Mitosis

Prokaryotes and eukaryotes differ in their cellular structures, the pathways, the machinery, and different processes. The cellular reproductive capacity of the eukaryotes is much higher than the prokaryotes. The cells follow a typical cycle for undergoing division and proliferation. The cell cycle studies help in knowing the pattern of growth and reproduction. The cellular compartmentalization bifurcates the functioning of every important cellular component. The nucleus and the cytoplasm have different roles to play. They involve great interactions and function in an efficient way. The study of cell cycle not only helps us in knowing about the cell division but also its way of differentiation.

Different types of cells exist in eukaryotic organisms. The cells either show haploidy or diploidy or any other condition. Each of them shows the presence of well-defined chromosomes. The G1, S, G2, and the M phases constitute the phases of a cell cycle. The M phase or the mitotic phase belongs to the somatic cell cycle. It is the final phase of the cell cycle and lasts for one hour. The interphase, lasting for 23 hours involves the G1, S, and the G2 phases. The division phase or mitosis plays a crucial role in cellular division. It consists of four subphases such as prophase, metaphase, anaphase, and telophase. Studying mitosis helps in finding out the mitotic index. It helps in determining the fraction of cells undergoing mitosis in a given sample. The mitotic index study helps in determining the division of cancer cells undergoing rapid division.

Image: Different stages of Mitosis

Mitotic apparatus:

Before knowing about the mitotic phase in detail, it is important to know the components involved in the mitosis phase. The mitotic apparatus consists of three main components such as the asters, the spindle, and the traction fibers. Each centrosome involves the formation of the asters. The spindle apparatus consists of a gelatinous structure. The connection of the centromeres to the centrosomes occurs with the help of the traction fibers.

Prophase:

The initial phase of the mitosis involving chromosome visibility is known as prophase. The word prophase indicates the initial stage involved in the division. The chromosomes occur in the highly condensed state. They show clear visibility under the microscope. The formation of the spindle apparatus starts during this phase. The spindle apparatus looks like a slender structure tapering towards the ends. The slenderness of the spindle arises due to the tubulin fibers. These tubulin fibers help in the chromosomal movements during the other phases of the mitotic phase. The chromosomes get attached to these fibers with the help of kinetochores. Each chromosome becomes longitudinally double. Exceptions include the regions near or at the centromere. Three types of microtubules include the polar microtubules, the kinetochore microtubules, and the astral microtubules. Hence, these structures help in organizing the spindle formation. The assembly occurs outside the nucleus during the prophase. The prophase further involves three stages. They include the early prophase, the middle prophase, and the late prophase.

The centrioles are the other organelles involved in the spindle formation. These structures show a composition of tubulin. They help in organizing the mitotic spindle. During the early prophase, these structures move apart, thereby indicating the beginning of the spindle formation. The early prophase chromosomes show a reduced structure. After some time, their visibility gets clarified. The nucleolus gets disappeared in this phase.

The middle prophase leads to the movement of centrioles further apart. It proceeds with the mitotic spindle formation. The late prophase depicts the movement of the centrioles towards the opposite sides. The spindle formation starts. It is the time for the chromosomes to coil and produce a series of compact structures known as gyres.

Metaphase:

The nuclear envelope starts getting disappeared in this phase. The kinetochores are an example of well-defined proteins playing a crucial role in chromosome-spindle attachment. They attach themselves very well to the centromere (structures of the chromosomes joining the sister chromatids). Hence, the chromosomes get aligned on the equatorial plane of the spindle. It occurs between the two spindle poles. It is also known as a metaphase plate. The chromosome gets aligned perfectly on the spindle. The cellular processing technique helps in arresting the cells. Hence, they get visualized under an electron microscope. The microscopic observations reveal the scaffolding patterns of the proteins surrounding the uncoiled DNA. Thus, it helps in studying the double-stranded DNA.

Anaphase:

The centromeres of the sister chromatids undergo a separation during the anaphase. They form two daughter chromosomes. The traction fibers help in the separation of the chromatids. They separate in such a way that their movement follows towards the spindle pole. The kinetochores also separate during this process. The process of disjunction leads to the conversion of the sister chromatids into the independent chromosomes. The microtubules start getting shorter and shorter. Thus, the two independent chromosomes move toward the poles. Due to the disjunction, the chromosomes appear in different shapes. They include V, J, T, X or rod-shaped structures. The shape, however, depends on the position of the centromere. The J-shaped chromosomes are known as the sub-metacentric chromosomes. The V-shaped chromosomes are known as the metacentric. Improper centromere split leads to chromosomal abnormalities. The factor defining the movement of the chromosomes towards the poles is known as the mitotic center. Example of the mitotic center includes the centriole.

Telophase:

Now the chromosomes align in two groups. They place themselves at the opposite ends of the cell. The uncoiling of the chromosomes starts in telophase. The spindle apparatus disappears. This phase helps in the completion of the nuclear division. The dissolution of the kinetochore microtubules occurs during this phase. The polar microtubules get elongated in this phase.

Cytokinesis:

One cell undergoes a division to give rise to two cells. Cytokinesis occurs after the telophase of the mitosis. The two cells have their own set of nuclei and organelles. First, the cell cytoplasm separates and cleaves at the middle of the telophase cell. Thus the cell divides into two. The mitotic spindle helps in determining the cleavage site. The thin ring of the actin filaments gets cleaved first. Hence, it plays a crucial role in the process of cleavage.

References:

[1] The Cell, Bruce Alberts

[2] All About Mitosis and Meiosis, Elizabeth Cregan

[3] Mitosis and Meiosis, Part 1

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