Significance of Mitosis
- The consequence of mitosis is the production of two daughter cells which are identical to each other and also the parent cell.
- The exactness of replication and separation of chromatids ensures that the daughter cells are genetically identical to the parent cell (diploid, having 2n chromosomal number), both in quantitative and qualitative terms.
- Thus mitosis is the basis of continuity of unicellular as well as multicellular organisms.
- Mitosis helps a haploid cell (spore) to develop into a gametophyte and a diploid cell (zygote) into a sporophyte.
- Asexual reproduction in lower plants and animals would have been impossible without mitosis.
- Vegetative propagation in higher plants by grafting and raising of plants and animals by tissue culture technique are all results of mitosis.
In 1884, Van Beneden demonstrated that although each parent contributes an equal number of chromosomes in fertilisation, the chromosome number of the offspring is the same as that of each of the parents. Thus the contribution from each parent during fertilisation should be half or one set of chromosomes (haploid) so that the zygote will have two sets (diploid) of chromosomes. The offspring developed from the zygote through mitosis will have a diploid number in its cells (that is each chromosome represented twice). If we assume that each parent contributed a full (diploid) set, the chromosome number would double after every generation. But it never happens, and the formation of germ cells (gametes) must involve a special kind of reduction division, reducing chromosome number into half in gametes. This special kind of cell division which halves the number of chromosomes in the daughter cells is known as meiosis (also known as reduction division since the daughter cells have haploid /n chromosomal number).
Since the function of meiosis is the production of gametes, it is restricted to gamete producing organs of an organism such as testes and ovaries in animals, and anthers and ovules in higher plants (angiosperms).
Meiosis involves two successive divisions of the cytoplasm and nuclei (Meiosis I and Meiosis II) with only a single doubling of the chromosomes. The cell which undergoes meiosis is called a meiocyte. Like mitosis, meiosis is also a continuous process. However for the convenience of study, it is divided into various stages and substages. DNA synthesis occurs during the premeiotic interphase wherein G1, S and G2 periods similar to mitotic interphase are recognisable. However, G2 periods seems to be lacking but may be comparable to the early part of the long prophase of meiosis I.
Meiosis I First Nuclear Division
Meiosis I is divided into four stages prophase I, metaphase I, anaphase I and telophase I, Meiosis I is unique in having an extended prophase which is generally subdivided into five distinct stages, namely (a) Leptonema, (b) Zygonema, (c) Pachynema, (d) Diplonema and (e) Diakinesis.
Fig: Diagrammatic Representation of Different Stages of Meiosis I and Meiosis II in a Hypothetical Diploid Cell
- Leptonema or Thin-Thread Stage
This is the stage immediately following the S-period. Cells and nuclei are large in size with very long chromosomes. However, along the length of each chromosome are seen darkly stained regions (chromomeres) which represent highly coiled regions of DNA alternating with uncoiled regions. These chromomeres are constant in number, size and position, and seem to represent the beginning of the shortening and condensation of chromosomes.
- Zygonema or Yoked-Thread Stage
Shortening of chromosomes by further coiling makes them distinctly visible. Since the meiocyte possesses pair of morphologically and genetically similar or homologous chromosomes (one set contributed by male and another set by the female parent), the chromosomes start pairing lengthwise with each other. Finally, homologous chromosomes get synapsed to form a complex of four chromatids (two in each chromosome). Since each set of two chromatids is joined at a centromere, the four stranded complex is generally called a bivalent.
The mechanism by which chromosome pairing occurs during meiosis is not clear. However, electron microscopic studies have indicated the presence of a synaptinemal complex made of a set of small fibres running between the two chromosomes and helping in their intimate pairing. The synaptinemal complexes seem to be significant for chromosome pairing, because experimental prevention of pairing during meiosis leads to the disappearance of this complex.
- Pachynema or Thick-Thread Stage
Synapsis of bivalents which starts in zygonema, gets completed in pachynema. The chromosomes appear thicker and nucleoli become more evident and are seen attached to nucleolar organizer regions of certain chromosomes. The pachynema stage ends when synaptic forces of attraction between homologous chromosomes lapse and they seem to separate from each other.
- Diplonema or Double-Thread Stage
This stage begins with the movement of chromosomes apart. Longitudinal duality of each chromosome resulting from DNA duplication at interphase becomes clearly evident. Thus each bivalent seems to consist of four chromatids. The separation of bivalents is generally not complete, as the paired chromatids can be seen held together at one or more points along their length. These points of contact are called chiasmata (singlular: chiasma). Chiasmata represent the points where there is the possibility of exchange of chromosomal material between chromatids of homologous chromosomes. The number and position of chiasmata varies depending on the length of the chromosome and the species in question. As diplonema progresses, the chromosomes are still actively shortening. Although the nucleolus is present, it becomes extremely diminished in size.
The bivalents get condensed to the maximum and are evenly distributed throughout the nucleus. The nucleolus gets detached form its chromosome and degenerates. The contraction of chromosomes forces the chiasmata to move laterally, so that paired chromosomes are joined only at the ends. This process is called as terminalisation. In large chromosomes with many chiasmata, terminalisation is never complete and they may remain joined together until metaphase. The nuclear membrane gets broken into pieces and starts disappearing.
It is characterized by the complete disappearance of the nuclear membrane and the nucleolus, and the formation of the spindle apparatus as in mitosis. However, the pair of chromosomes moves as a 'single unit', behaving as if it were one chromosome. The centromere of one chromosome of the bivalent gets connected by a spindle fibre to one pole, and the centromere of another chromosome to the opposite pole. The bivalent then moves into position midway between the poles, and arranges in the equatorial plane to form a metaphase plate in such a way that one centromere point to one pole and the other to the opposite pole. Thus the orientation of the paternal and maternal chromosomes of different bivalents is independent of each other.
It is characterized by the movement of chromosomes from metaphase plate to their respective poles. The centromeres are undivided so that the whole chromosomes move poleward. With respect to the number of centromeres, the number of chromosomes is haploid and represents one anaphase group. However, DNA content will be equal to that of the meiocyte since each chromosome is a bivalent. This way meiosis-I results in the reduction of chromosome number.
As the chromosomes move towards the poles, the chaismata lose their retentive influence and free the separating chromatids. Because of crossing over between chromatids, the set of chromosomes that move to one pole consists of a mixture of parts of paternal and maternal chromosomes.
This stage begins with the arrival of chromosomes at their respective poles. The nucleolus reappears and the nuclear membrane is reorganized. The chromosomes start uncoiling and become invisible.
The interphase between meiosis I and meiosis II is very brief. There is no doubling of DNA of the chromosomes which had already duplicated in premeiotic interphase. However short the interphase may be, it seems responsible for the synthesis of RNA and proteins required to complete the process of meiosis II.
Meiosis II - Second Nuclear Division
As in meiosis I, meiosis II is also divided into four stages namely, prophase II, metaphase II, anaphase II and telophase II.
It is very short, and is characterized by the disappearance of the nucleolus and nuclear membrane, and the coiling of chromosomes leading to their shortening. The chromosomes show a unique feature wherein the chromatids are flaring apart because of their non-identical nature, in certain segments as a result of crossing over in prophase I.
Metaphase begins with the reorganization of spindle fibres. The chromosomes arrange themselves in the equatorial plane to form a metaphase plate. The centromeres get divided; one of the daughter centromeres is connected by spindle fibres to one pole and the other to the opposite pole.
The spindle fibres connected to centromeres start contracting and pull the system chromatids (now chromosomes) to their respective poles.
As soon as the chromosomes reach their respective poles, they start uncoiling and become elongated. The nucleoli and the nuclear membranes are reorganized. Thus the end result of meiosis is the production of four nuclei, each having one half the chromosome number of the parent nucleus.
Division of the cytoplasm can occur after each nuclear division (meiosis I and meiosis II) or can be delayed until the four nuclei are formed. In animal cells cytokinesis occurs by furrowing whereas in plants by cell plate formation. Finally in plants, these haploid cells develop into gametophytes which later may bear gametes, whereas in animals haploid cells directly act as gametes.
Significance of Meiosis
- In sexually reproducing organisms, meiosis helps in maintaining the chromosome number. One important event that happens during prophase I (diplotene of diplonema) of meiosis is the pairing of homologous chromosomes.
- This leads to the formation of chiasmata between non-sister chromatids. At these chiasmata, breakage and exchange of chromosomal segments occurs between the chromatids of paternal and maternal chromosomes (crossing over). This exchange of chromosomal material results in exchange of genetic material (genes) which forms the basis for the production of new variations in nature.
- The inheritance of such variations leads to the formation of new species during the course of evolution. Thus meiosis forms the basis of speciation in nature.