What is Mitosis? Phases of Mitosis, Diagrams, Cytokinesis

Mitosis phase

Have you ever wondered what the yeast in bread dough and a growing frog have in common? Apart from other things, they all carry cells that undergo mitosis, the process of cell division to produce more cells genetically identical to the parent cells. Why do all these cells undergo mitosis? Well, intestinal cells need to be replaced after they wear out; yeast cells must reproduce to keep their population growth, and a tadpole needs to produce new cells as it gets bigger and becomes more complex.

Mitosis is the process during which the nucleus of a eukaryotic cell splits in two, followed by the splitting of the parent cell to form two daughter cells. The meaning of the word “mitosis” is “threads,” which refers to the threadlike structure of chromosomes before the cell prepares for division. Early microscopists first observed this structure. They also noted the appearance of a specialised network of microtubules during the mitosis process. These tubules, collectively called spindles, extend from structures called centrosomes. 

One centrosome is located at the opposite ends of a cell. As phases of mitosis progress, the microtubules fix to the chromosomes, which have already replicated their DNA and are lined up across the centre of the cell. The microtubules then shorten and march toward the ends of the cell. In the process, they carry one copy of each chromosome to opposite poles of the cell. This process ensures that each daughter cell contains one exact copy of the parent cell’s DNA.

What are the phases of mitosis?

• Before a cell enters the phases of mitosis, it undergoes a period of growth referred to as interphase. About 90% of the total time a cell spends in the cell cycle is spent in the interphase. Interphase is further divided into phases:
• G1 phase: is the period before DNA synthesis, during which the cell mass increases to prepare for cell division. It is referred to as the first gap phase.

• S phase: It is the period when the DNA is synthesised. S here stands for synthesis.
• The G2 phase is the period after the synthesis of DNA, after which the prophase starts. During this period, the cell synthesises proteins, and its size continues to increase. It is the second gap phase. 
• Mitosis comprises five morphologically distinct phases. The mitosis phase order is— prophase, prometaphase, metaphase, anaphase, and telophase. Each phase consists of characteristic steps in the process of alignment and separation of chromosomes. Once mitosis is over, the entire cell divides in two by a process called cytokinesis.

Prophase

• Prophase is the first stage of mitosis that occurs after the G2 portion of the interphase is over. During prophase, the chromosomes of the parent cell, duplicated during the S phase, condense and become much more compact than they were in interphase. This is because every duplicated chromosome contains two similar-looking sister chromatids joined at an area called the centromere; these structures now seem like X-shaped bodies when visualised under a microscope.
• The condensation process is catalysed by several DNA binding proteins, including cohesin and condensin. Cohesin produces rings that hold the sister chromatids together, while condensin produces rings that coil the chromosomes into highly compact forms.
• The mitotic spindle also starts to develop during the prophase. As the two centrosomes of the cell move toward opposite poles, microtubules gradually collect between them, forming a network that later pulls the duplicated chromosomes apart.

Prometaphase

• Once the prophase is over, the cell enters the second stage for mitosis— prometaphase. During this phase, phosphorylation of nuclear lamins by M-CDK breaks down the nuclear membrane into numerous small vesicles. Consequently, the spindle microtubules now get direct access to the genetic material of the cell.
• Each microtubule is extremely dynamic, growing outward from the centrosome and falling backwards as it tries to find a chromosome. Eventually, the microtubules locate their targets and attach to each chromosome at its kinetochore, a complex of proteins situated at the centromere. 
• The actual number of microtubules that connect to a kinetochore differs between species, but at least one microtubule from each pole connects to the kinetochore of each chromosome. Then a tug-of-war ensues as the chromosomes pass back and forth toward the two poles.

Metaphase 

• As prometaphase ends and metaphase commences, the chromosomes line up along the cell equator. Every chromosome has at the minimum two microtubules that extend from its kinetochore and at least one microtubule attached to each pole.
•  At this point, the tension inside the cell is balanced, and the chromosomes no longer pass back and forth. Additionally, the spindle is now complete, and three groups of spindle microtubules are visible.
• Kinetochore microtubules link the chromosomes to the spindle pole; interpolar microtubules lengthen from the spindle pole and extend up to the equator to the spindle end opposite to it, and astral microtubules stick out from the to the cell membrane.

Anaphase

• After metaphase, anaphase occurs, during which the sister chromatids of each chromosome part go to opposite ends of the cell. Enzymatic degradation of cohesin — which is attached to the sister chromatids during prophase — facilitates this separation. After separation, every chromatid forms a separate chromosome. 
• Meanwhile, changes in the microtubule length provide the mechanism for the movement of the chromosomes. More specifically, during the first portion of anaphase (sometimes referred to as anaphase A), the kinetochore microtubules condense and pull the chromosomes toward the spindle ends. Then, during the next portion of anaphase (sometimes referred to as anaphase B), the astral microtubules tied up to the cell membrane draw the poles apart, and the interpolar microtubules slip past each other, exerting an additional pulling force on the chromosomes.

Telophase

• During telophase, the chromosomes come at the cell poles, the mitotic spindle dismantles, and the vesicles carrying the fragments of the parent nuclear membrane gather near the two sets of chromosomes. Then lamins at each end of the cell are then phosphorylated by the phosphatases. A new nuclear membrane is formed around each group of chromosomes due to this phosphorylation.

Cytokinesis

• Cytokinesis is the final process in the mitosis phase order that divides the parent cell into two identical daughter cells. During this process, the cell membrane tweaks in at the cell equator, giving rise to an aperture called the cleavage furrow. The location of the furrow is based on the location of the astral and interpolar microtubules while in anaphase.
• The cleavage furrow is formed due to the action of a contractile ring of overlapping actin and myosin filaments. As the actin and myosin filaments shift past each other, the contractile ring gets smaller, similar to pulling a drawstring at the top of a purse. When the ring advances to its smallest point, the cleavage furrow bisects the cell at its centre, giving rise to two separate daughter cells of equal size.
• The process of cytokinesis is different in plants and animal cells. In animals, cytokinesis occurs by constricting the cell by a band of contractile microfilaments called actin and myosin, which are the proteins responsible for muscle contraction and other kinds of cell movement. In-plant cells, cytokinesis occurs by forming a new cell wall between two daughter cells called a cell plate. The cell plate is formed from small Golgi-derived vesicles that merge in a plane across the equator of the telophase spindle to give rise to a disk-shaped structure. 

Importance of stages for mitosis

• The process of mitosis is vital for the growth and replacement of the damaged and worn-out cells of an organism. In single-celled organisms like yeast, mitosis is carried out asexually and adds new individuals to a population. In humans also, many cells divide through mitosis.

Conclusion

Therefore, mitosis is the process of nuclear division, occurring just before cell division or cytokinesis. During this process of the mitosis phase order, the cell’s chromosomes compress, and the spindle gathers. The replicated chromosomes then connect to the spindle, line up at the cell equator, and drift apart as the spindle microtubules withdraw toward opposite poles of the cell. Each set of chromosomes is then enclosed by a nuclear membrane, and the parent cell is cleaved into two complete daughter cells.

Frequently Asked Questions

1. What is a centromere?

A centromere is a structure in a chromosome that holds two chromatids together. It is the point of attachment of the kinetochore, a structure to which the microtubules of the mitotic spindles are attached. 

2. What does a spindle do?

The spindle pulls the chromatids to opposite poles of the cell during the cell division processes of mitosis and meiosis. After separation, each chromatid becomes a chromosome. Therefore, after dividing, both daughter cells have complete sets of chromosomes.

3. What is an embryo?

An embryo is a stage in the initial development process of an animal while it is in the egg or within the uterus of the pregnant female. In humans, the term embryo refers to an unborn child until the seventh week following conception. From the eight weeks onwards, the unborn child is referred to as a fetus.