In this article we will discuss about Mitosis which is an indirect type of cell division:- 1. Subject-Matter of Mitosis 2. Process of Mitosis 3. Significance.
Subject-Matter of Mitosis:
Walter Fleming (1878), a German biologist, was the first to study the cell division. He stated that the nucleus of a dividing cell undergoes a series of changes. To this process he called mitosis. Thus mitosis refers only to karyokinesis.
In this process all the chromosomes undergo longitudinal doubling, the halves of the chromosomes then separate into two similar groups and the latter finally give rise to two daughter nuclei. In other words, the daughter nuclei have qualitatively and quantitatively the same types of chromosomes which the original nucleus possessed.
‘Mitosis’ in its loose sense has been used by the biologists to denote the division of the nucleus and cytoplasm both. Some cytologists define mitosis as a process involving equal division of the nucleus followed by equal division of cytoplasm. It is also called equational division. Mitosis commonly occurs in all the somatic cells of body.
The suitable materials for the study of this process in plants are the root tips of onion and marginal tissue of the young leaf of tradescantia (Fig. 11.3).
Process of Mitosis:
The process of mitosis is described below:
1. Karyokinesis or Division of Nucleus:
The process is continuous one, but for the convenience in description it is divided into several successive steps. These stages are as follows:
(a) Interphase or metabolic state or resting state,
(b) Prophase,
(c) Metaphase,
(d) Anaphase, and
(e) Telophase.
(a) Interphase:
When the cell is not in division stage its nucleus is said to be in interphase or metabolic state (Figs. 11.4 A and 11.4 B). In this stage the nucleus has a definite nuclear wall. The karyolymph (nuclear sap) is very dense with one or more nucleoli and inconspicuous chromatic reticulum in it.
(b) Prophase:
This is the first visible step in the nuclear division (Figs. 11.4 A and 11.4 B). During this stage, several changes take place in the nucleus. The nuclear reticulum and nucleoli become clear due to dehydration of the nuclear sap at this stage. Stain-ability of the reticulum increases.
The chromatic network dissociates and the chromosomes become distinct. The chromosomes show relic coiling and appear as if they are longitudinally split into two parts.
The two components of the chromosome are called chromatids. The two chromatids lie closely parallel and are still connected with each other at a single point, called the centromere or kinetochore. In the beginning, the chromatids of the chromosomes are spirally coiled. This type of coiling is known as relational or plectonemic coiling (Fig. 11.5) but uncoiling takes place at the end of prophase.
Each chromatid consists of many deeply staining bodies, called the chromomeres. The chromomeres are held together by much finer thread named the chromonema. Probably, this duplication takes place during interphase, sometimes before prophase.
The nucleolus remains attached to a chromosome near the centromere, but in the late stage it begins to disappear and simultaneously the matrix sheaths appear around the chromosomes.
The chromatids now become shortened and thickened. The centrosome, which is found only in the cells of some lower plants and all the animals, divides in the early stage into two star shaped centrosomes (Figs. 11.6 B and D). The centrioles move and come to lie on the two opposite sides of the nucleus. The disappearance of nuclear membrane finally terminates prophase.
(c) Metaphase:
The beginning of the metaphase is marked by the dissolution of nuclear membrane and simultaneous appearance of spindle fibres (Fig. 11.4 B and 11.6 D). The spindle is a structure that is formed during nuclear division or mitosis and is the part of the machinery responsible for equidistribution of the hereditary material into two daughter cells.
It was long a subject of dispute whether the spindle fibres exist in the living cell and whether they are responsible for the anaphasic movements of the chromosomes.
The reality of fibres in the living cells has now been proved. By electron microscopy, the spindle fibres appear to be microtubules. At metaphase stage, the chromosomes can be easily counted and their size and shape can be determined.
The chromosomes at this stage become arranged in the equational plane of the cell and their kinetochores (centromeres) lie in the centre of the spindle. Some of the spindle fibres now become connected with the centromeres of the chromosomes. They are called chromosomal fibres or half fibres.
The other fibres run uninterruptedly between the two poles of the spindle. They are termed continuous fibres. The spindle attachment region determines the future shape of chromosome. If the centromere is located terminally the chromosomes will be rod shaped, if it is sub-terminal then the chromosomes will be ‘j’ shaped, and if it is median then the chromosome will be ‘V’ shaped at the anaphase.
At the late metaphase, the centromere of each chromosome divides in the longitudinal plane forming two daughter centromeres which are attached to the chromatids of their respective sides. Thus, the two chromatids become completely free from each other. One chromatid of each chromosome is attached with spindle fibre of one pole and the other chromatid is connected with the fibre of the opposite pole.
(d) Anaphase:
The beginning of anaphase is marked by doubling of the centromeres and appearance of fibrils between the daughter centromeres. Now the sister chromatids of the chromosomes begin to move apart towards opposite poles of the spindle (Figs. 11.4 A and 11.4 B, 11.6 F).
The movement of the chromatids appears to result from the interaction of special region on the chromosomes (centromeres) with the microtubules. It seems as if some sort of repulsive force develops between two sister centromeres.
The nature of repulsive force is not clearly understood, but it seems possible that the movement is due to changes in the configuration of the protein chains which make up the spindle fibres. The spindle fibres are contractile in nature. This exerts a contractile force on the centromeres. The contractile force, probably, causes the anaphasic movement of daughter chromosomes (chromatids).
At the late anaphase stage, one of the two chromatids of each chromosome reaches to one spindle pole and the other moves in the opposite direction to the other spindle pole. During the anaphasic movement of the chromatids, the gel fibrils (interzonal fibres) between the two centromeres lengthen and the chromosomal fibrils contract and finally disappear.
When the two sets of chromosomes reach to the opposite poles, the interzonal fibres of the spindle between the two poles form the ‘stem body’.
(e) Telophase:
The last visible stage of nuclear division is telophase. This follows the anaphase (Fig. 11.4 B and 11.6 F). At this stage of mitosis, the chromatids at each pole behave like daughter chromosomes. The daughter chromosomes begin to lose their matrix sheaths. This disappearance of matrix coincides with the reappearance of nucleolus.
Spindle fibres disappear and nuclear membranes appear around the two opposite chromosomal sets. The telophase ends when the two daughter nuclei are completely organized from the two sets of the daughter chromosomes. The karyokinesis is now followed by cytokinesis.
2. Cytokinesis:
The division of cytoplasm is known as cytokinesis. It is accomplished either through the formation of a cell plate in between two newly formed daughter nuclei or by means of peripheral furrowing.
(a) Cell plate formation:
Earlier it was thought that the spindle fibres in the equatorial region of the cell after karyokinesis thickened to form an intermediate cell plate dividing the cytoplasm into two.
This cell plate later on splits longitudinally to form two cytoplasmic membranes for daughter protoplasts. The daughter protoplasts in plant cells then secrete calcium pectate which is deposited in between two membranes in the form of middle lamella.
In recent years it has been suggested that a transverse cleft or fluid film is formed as a result of disappearance of spindle fibres in the region of equational plate (Fig. 11.4). This fluid film if believed to be formed by fusion of many small vacuoles or droplets. The daughter protoplasts secrete calcium pectate in the cleft to form the middle lamella. This type of cell wall formation is exemplified by bryophyta and tracheophyta.
(b) Furrowing:
In this method, a peripheral cleavage furrow appears gradually between the two daughter nuclei (Fig. 11.41). The furrow deepens and when the edges of the furrow meet in the centre of the cell, the protoplast is divided into two daughter cells. The daughter protoplast the secrete intermediate cell wall. This type of cross wall formation is seen in bacteria, fungi an animal cells.
Significance of Mitosis:
Mitotic division is a common method of multiplication of cells in the body. During mitosis the chromosomes split longitudinally and the chromatids of these chromosomes separate into two equal groups which finally form two daughter nuclei. In this way the same genetic constitution is maintain qualitatively as well as quantitatively in the chromosomes of the two daughter nuclei formed by this process.