Mitosis: Useful notes on Mitosis (2056 Words)

This article provides information about the Mitosis: type of Cell division in Animals!

The growth and development of every living organism depends on the growth and multiplication of its cells. In unicellular organisms, cell division is the means of reproduction and by this process two or more new individuals arise from the mother cell.

In multicellular organisms, new individuals develop from a single primordial cell, the zygote.

Organisms grow and repair themselves through the medium of cell division.

The cell increases in size as a result of growth, which is the characteristic feature of all the living beings. After attaining maximum growth, the cell begins to divide.

According to the cell theory, put forward by T. Schwan (1839), K. Nageli (1846), and confirmed by R. Virchow (1855), new cells originate from the division of pre-existing cells.

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The growth of living material progresses rhythmically and according to a geometric progression, i.e., Mn, 2Mn, 4Mn, MC, 2MC, 4MC, etc. Mn is the nuclear mass and MC is the cytoplasmic mass of the cells. The two masses are in a state of equilibrium, the so called nucleoplasmic index (NP).

NP = Vn D / Vc – Vn

Vn is the nuclear volume and Vc is the cell volume. Cell division is precisely regulated so that production of new cells compensates exactly the loss of cells in adult tissues. Cell division is essential for perpetuation of the species. Meiosis leads to over-production and allows biparental inheritance and variation, which is important for evolution and continuation of species.

Three kinds of cell-divisions occur in animals —

1. Mitosis:

It takes place regularly in ordinary (somatic) cells of the body.

2. Meiosis:

It occurs in germ cells in gonads and results in gamete formation.

3. Amitosis:

It is rare and of little importance, and occurs in some primitive acellular organisms, involving a sort of mass division of the nucleus.

Mitosis:

The multiplication of a body cell into two daughter cells of equal size and containing the same number of chromosomes as the parent cell is called mitosis or somatic division or karyokinesis. The term ‘mitosis’ was proposed by W. Fleming in 1882 showing longitudinal splitting of chromosomes during nuclear division.

It is derived from Greek word for thread (mitos) and refers to thread-like appearance of chromosomes early in cell division. All somatic cells divide by this process, forming the multicellular body. Mitosis thus is characteristic of somatogenic reproduction maintaining exactly the same number and kind of chromosomes as were originally present in the parent cell.

For the sake of convenience of study, mitosis can be divided into two stages, namely preparative stage and distributive stage.

Preparative Stage (Interphase—G₁, S and G₂ phases):

It occurs during interphase which occupies large portion of interkinesis. Interphase is the period between two nuclear divisions. It is a time of important metabolic reactions both in the nucleus and in the cytoplasm, marked by DNA replication as well as the duplication of the chromosomes. The time between the end of telophase and the beginning of the next phase is called the interphase. Its duration varies from organism to organism.

A typical cell cycle, including interphase, lasts from 20 to 24 hours. Cell cycle is comprised of two periods— (i) interphase (period of non-apparent division), and (ii) period of division. Interphase is the longest period in the cell cycle. The necessary events of the preparative stage are the following —

Doubling of chromosomal constituents:

During this, DNA histones and kinetochores are duplicated. DNA contents are doubled and organise themselves into two equal bodies, chromatids, which can be separated by splitting. Most evidence suggests that chromosomes replicate in conservative manner, i.e., parental DNA unit retains integrity and all new DNA goes to form daughter chromatid. Synthesis period of inter-phase is called S-stage and preceding it is G₁ stage, while following it is G2 stage.

Synthesis is one of the necessary steps for initiating mitosis. Mitosis will not begin until synthesis is finished.

The interphase, which is also described as resting phase or preparative stage, can be subdivided into three distinct periods on the basis of synthetic activities —

1. First growth period (G₁-period):

It is the post-mitotic period during which young daughter cell grows in size. Most of the non-dividing cells remain permanently in this stage. During this phase, various enzymes and substrates necessary for DNA synthesis are produced. Therefore G₁ is marked by the synthesis of RNA and protein. DNA synthesis does not occur in this period. G₁ is the period between the end of mitosis and the start of DNA synthesis.

2. Synthetic period (S-period):

In it, DNA molecules are replicated. The S-phase cells contain a factor that induces DNA synthesis. This period varies from 6-8 hours duration in cultured human cells, and without this synthetic phase, no cell division will occur further.

3. Second growth period (G2-period):

It is premitotic period characterized by increased nuclear volume. It has an average duration similar to that of M (1 to 4 hours). r-RNA and mRNA all are synthesized in G₂ period. Nucleolar RNA synthesis immediately prior to M is not required. G₂ is the interval between the end of DNA synthesis and the start of mitosis.

During G₂ a cell contains two times (4C) the amount of DNA present in the original diploid cell (2C). Following mitosis the daughter cells again enter the G₁ period and have a DNA content equivalent of 2C.

During interphase, chromosomes exhibit a minimum degree of condensation or coiling and are so entwined that they cannot be distinguished individually. The slight coiling evident in the chromosomes is a hold over from the previous mitosis. The coils are thus known as relic coils.

The volume of interphase nucleus as well as nucleolus increases in size. Nucleolus is at maximum in size. The cell may also show increase in its volume before division.

The cell is quite active metabolically during interphase, and several processes, including DNA synthesis, occurring at this time so affect the cell that there is apparently no alternative to division. At the end of interphase, a series of changes begins in the physical and chemical organization of the chromosomes.

Distributive Stage:

In this phase, chromosomes are distributed to their respective poles and total time period required for it ranges from half an hour to 3 hours. It can be divided into five phases —

(1) Prophase

(2) Prometaphase

(3) Metaphase

(4) Anaphase

(5) Telophase

[I] Prophase: Chromatid coiling, nucleolar disintegration and spindle formation:

After interphase, cell nucleus starts the first phase of division. This stage is the longest mitotic stage lasting from several minutes to more than an hour. In case of onion root, it lasts for 71 minutes, in grasshopper neuroblast for 102 minutes, and in case of pea endosperm for 40 minutes. During this stage, mitotic apparatus is assembled with the visibility of chromosomes. Prophase is characterized by the following features —

Differentiation of chromosomes:

The beginning of prophase is indicated by the appearance of the chromosomes as thin threads inside the nucleus. The condensation occurs by a process of coiling or folding of the chromatin fibres. At the same time the cell becomes spheroid, more refractile and viscous.

Each prophase chromosome is composed of two coiled filaments, the chromatids, which are a result of the replication of DNA during S-period. As prophase progresses, the chromatids become shorter and thicker, and the primary constrictions, which contain centromeres and kinetochores, become clearly visible.

During early prophase, chromosomes are evenly distributed in the nucleus, and as prophase progresses, chromosomes approach the nuclear envelope, causing the central space of nucleus to become empty.

Now each chromosome appears to be composed of two cylindrical, parallel elements that are in close proximity. The secondary constrictions along some chromosomes may also be observed along with primary constrictions. Nucleoli disintegrate within nucleoplasm. At the end of prophase, nuclear envelope disintegrates and disappears, and nuclear material is released into cytoplasm.

In early prophase there are two pairs of Centrioles in cytoplasm, each one surrounded by the aster, composed of microtubules that radiate in all directions.

The two pairs of Centrioles migrate along with asters describing a circular path toward poles, while the spindle lengthens between. Migration of asters continues until they become situated in antipodal positions.

Centrioles replicate during interphase, generally during the S-period. At the beginning of prophase, there is a single aster surrounding the two pairs of Centrioles. One of the pairs remains in position with half the original aster, while the other, along with the other half aster, migrates about 180° around the periphery of nucleus to reach the opposite pole.

[II] Prometaphase and metaphase:

Chromosomal orientation at the equatorial plate:

White (1963) defined prometaphase as the period during which spindle is produced and chromosomes try to reach the equator of developing spindle.

Sometimes the transition between prophase and metaphase is called prometaphase (Gr., Meta, between). This is a very short period in which nuclear envelope disintegrates and chromosomes are in apparent disorder.

After that, spindle fibres invade the central area and their microtubules extend between the poles. Chromosomes become attached by kinetochores (centromeres) to some of the spindle fibres and become radially oriented in the equatorial plane and form the equatorial plate.

All chromosomes are independently oriented on the equator. Metaphase chromosomes exhibit a high level of coiling and may therefore be at their shortest and thickest. Somatic coils are at their greatest diameter and smallest number, and there is no longer much relational coiling. Chromatids are, as a result, no longer twisted about each other but lie side by side.

Within a comparatively short time, kinetochore regions of each chromatid separate so that the sister chromatids become independent of one another. Upto this point they were in close apposition.

[III] Anaphase (Gr:, ann, back):

Movement of the daughter chromosomes toward the poles:

It is the shortest of all the stages in mitotic cycle. Centromeres in all chromosomes separate and then move apart and chromatids separate and begin their migration toward the poles (telokinesis).

Afterwards, all the daughter chromosomes or chromatids reach at their respective poles. During movement towards poles, the centromere moves first along the spindle axis carrying with it the chromatids. These chromatids thus assume either ‘J’ or ‘V’ shapes, called heterobrachial or isobrachial chromosomes, respectively.

During anaphase, microtubules of chromosomal fibres (fibres of spindle that connect to chromosomes) of spindle shorten one third to one fifth of the original length. Simultaneously, microtubules of continuous fibres (those that extend without interruption from one pole to other) increase in length.

Some of these stretched spindle fibres now constitute the so-called interzonal fibres (fibres between daughter chromosomes and nuclei in anaphase and telophase). Thus, chromosomes are pulled to the poles due to mechanism analogous to muscle contraction. It is possible; kinetochore activity participates by gliding along half spindle fibres.

[IV] Telophase (Gr., telo, end):

This is last phase in which two daughter cells are formed by the following changes—

Chromosomes with their centromeres at poles begin to uncoil and lengthen as a result they become invisible. At the same time, chromosomes gather into masses of chromatin which become surrounded by discontinuous segments of nuclear envelope. These segments fuse to make the two complete nuclear envelopes of daughter nuclei. During final stages, nucleoli reappear at the sites of nucleolar organizers.

Simultaneous with uncoiling of chromosomes and formation of nuclear envelope, cytokinesis occurs. This is the process of segmentation and separation of cytoplasm. At the time of cleavages of animal cells, a dense material around the microtubules (aster) at the equator of the spindle at either mid- or late anaphase, while most spindle microtubules tend to become disorganized and disappear during telophase, some may persists, and even increase in number at the equator, which frequently being intermingled with a row of vesicles and dense material—the entire structure is called the midbody.

Simultaneously, a depression appears on the cell surface—“a constriction” that deepens gradually until reaching the midbody. With the completion of furrowing the sepration of the cell is concluded. Surface constriction involves a contractile mechanism at the cell cortex. It is due to actin and myosin present in cell cortex.

In plant cells cytokinesis starts with the formation of phragmoplast (i.e., interzonal microtubules and Golgi vesicles). This forms the cell plate.