In this article we will discuss about the two parts of cell division: 1. Mitosis 2. Meiosis.
The process in which a cell divides to form two new cells, each containing a nucleus, is called cell division. It is of two types i.e., mitosis and meiosis.
In mitosis, which is also called vegetative cell division, the chromosomes in the nucleus are duplicated into two chromatids. The nuclear membrane breaks down, the centromeres divide, and the chromatids move to either end of the cell on the spindle.
There is the reformation of nuclear membrane around each group of chromatids, and a new cell wall is laid down between them. Because of this entire process, each new cell gains exactly the same chromosomes and genetic material. Prophase, metaphase, anaphase and telophase are the four stages of mitosis. Two daughter cells are produced after mitosis.
In meiosis, which is also called reduction division, the haploid sex cells are produced from the diploid cells. In this division, the daughter cells receive a haploid set of chromosomes from the diploid parent cell.
Meiosis involves two cell divisions, and usually there is no inter-phase between the two divisions. In meiosis division I, the replicated homologous chromosomes pair with each other on the spindle, and at this stage the crossing over takes place.
Soon, the chromosomes are separated to either end of the spindle. Meiosis division I consists of a long prophase I (including stages such as leptotene, zygotene, pachytene, diplotene and diakinesis), metaphase I, anaphase I and telophase I.
In meiosis division II, the chromatids of each clironioiome come apart at the centromere, and separate to each end of the second spindle. Meiosis division II consists of prophase II, metaphase II, anaphase II and telophase II. Four daughter cells are produced after meiosis.
Parts of Cell Division:
The mitosis is a part of somatic cell division which includes the division of the nucleus (called mitosis or katyokinesis) and the division of the cytoplasm (called cytokinesis). Strasburger (1875), a German botanist, was the first to work out the details of mitosis. Mitosis can be studied best in the root tip and shoot up of several plants. But the most favourable material is the apices of onion roots.
In mitosis, the metabolic nucleus (Fig. 25 A) passes through a complicated system of changes in the form of four different stages, viz., prophase, metaphase, anaphase and telophase.
Some important aspects of all these stages are discussed below:
In this longest stage of mitosis, the metabolic nucleus (Fig. 25A) contains several delicate and coiled thread-like structures called chromonemata, which are not distinguishable as separate structures.
Some distinct, slender, thread-like chromosomes start appearing in the early stages of prophase (Fig. 25B). Longer chromosomes become spirally coiled. The individual chromosomes are always longitudinally double, and each longitudinal half of the chromosomes is called a chromatid.
Both the chromatids of a chromosome remain coiled around each other almost throughout their length. In the later stages of prophase the chromosomes become somewhat thickened, and their double nature becomes quite clear (Fig. 25C).
The chromatids become irregular and hairy. But their hairiness is soon lost and they become smooth and more thick. Each chromatid soon divides longitudinally into two, and thus at this stage a chromosome consists of four thread-like structures called chromonemata (Fig. 25 D), of which two each belong to two chromatids.
Around each chromatid is accumulated a chromosomal substance in the form of a sheath or matrix. Some constrictions or attachment regions are seen in the chromosomes at this stage.
These constrictions are called centromeres. The nucleolus or nucleoli start decreasing in size and ultimately disappear as the prophase stage ends. This is now the starting point of the next stage, i.e., metaphase.
The metaphase stage is initiated by the disappearance of the nuclear membrane and the appearance of a spindle-like body called nuclear spindle (Fig. 26). The nuclear spindle is usually bipolar, consists of fine and delicate fibrils, and originates either from the nuclear sap or from the cytoplasm. The nucleolus also completely disappears.
The chromosomes become shorter and thicker, and move to the equatorial plane of the spindle becoming completely apart from one another. In metaphase, the centromeres of the chromosomes position themselves at the centre of the spindle or along the line of the equator forming the metaphase plate.
The chromosomes are observed best during the metaphase stage. The centromere of each chromosome divides so that each chromatid contains its own centromere. The centromeres of each pair of chromatids get attached to the spindle fibres passing towards the poles of opposite sides. The centromeres of each pair of chromatids appear to repel each other at the end of metaphase and beginning of the next stage i.e., anaphase.
In the beginning of this shortest phase of mitosis, the centromeres of each pair of chromatids start repelling and moving towards the two opposite poles. Soon, the chromatids get separated from each other and the spindles get elongated.
Ultimately the two sets of the chromatids are separated completely from each other and reach to the two opposite poles of the spindle (Fig. 27). Anaphase is followed by the next stage, i.e., telophase.
In the beginning of telophase the chromatids form a close group at each pole (Fig. 28 A) and are now considered as chromosomes.
This is followed by the following changes:
(i) Disappearance of polar caps of the spindle;
(ii) Formation of a nuclear membrane around each group of chromosomes (Fig. 28 B);
(iii) Reappearance of nucleoli;
(iv) Disappearance of spindle body and matrix;
(v) Reorganization of the chromosomes in the two nuclei;
(vi) Reappearance of nuclear sap and;.
(vii) Enlargement of both the newly formed nuclei (Fig. 28C).
It includes two phenomena, i.e., the division of the cytoplasm and the formation of the cell wall. The cytoplasm divides usually by the formation of new cell wall in the equatorial region (also called cell-plate method), or sometimes by furrowing i.e., by cleavage of the cytoplasm.
The cell-plate formation starts usually in the late telophase stage (Fig. 28C) by the deposition of the new cellulose particles in the equatorial region. A delicate membrane soon develops by the fusion of these cellulose particles, and divides the cytoplasm into two young and new cells (Fig. 28 D).
All stages of mitosis are together shown in Fig. 38.
Significance of Mitosis:
i. Mitosis results in the formation of two daughter cells identical with that of the parental cell.
ii. By this process, DNA, the main component of chromosomes, is distributed equally among the two newly formed nuclei.
iii. Both the daughter cells formed after mitosis are identical and have the same genetic constitution, qualitatively as well as quantitatively, as the parent cell.
iv. The number of chromosomes remain the same from one generation to another generation.
v. Resulted daughter cells have the same characters as were present in the parent cell.
vi. The characters of the plants grown by vegetative reproduction may be preserved for a long period.
The meiosis is a process of cell division by which the chromosomes are reduced from the diploid to the haploid number. It takes place in all sexually reproducing organisms. Haploid sex cells are produced from the diploid cells in meiosis.
Van Benedin, while working on the horse thread-worm (Parascaris equorum), observed in 1883 that there were twice as many chromosomes visible during mitosis in the fertilized egg as there had been in the sperm and egg nuclei before the mitosis.
By this observation, Van Benedin concluded that the contribution of each of the female and male gametes was half the chromosome number to the zygote. Weismann suggested in 1887 that in each generation there must occur reduction division at some stage in which the chromosome number is reduced to half.
Flemming (1887) and Strasburger (1888) observed that two nuclear divisions take place in rapid succession just prior to the formation of mature eggs and sperms in animals and formation of pollen grains in angiosperms. The entire process of reduction division leading to the formation of gametes was termed as “meiosis” in 1905.
‘Meiosis’ consists of two successive divisions of the diploid mother nucleus, i.e.:
(i) Meiosis division I in which the diploid chromosome number (2n) is reduced to haploid chromosome number n, and
(ii) Meiosis division II which is a mitotic division.
Meiosis Division I:
In this division the chromosomes are reduced to half, and therefore this is a reduction division.
Meiosis division I is divisible into four major stages (Prophase I, metaphase I, anaphase I and telophase I) which are briefly discussed below:
A. Prophase I:
This is a complicated and prolonged phase of meiosis which can be subdivided further into five substages, i.e., leptotene, zygotene, pachytene, diplotene and diakinesis.
The important features of all these five substages are under-mentioned:
The diploid nucleus enlarges in volume. The chromosomes appear as long, thin and single threads which soon begin to coil. Several, small, bead-like granules (chromomeres) appear in each thread-like chromosome (Fig. 29A).
The homologous chromosomes come together, get themselves arranged side by side, and form pairs or bivalents (Fig. 29B). This pairing is also called synapsis. The pairing chromosomes soon begin to shorten and get thickened, but there is no actual fusion.
In this stage the chromosomes become shorter, thicker and get splitted into chromatids linked at the centromeres (Fig. 30). From a pair of each homologous chromosomes are thus produced four chromatids. Identification of the homologous chromosomes can be made in pachytene, which is a long stage of prophase I.
Centromeres of paired chromosomes move away from each other (Fig. 31). This movement is because of the development of some repulsive force between the homologous chromosomes. However, the homologous chromosomes remain connected at one or more points called chiasmata.
The physical exchange of genetic material takes place at each chiasma under the process called crossing over. Further coiling and shortening of chromosomes is also seen in late stage of diplotene which soon changes into the diakinesis.
In this last stage of the first meiotic prophase the chromosomes are shortest and thickest. The nuclear membrane starts disintegrating. The nucleolus also disintegrates and disappears (Fig. 32).
The chromosome bivalents move towards the periphery of the nucleus and remain connected only at the points of chiasmata. The chromosomes are finally released into the cytoplasm.
B. Metaphase I:
Two major events of metaphase I include complete disintegration of nuclear membrane and the formation of spindle (Fig. 33). All the chromosomes, each along with their two chromatids, move to the equatorial region of the newly formed spindle.
Differing from the metaphase stage of mitosis, the centromeres of chromosome pairs in metaphase stage of meiosis I become attached with the spindle fibres near the equatorial region. The centromeres remain clearly apart from each other and face the opposite poles while the arms of the chromosome pairs lie towards the equator.
C. Anaphase I:
There is first a repulsion and then movement of the two centromeres of the homologous chromosomes towards the opposite poles of the spindle in anaphase I (Fig. 34A).
A centromere carries either a paternal or a maternal chromosome to one pole but not both the chromosomes. This actually reduces the chromosome number from diploid (2n) to haploid (n), which is the main feature of meiosis or reduction division.
D. Telophase I:
A nuclear membrane develops around each group of homologous chromosomes present on the two opposite poles in the form of a compact group in telophase I (Fig. 34 B). The nucleolus reappears.
Both the so formed daughter nuclei contain haploid number (n) of chromosomes, and each chromosome contains a pair of chromatids. Both the daughter nuclei may or may not be separated by a plasma membrane and soon pass on to the next division, i.e., meiosis division II.
Meiosis Division II:
This division includes almost all the phases found in mitosis.
Four different phases which constitute meiosis division II are prophase II, metaphase II, anaphase II and telophase II, and main events of all these four phases are discussed below:
A. Prophase II:
The chromosomes split into chromatids (Fig. 35 A) in both the haploid nuclei and cells formed after meiosis division I. The splitted chromatids remain connected only at the centromeres. The chromosomes start coiling and become shorter and thicker. The nuclear membrane and nucleolus start disintegrating and some spindle fibres also start appearing.
B. Metaphase II:
The chromosomes get arranged in an equatorial position in the newly-formed spindle (Fig. 35B). Very soon, the chromosome pair separates, of which each contains its own centromere. This is a very short phase of meiosis division II.
C. Anaphase II:
In this phase, the two sister chromosomes of each pair start to move towards the opposite poles of the spindle (Fig. 35C). They are being drawn towards the opposite poles by their centromeres.
D. Telophase II:
Each polar group of chromosomes get enveloped by a nuclear membrane, and there is the reappearance of nucleolus. Four cells are formed by cytokinesis, and the nucleus in all these so formed four young cells contain haploid number (n) of chromosomes.
In this way, four haploid cells are resulted from a single diploid cell in the process of meiosis (Fig. 38).
Significance of Meiosis:
In the process of sexual reproduction the male and female gametes fuse to form a zygote which gives rise to the new offsprings.
If the gametes contained the same number of chromosomes as that of their parents, the offsprings would have an ever- increasing chromosomes number in all future generations to come, and this might have resulted always in the formation of new and peculiar types of offsprings, much different from that of their parents.
To solve this problem, nature has provided the phenomenon of meiosis to all sexually reproducing plants and animals. Meiosis maintains the haploid nature of gametes.
DNA, the sole hereditary material, is distributed equally among the gametes by the process of meiosis.
Meiosis forms spores (n) from the spore mother cells (2n) and thus maintains the alternation of generations in organisms.