “interface”).
  • M phase (mitosis): mitosis involves the segregation of sister chromatids. A structure of protein filaments called the mitotic spindle latches onto the centromere and begins to contract. This separates the sister chromatids, slowly moving them to opposite poles of the cell.

Chromosomes become very compact during mitosis and can be clearly seen as dense structures under the microscope.

The resulting daughter cells can re-enter the G1 phase only if they are destined to divide.

Not all cells need to continually divide. For example, human nerve cells stop dividing in adults.

Cells in internal organs such as the liver and kidney divide only when necessary: ​​to replace dead or injured cells.

These types of cells enter the G0 phase (resting phase).

They remain metabolically active and only enter the G1 phase of the cell cycle when they receive the necessary molecular signals.

Stages of mitosis

Mitosis is the process by which a cell secretes its duplicated DNA, eventually dividing its nucleus in two.

Mitosis follows G2, and it is the time when cells separate their duplicated contents and divide.

Cell division at the end of mitosis produces identical diploid cells.

Although cell division is the defining feature of mitosis, several events must occur during mitosis before the cell is ready to divide.

Mitosis involves a five-step process, and then a final, culminating sixth step, called cytokinesis.

The five steps of mitosis and cytokinesis are often considered two distinct subphases within the general phase of the cell cycle which is called mitosis, or M phase.

The five steps of mitosis, called prophase, prometaphase, metaphase, anaphase, and telophase, constitute the period in which the cell prepares for cell division.

The five phases are differentiated by specific events of preparation for cell division.

Cytokinesis refers to the actual cleavage event, dividing the cell in two.

Cells cannot perpetually perform their cellular functions without maintaining normal wear and tear.

Thus, when the cell completes the interface (that is, cellular functions and prepares the chromatin within the nucleus of the cell into chromosomes), the cell will begin mitosis, which is made up of the four main phases: prophase, metaphase, anaphase, and telophase. .

The different phases of mitosis that occur during cell division occur as follows:

Profase

Before mitosis begins, the chromosomes replicate and the proteins that will form part of the mitotic spindle are synthesized.

Mitosis begins in prophase.

Prophase immediately follows the S and G2 phases of the cycle and is marked by the condensation of genetic material to form compact mitotic chromosomes composed of two chromatids joined at the centromere.

Sister chromatids can easily be visualized at this stage.

The completion of prophase is characterized by the beginning of the assembly and organization of a group of fibers to form the mitotic spindle.

The mitotic spindle is a network of protein filaments, emerging from structures called centrioles, located at each end of the cell.

The mitotic spindle is flexible and made of microtubules, which in turn are made up of the protein subunit tubulin, proteinaceous components of the cytoplasm that aid in the process.

The nuclear envelope begins to disintegrate.

The nucleolus, which is a rounded structure, disappears.

Metafase

The nuclear envelope completely disintegrates at the beginning of metaphase and the condensed chromosomes extend throughout the cytoplasm.

These chromosomes are composed of a double structure consisting of two sister chromatids.

The mitotic spindle attaches to sister chromatids at the centromere.

The mitotic spindle can now move the chromosomes around the cell held in the center by centromeres.

The spindle fibers are attached to a disc-shaped structure on the surface of the centromeres, known as kinetochore.

The spindle fibers of a centriole are attached to a single sister chromatid.

At the end of metaphase, all chromosomes line up along the midline or equator of the cell’s cytoplasm, a characteristic metaphase arrangement known as the metaphase plate.

Anaphase

The onset of anaphase is marked by the division of sister chromatids.

These sister chromatids become two identical chromosomes from daughter nuclei.

Chromosomes are drawn to opposite ends or poles of the cell by spindle fibers attached to the kinetochores of each chromosome.

The centromere of each chromosome leads to the edge as the arms follow it.

Telophase

During telophase, the chromosomes that are grouped at the two poles begin to join together forming an undifferentiated mass, the spindle is broken.

As the nuclear envelope begins to form around it, the nucleolus, Golgi bodies, and the endoplasmic reticulum complex, which had disappeared at the end of prophase, begin to reappear.

The phase, which occurs after telophase, is called interface and marks the completion of cell division or mitosis.

In it, cytokinesis occurs, which denotes the division of the stem cell’s cytoplasm to form two daughter cells, each one has the same number and type of chromosomes that the stem cell had.

At the end of mitosis, a cell produces two genetically identical daughter cells.

A powerful light microscope captures these scenes during the mitosis process.

Mitosis and inheritance

When Gregor Mendel formulated his laws of heredity, he postulated that the determinants of heredity are particulate in nature.

Today scientists call them genes, and they understand not only the physical location of hereditary units, but also their molecular makeup.

Each individual in a sexually reproducing species inherits two alleles for each gene, one from each parent.

Furthermore, when such an individual forms sex cells, each of the resulting gametes receives a member of each allelic pair.

Gamete formation occurs through a process of cell division called meiosis.

When the gametes unite in fertilization, the double dose of hereditary material is restored and a new individual is created.

This individual, consisting initially of a single cell, grows through mitosis, a process of repeated cell divisions.

Mitosis differs from meiosis in that each daughter cell receives a complete copy of all the hereditary material found in the parent cell.

Mitosis occurs in somatic cells as opposed to germ cells, which undergo meiosis.

Although the four characteristic stages are also observed in the second half of meiosis.

Each species has a unique set of chromosomes that are carriers of genetic information.

One cell gives rise to two genetically identical daughter cells during the mitosis process.

It is clear that genes must physically reside in cellular structures that meet two criteria.

First, these structures must be replicated and passed on to each generation of daughter cells during mitosis.

Second, they must be organized into homologous pairs, one of which is shared between each gamete formed during meiosis.

As early as 1848, biologists had observed that cell nuclei resolve into small rod bodies during mitosis, later these structures were found to absorb certain dyes and therefore came to be called chromosomes (colored bodies).

During the early years of the 20th century, cell studies using ordinary light microscopes clarified the behavior of chromosomes during mitosis and meiosis, leading to the conclusion that chromosomes are the carriers of genes.

When chromosomes condense during cell division, they have already reproduced.

Each chromosome therefore consists of two identical replicas, called chromatids, joined at a point called the centromere.

During mitosis, the sister chromatids separate, one going to each daughter cell.

Therefore, chromosomes meet the first criterion to be the gene pool: they replicate and a complete copy is passed to each daughter cell during mitosis.

During the early development of an embryo, the original egg, usually a very large cell, undergoes repeated series of cell divisions without any intermediate growth period, as a result, the original egg divides into thousands of small cells.

The usual pattern of growth and mitosis takes place only after the embryo is able to obtain food from its environment.

Growth and mitosis

The increase in size and changes in the shape of an organism in development depend on the increase in the number and size of the cells that make up the individual.

The increase in the number of cells of an individual is carried out through a very precise cellular reproductive mechanism called mitosis.

During the process of mitosis, the chromosomes that possess the genetic material are duplicated within the nucleus, and then they are exactly distributed in the two daughter cells, one of each chromosome type.

At each end of the cell to be divided by mitosis, there is a complete set of chromosomes.

The cell is then divided by a pinch of the cell membrane called cytokinesis.

In the life span of the cell that precedes chromosome duplication, the stem cell usually doubles its original size.

Establishing a cycle that constitutes the growth and division of the cell.

Cell growth: which is an increase in cytoplasmic mass, number of chromosomes, and cell surface, followed by cell division, in which the cytoplasmic mass and chromosomes are distributed to daughter cells.

Mitosis is important for:

  • Increase the number of cells in a particular tissue.
  • Protection against harmful microorganisms in the event of a cut or wound.
  • The replacement of dead or inefficient cells in a tissue.
  • To maintain the cytoplasm to nucleoplasm, as well as the surface area to volume ratio.

Regulation of the cell cycle and cancer

The various events of the cell cycle are tightly regulated. If errors occur at any stage, the cell can prevent cell division from progressing.

These regulatory mechanisms are known as cell cycle checkpoints.

There are three checkpoints within the G1, G2, and M phases. Damaged DNA stops cell cycle progression in the G1 phase, ensuring that an aberrant cell will not replicate.

The G2 checkpoint responds to incorrectly duplicated or damaged DNA.

It prevents cells from moving into M phase until DNA replicates properly or until damage is repaired.

The M-phase checkpoint can stop the cell cycle in metaphase.

It ensures that all sister chromatids are properly attached to the mitotic spindle and that sister chromatids move to opposite ends of the cell.

Sometimes abnormal cells manage not only to survive, but also to proliferate. Most often, these cells are involved in cancer.

Cancer cells are known to go through rampant and unregulated cell divisions.

The relationship between the cell cycle and cancer has led to the development of a class of anticancer drugs that specifically target cancer cells during mitosis.

However, the results have been mixed, with some drugs being more successful in a laboratory setting than in the real world.

Cancer is essentially a disease of mitosis.

The normal “checkpoints” that regulate mitosis are ignored or bypassed by the cancer cell.

Cancer begins when a single cell transforms, or turns from a normal cell to a cancer cell.

This is often due to a change in the function of one of several genes that normally function to control growth.

That is, the cell cycle gene p53, the “guardian of the genome” that is mutated in more than 50% of all human cancers or suppresses tumor formation.

Once these crucial Cell Cycle genes begin to behave abnormally, cancer cells begin to proliferate violently from repeated and uncontrolled mitosis.

Unlike normal cells, cancer cells ignore the usual density-dependent growth inhibition, multiplying after contact with other cells, accumulating until all nutrients have been depleted.

Cell growth and tumors

Cancer cells grow to form a mass of cancer cells called a tumor.

As the tumor grows, it begins to release proteins from the cell to attract the growth of new blood vessels (this is called “angiogenesis”).

When the tumor is benign, the tumor cells remain in the original site.

And when the tumor is malignant, some tumor cells send signals that tell the body to make a new blood vessel at the site of the tumor.

These cells not only have a supply of food and oxygen, but they also have a pathway to spread to a new part of the body, due to the new blood vessel and through the bloodstream.

Cells that are shed from the tumor begin to spread to surrounding tissues (through the bloodstream or lymph) and new tumors begin, metastasis occurring.

Cancer cells are frequently immortal, whereas normal cells divide about 50 times and die, cancer cells can continue dividing indefinitely if they are supplied with nutrients.

Cancer cells often have an unusual number of chromosomes or mutations in the chromosomes.

Aging (production of toxic oxygen ‘free radicals’), exposure to toxins (such as components of tobacco), mutagens (such as ultraviolet light) cause mutations in genes, but normal errors in DNA replication can lead to cell transformation if they occur in a crucial gene.