What is mitosis? Definition, phases, diagram, and importance.

 

What is mitosis? Definition, phases, diagram, and importance

Mitosis

Mitosis is the process of cellular division in eukaryotic cells where a single parent cell replicates into two daughter cells with the exact no of chromosomes. Mitotic cell division is the phase of the cell cycle where mitosis refers to the division of the nucleus. Mitosis is the primary mechanism of cell division, in which a cell usually grows in number and enables an organism to grow in size. It is therefore involved in growth and repair. It is a method of asexual reproduction in lower organisms (single-cell eukaryotes) that reproduces offspring having exactly the same genetic make-up as their parent cell. Prokaryotes lack the membrane-bounded nucleus and hence divide by binary fission. Mitosis is also known as equational division because two cells are produced at the end of the process identical to each other, having the exact same genetic make-up as their parent cell and having the same ploidy level. Before entering into the mitotic phase, the cell has already gone through interphase where it duplicates its DNA, accumulates necessary proteins for division and formation of some organelles, etc. The German biologist Walther Flemming discovered mitosis for the first time in 1882.

Definition of mitosis 

"Mitosis is a process of cell duplication where a mature parent cell divides into two identical daughter cells, with the exact genetic makeup and number of chromosomes to their parent cell."

Mitosis is generally the process of cell reproduction or duplication where DNA is duplicated first during interphase and then the cell segregates its duplicated DNA during cell division. As a result, two daughter cells are formed that are genetically identical to their parent cell with the exact chromosome no and genetic makeup. Karyokinesis (division of the nucleus) is the main event that occurs during mitosis. Mitosis is comprised of four main phases, which are:

Prophase

Metaphase

Anaphase

Telophase

Prophase

• Prophase of mitosis begins after the G² phase. It starts as the pairs of centrioles (centrioles duplicated by the cell before the cell enters into mitosis) separate from each other and their migration alongside the nuclear membrane to the opposite poles.
• In this initial stage of mitosis, condensation and contraction of chromatin material of nucleus occur, and chromosomes appear as thread-like structures.
• Each chromosome consists of two parallel subunits called chromatids. Chromatids are attached to each other at the centromere.
• Nuclei start disappearing and holes start appearing in the nuclear envelope. As a result, the nuclear envelope gradually disintegrates and liberates the chromosome. 

Late prophase

Lateprophase is sometimes also known as prometaphase.

• In lateprophase, the nuclear envelop and nucleoli vanish, and the mitotic spindle apparatus starts to appear.
• The spindle fibers start radiating at two poles and are known as spindle fiber apparatus, which are of three types (the aster microtubules that radiate outward from centrioles; the polar microtubules that extend from one pole and are attached to the polarmicrotuble of the opposite pole’s centriole; and kinetochore microtubules that extend from one pole and are attached to the centromere of the chromosome).
• With continued contraction and condensation of the chromosomes, it appears as an X-shaped structure. Each chromosome consists of two sister chromatids that have identical genetic information.

Metaphase

• The metaphase of mitosis begins soon after as the centrioles reach the opposite poles and the spindle apparatus's formation.
• Chromosomes are at maximum contraction and condensation at the metaphase stage and can easily be visible under the microscope at this stage.
• The distinct event that happens in metaphase of mitosis is that chromosomes arranged themselves midway between two poles, which is called the metaphase plate or equatorial plate.
• Centromeres of chromosomes are attached fully to the kinetochore microtubules of both poles, and the spindle fibers of both poles are ready to pull the chromosomes at their respective poles.
• Metaphase checkpoints ensure full attachment of the kinetochore to the centromere and proper alignment of chromosomes along the metaphase plate. After that, the cell enters into the next succeeding phase, the Anaphase.

Anaphase

• Mitotic Anaphase begins as the centromeres divide. In early anaphase, the kinetochore spindle fibers of two poles start contracting and pulling the chromosomes at their respective poles. The centromeres of chromosomes split into two.
• After the division of centromeres, chromatids start separating gradually because of the constant pulling of spindle fibers. These chromatids will become the chromosomes of daughter nuclei later.
• In late anaphase, chromosomes segregate equally and two sets of chromosomes are generated, which continue their journey at their respective poles.
• The polar microtubules elongate and are pushed apart by another protein family called the kinesin motor protein family, which pushes the polar microtubules, separating the two poles, making the cell elongated. The main purpose of the polar microtubules is to maintain kinetochore stability for reaching to the opposite poles.

Telophase

• During telophase, a set of daughter chromosomes reaches to the opposite poles and kinetochore microtubules start disappearing.
• Telophase of mitosis is the reversal of prophase and prometaphase. The polar microtubules become more elongated in early telophase.
• The nuclear membrane starts reappearing from the membrane vesicles around the two sets of daughter chromosomes, and nucleoli also start reappearing.
• Chromosomes start uncoiling, becoming thinner and thinner, and start disappearing.
• Mitotic Spindle fiber apparatus gets dissolved at the end of telophase and two sets of daughter nuclei appear in one parent cell. Mitosis is completed and two sets of daughter nuclei form with the exact number of chromosomes.
• Cytokinesis also begins during the telophase.

Cytokinesis

Cytokinesis is the division of cytoplasm as the two daughter nuclei form. During anaphase, the cytoplasm also begins to divide, separating the cell into two daughter cells. Although cytokinesis is not a part of mitosis, it is required to complete cell division. Both plants and animal cells have different mechanisms of cytokinesis.

In animals, the cytoplasm divides by inward movement or inward folding of the plasma membrane. The plasma membrane starts moving inside at the equator of the parent cell. The invagination of the cell membrane deepens and deepens, which lets the separation of two daughter cells with the exact number of chromosomes and genetic material.

In plant cells, cytokinesis occurs via different methods. During the metaphase stage, a small cylindrical structure appears at the equatorial place, which is a complex assembly of microfilaments, microtubules, and endoplasmic reticulum called Phragmoplast. Phragmoplast serves as a scaffold for cell plate assembly and the formation of the cell wall. These cylindrical barrels originated from the Golgi complex and started lining up parallel to each other during anaphase. Phragmoplast guides the golgi-driven vesicles to the site of cell plate assembly. Vesicles also contain material like pectin and cellulose for cell wall formation.

The importance of mitosis

• In mitotic cell division there is no crossing over and no change in the number of chromosomes, so the genetic information remains unchanged generations after generations.
• It is a type of asexual reproduction in the majority of lower eukaryotes (single-celled organisms).
• It helps in the growth and development of an individual. The development of a zygote into an adult occurs by mitosis.
• It is also responsible for the regeneration of organs in the majority of animals and plants.
• Healing of wounds and replacement of cells take place via mitosis.

The label diagram of mitosis cell division.