Cell division is a process by which the cell duplicates itself either for growth and repair or for reproduction of organism.
Chromosome plays an important role during cell division. This is so because chromosomes are means by which hereditary characters are transferred from parents to next generation in sexual reproduction or from parent cell to daughter cells.
Based on the behavior of chromosomes, the cell divisions are of two types:
Mitosis is the mechanism by which the chromosome content of a somatic cell (haploid or diploid) is kept constant through successive cell divisions. The division of the cell is initiated by division of the nucleus i.e. Karyokinesis followed by division of cytoplasm i.e. Cytokinesis. The stages of karyokinesis are – prophase, metaphase, anaphase and telophase (Fig. 2.4).
In the resting nucleus the chromatin is spread out as a network. Gradually the chromosome becomes thick and condensed and each of them splits lengthwise forming two chromatids. The chromatids remain coiled around each other throughout their length. The chromatids coil around each other spirally. The chromosomes become distinct as individual units due to coiling. The nucleolus and nuclear membrane gradually disappear.
A new structure, the spindle fibres, appears in the cytoplasm, which chemically, consists of long chain protein molecules oriented longitudinally between two poles. The chromosomes move towards the equatorial plate and get arranged at the equatorial plane. Centromeres divide and each chromatid moves to align itself on the equatorial plate.
Anaphase follows metaphase. At the end of the metaphase the chromatids of each chromosome start moving apart in opposite directions. This helps in identical division of chromatids between two poles and the number of chromosomes remains constant. At the termination of anaphase, chromosomes form densely packed group at the two poles.
This is reorganization phase resulting in the formation of two daughter nuclei. Nuclear membrane and nucleoli reappear and surround the chromosomes. The newly formed nucleus contains the same numbers of chromosomes, as this was in parent nucleus.
Just after the nuclear division, the division of cytoplasm takes place which is known as cytokinesis. The cytokinesis takes place in two ways- by the formation of cell plate m the centre extending towards the cell wall or by formation of cytoplasmic cleavage or furrow in equatorial region that deepens to form a wall separating the two daughter nuclei.
Significance of Mitosis:
1. Mitosis contributes in growth of living matter. Each new cell formed formed after mitosis receives a set of chromosomes to regulate the cellular activities.
2. The two identical cells formed during mitosis have the same genetic constitution, qualitatively and quantitatively, as the parental and ultimately of the organism. Mitosis helps in maintaining uniformity within the species.
3. Mitotic division helps in replacing the old and damaged tissue by the new cells. This helps in repair, healing and regeneration of damaged parts.
The sexual cycle of a diploid organism involves the alternation of haploid and diploid states. Meiosis is the process by which haploid gametes or spores are produced by two successive divisions of diploid nucleus. During meiosis, homologous chromosomes pair, replicate once and undergo assortment so that each of the four meiotic products receives one representative of each chromosome. The two nuclear divisions are called first (meiosis- I) and second meiotic division (meiosis- II).
First meiotic division (meiosis-l):
In meiosis I, the chromosome number is reduced from diploid to haploid. The mechanism consists of four important phases – prophase l, metaphase I, anaphase I and telophase I (Fig. 2.5).
The most complex phase of meiosis I is prophase I, it is longer in duration, and consists of five sub stages – leptotene, zygotene, pachytene, diplotene and diakinesis. Leptotene is marked by the appearance of the chromosomes as long threads. In zygotene homologous chromosomes pair side by side and gene by gene with each other. This process of lateral association of homologues is called synapsis. When the two homologous chromosomes consisting of four chromatids are paired, this structure is called a bivalent.
In pachytene stage, shortening and thickening of chromosomes takes place. During this stage crossing over takes place resulting into exchange of portions of homologous chromosomes. In diplotene, homologous chromosomes begin to separate, particularly in the region surrounding the centromere. The sister chromatids remain attached at the centromeric region, at some points homologous chromosomes remain in close contact, these points are known as chiasmata. The last stage of Prophase I, diakinesis, is characterized by shortened chromosomes and the terminalization of chiasmata.
The homologous chromosomes which are joined through the chiasmata become oriented on the spindle, with the centromeres of each chromosomes lying towards poles but the ends of chromosomes towards the equatorial plate.
The chromosomes in each bivalent separate at this stage so that homologous pairs disjoin and migrate towards the opposite poles. As a result, the maternally and paternally derived homologues are segregated.
It is a reorganization phase. Nuclear membrane and nucleolus reappear and thus at each pole a haploid nucleus is formed.
Second Meiotic division (meiosis -II):
It is similar to mitotic division; the sub stages are prophase II, metaphase II, anaphase II and telophase II. In prophase II, the chromosomes condense, and the centromeres divide. In metaphase II, a spindle apparatus is organized and the chromosomes become aligned at the equatorial plate.
In anaphase II the centromeres migrate to the opposite pole of the spindle, pulling the chromatids with them (Fig. 2.5 (ii)). Each of the two cells produced by the first division divides in telophase II, resulting into formation of four haploid cells. The chromosomes become less condensed and a nuclear membrane forms.
(i) Meiosis is necessary part of the life cycle of sexually reproducing animals and plants as it helps in restoring the definite number of chromosomes, the characteristic of a species.
(ii) The crossing over of genes between homologous chromosomes helps in exchange of genes leading to formation of new recombinants.
(iii) Meiosis is most essential for the completion of life cycle of a plant as it brings a change from diploid to haploid generation.