Growth and multiplication of cells of a living organism are the basic necessities for its proper growth.
The process of cell division is found same in all living organisms and the events are chiefly centre in the nucleus.
The following three types of cell divisions have been recognized in plants and animals:
1. Amitosis – Direct cell division
2. Mitosis – Indirect cell division
3. Meiosis – Reduction division or equational division
Amitosis is characterized by the splitting of nucleus followed by cytoplasm. It is the means of asexual reproduction in unicellular organisms like bacteria and protozoa and also in the cells of foetal membrane. A depression appears in the nucleus which increases in size and splits the nucleus into two nuclei. Simultaneously, cytoplasm also constricts into two equal or approximately similar halves. Amitosis division does not include any nuclear events.
Mitosis involves the division into two daughter cells which are identical and contain the same number of chromosomes as found in the parent cell. This division was first observed by Strassburger (1870) in plant cells and by Fleming (1882) in animal cells. The basic outline of mitosis remains the same in all living organisms.
Process of Mitosis:
The process of mitosis is characterized by the duplication of chromosomes, their separation into two and then their movement to opposite poles so as to construct two daughter nuclei. It is followed by the constriction of cytoplasm to form two daughter cells. Mitosis can be divided into two stages, namely preparative stage and a distributive stage.
1. Preparative Stage (Interphase):
Interphase is the interval period between two successive cell divisions. Though apparently inactive, the nucleus and cytoplasm are very active metabolically and synthetically. This phase, therefore, is described as preparatory. Interphase is longest phase taking one or two days in its completion. During Interphase, the following events occur:
(i) Nuclear envelope remains intact.
(ii) DNA amount becomes double.
(iii) A daughter centrioles originates near the existing centrioles.
2. Distributive Stage (Mitotic Phase):
The main divide on of cell occurs during this phase. It is divided into the following phases:
Nuclear division begins with prophase. The important events during this phase are: (l) Chromatin material of nucleus condenses into distinct chromatin threads by losing water.
(ii) Chromatin threads coil and become shorter to form chromosomes.
(iii) Each chromosome is already doubled due to the doubling of DNA contents in Interphase.
(iv) The nucleolus starts to disappear.
(v) Each centriole migrates towards the opposite pole and duplicates so that both poles of the cell contain paired centrioles.
(vi) The Centrosomes forms an elongated body or bridge known as centrodesmus in between the two centrioles.
(vii) Asters arise from the centrodesmus and form the spindle.
Metaphase is marked by the appearance of the spindle and arrangement of chromosomes on the equator of the spindle. The main events during this phase are:
(i) Microtubules form the amphiaster spindle.
(ii) The chromosomes migrate towards the equator of the spindle.
(iii) Each chromosome becomes more compact and short and its two chromatids separate except at the centromere which has not divided so far.
The following changes occur during anaphase:
(i) The centromere of each chromosome divides and allows the separation of two sister chromatids into two daughter chromosomes.
(ii) The daughter chromosomes move apart and migrate towards opposite poles.
(iii) The arms of daughter chromosomes are directed towards the equator and centromere towards the pole of the equator.
The main events during this phase are as follows:
(i) Chromosomes reach the poles of the spindle and form two groups.
(ii) Chromosomes begin to uncoil and form a chromatin net.
(iii) The nuclear wall and nucleolus reappear.
Cytokinesis is the division of cell-cytoplasm into two separate cells. It usually occurs in Telophase along with the formation of daughter nuclei after the nuclear division. The process of Cytokinesis differs in plant and animal cells.
(A) Cytokinesis in Animal Cells:
In animal cells, Cytokinesis starts by the appearance of a shallow groove or furrow in the cytoplasm at the equator of the spindle. Slowly and slowly it deepens and constricts the cytoplasm and the cell into two parts.
(B) Cytokinesis in Plant Cells:
In plant cells, Cytokinesis is accomplished by the formation of phragmoplast and cell-plate at the equator of the dividing cell.
The importance of mitosis for the organisms has been summarised as follows:
1. It helps in maintaining the cells in their proper size.
2. It maintains equilibrium in the amount of DNA and RNA contents.
3. It provides an opportunity for the growth and development of organs and the body of organisms.
4. Old, decaying and dead cells are replaced by mitosis.
5. It helps the organisms in the asexual reproduction.
6. Sex-cells (sperm and ova) also depend on mitosis for the increase in their number.
Meiosis is a type of cell division occurring only in diploid reproductive cells and results into the formation of haploid sex-cells or gametes. The cells undergoing meiosis are known as meiocytes. In animals, the meiocytes are the primary spermatocytes and primary oocytes, while in plants, these are represented by sporocytes.
Process of Meiosis
Meiotic division includes two complete divisions following in close sequence with or without a short Interphase between them. The first division is known as heterotypic reduction division and the second one is known as homeotypic. Each of the two divisions is further distinguished into phases. These are prophase, metaphase, anaphase and Telophase.
A. Heterotypic (Reduction) Division
1. First Prophase:
The first prophase of meiotic division is of longer duration and is distinguished into the following six sub-stages:
The meiotic cell is comparatively larger in size and possesses a longer nucleus. It contains diploid number of chromosomes which form a network.
The leiptotene stage initiates meiosis. The chromosomes appear like thin and uncoiled threads of, slender filament. Each chromosome presents a beaded appearance due to the presence of dense bead like swellings called chromomeres. The nucleolus is well marked.
Zygotene commences with the movement of chromosomes. It is affected by the forces of attraction between the two homologues of a chromosome pair. Thus, chromosomes of a pair approach each other. The pairing of homologous chromosomes is known as synapsis. At zygotene, nucleolus increases in size and the centrioles move apart initiating the spindle formation.
With the pairing or synapsis of homologues, the nucleus enters the pachytene stage. It represents the stable period in cell division. During this stage, the paired chromosomes get shortened and thickened and twist or twine round each other forming relational coils. Each starts splitting into two sister chromatids by a vertical or longitudinal furrow. As a result, the bivalent is now converted into tetrad. The exchange and recombination known as crossing over occurs between matching chromatids during this phase.
The separation of homologous chromosomes initiates diplotene. Separation remains incomplete since the homologues remain in contact at one or more points. These points of contact are known as chiasmata. By the end of diplotene, the chiasmata begin to move along the length of chromosomes from the centromere towards the end. This displacement of chiasmata is termed as terminalization.
The two chromatids of each chromosome become closely oppressed together losing their identity. At the same time, the homologues move still apart due to the force of repulsion developed between their centromere. In doing so, the chiasmata move towards the ends. During this stage, nucleolus and nuclear membranes disappear and the formation of nuclear spindle starts.
2. First Metaphase:
In metaphase, bivalents move to the equator. They orient themselves on the equator in such a way that their centromere lie one on either side and face the pole of spindle while the arms are directed towards the equator.
3. First Anaphase:
During this stage, bivalents move apart towards the opposite poles of the spindle. The tetrad which was having four chromatids now separates into two dyads due to complete separation of maternal and paternal chromosomes of the bivalent. This process of separation is known as disjunction.
4. First Telophase:
The first Telophase commences with the formation of nuclear wall around the haploid group of chromosomal dyads. The chromosomes elongate and uncoil, and the nucleus is again formed. The cell cytoplasm also segments into two. Thus, two daughter cells are formed, each of which contains haploid number of chromosomes.
It is the resting stage and its duration depends upon the species involved. It may be totally absent, but if it is present, it is of a very short duration.
B. Homeotypic Division
The second meiotic division takes place independently in both the haploid sister cells. It includes the followi-3 phases:
1. Second Prophase:
During this phase, nucleus and nuclear wall disappear in both the daughter cells and the formation of spindle starts.
2. Second Metaphase:
It is of short duration. The chromatids move towards the centre of the spindle and orient themselves on the equator. Their centromere touches the equator and, later on, centromere in each dyad divides into two.
3. Second Anaphase:
The chromatids with their independent centromere form sister chromosomes and move apart towards the opposite poles of the spindle.
4. Second Telophase:
The chromosomes at each pole uncoil and thin out to form the nuclear net. Each group gets surrounded by a nuclear membrane. The nucleolus reappears, thus, two nuclei are recognised in each cell. This is soon followed by Cytokinesis and two cells are formed from each daughter cell.
Thus, as a result of meiosis, four cells are produced, each with a haploid set of chromosomes i.e., each contains just one member of each homologous pair.
Significance of Meiosis
1. It is a necessary part in the life cycle of sexually reproducing animals since it leads to the formation of gametes (sex-cells).
2. The gametes produced as a result of meiosis are haploid and the zygote formed by their fusion is diploid. Thus, it is the only means for restoring the chromosome number characteristic of the species.
3. Meiosis provides for new combinations of genetic material. During crossing over, the hereditary factors from male and female parents get mixed due to breakage and exchange of chromatids.