The following points highlight the three main types of cell division in animals and plants. The types are: 1. Amitosis 2. Mitosis 3. Meiosis.
Cell Division: Type # 1. Amitosis:
The amitosis or direct cell division is the means of asexual reproduction in unicellular organisms like bacteria and protozoans and also a method of multiplication or growth in foetal membranes of some vertebrates. In amitosis type of cell division, the splitting of nucleus is followed by cytoplasmic constriction.
During amitosis, the nucleus elongates first and then assumes dumb-bell-shaped appearance. The depression or constriction increases in size and ultimately divides the nucleus into two nuclei. The division of nucleus is followed by the constriction of cytoplasm which divides the cell into two equal or approximately similar halves. Therefore, without the occurrence of any nuclear events two daughter cells are formed.
Cell Division: Type # 2. Mitosis:
The mitosis (Gr., mitos/thread) occurs in the somatic cells and it is meant for the multiplication of cell number during embryogenesis and blastogenesis of plants and animals. Fundamentally, it is related with the growth of an individual from zygote to adult stage.
One of the basic characteristics of mitotic cell division which is meant for growth due to multiplication is that it gives rise to two daughter cells, which resemble each other and also the parent cell qualitatively and quantitatively (i.e., the chromosome number of mitotic product cells remain the same like the parent cell).
The mitosis is composed of two apparatuses, viz., chromatic apparatus which includes the chromosomes and the nucleolus and the achromatic apparatus which, in its turn, includes the centrioles and spindle.
The basic outline of mitosis is the same in all living organisms. It includes four phases, namely, G1phase, S phase, G2 phase and mitotic phase, which occur in succession and forming the so called cell cycle. The G1 phase, S phase and G2 phase are collectively forming the interphase.
Thus, in continuously dividing cells, an individual cell passes through following two main phases of cell or mitotic cycle:
B. Mitotic phase
The resting phase or stage between the two mitotic divisions is known as the inter-mitotic phase or interphase (L., inter = between, Gr., phasis = appearance). In interphase, no division occurs but in the nucleus and cytoplasm, active metabolic activities occur and also increase in the volume of the cytoplasmic and nuclear substances takes place.
The interphase is the longest phase of the mitotic cycle and it takes one or two days in its completion.
During the interphase, following events take place:
1. The nuclear envelope remains intact.
2. The chromosomes occur in the form of diffused, long, coiled and indistinctly visible chromatin fibres.
3. The DNA amount becomes double.
4. Due to the accumulation of ribosomal RNA (rRNA) and ribosomal proteins, the nucleolus attains the maximum size.
5. A daughter centriole is originated near the already existing centriole and, thus, an interphase cell has two centrioles or a diplosome (Fulton, 1971).
B. Mitotic Phase:
The main mitotic cell division occurs during mitotic phase or M phase which includes the following phases:
The prophase (Gr., pro=before; phasis=appearance) is the actual first phase of the mitosis.
During the prophase, following events take place in the cell:
1. The cell becomes spheroid, refractive and viscous.
2. The disintegration of nuclear envelope starts.
3. Due to the DNA duplication in the interphase, each chromosome now possesses two chromatids. Each chromatid consists of a single DNA molecule wrapped in the nucleoproteins.
4. Both chromatids remain connected with each other by the centromere and both remain closely associated along their entire lengths.
5. The chromatids become shortened and thickened.
6. The nucleolus starts to disappear.
7. Each centriole separates and migrates towards the opposite poles of the cell. Each centriole duplicates, so that both poles of the cell contain paired centrioles or diplosome.
8. The centrosome forms an elongated body or bridge known as the centrodesmus in between the two centrioles.
9. From the centrodesmus, delicate filaments (microtubules) or asters arise and form the spindle.
In, prometaphase (Gr., pro=before; Meta=after; phasis = appearance), following changes usually occur in the cell:
1. The nuclear membrane completely disintegrates.
2. A clear zone known as the equator appears in between the mid-line of the spindle and the nucleus.
3. The chromosomes move towards the equator.
The metaphase (Gr., Meta=after; phasis=appearance) follows the prometaphase and during this phase following events occur in the cell:
1. Each chromosome reaches to the equator and all arrange themselves radially at the periphery of the spindle.
2. The smaller chromosomes usually remain towards the interior, while the larger chromosomes remain at the periphery.
3. Some of the fibres of microtubules of the spindle attach with the centromere of each chromosome and are known as the chromosomal fibres.
5. Certain fibres occur in between the chromosomes and are known as inter-zonal fibres or inter-chromosomal fibres.
In the anaphase (Gr., ana=up; phasis=appearance), following changes occur in the cell:
1. The centromere of each chromosome divides into two.
2. The chromatids of the each chromosome are separated and form two chromosomes.
3. The chromosomes become shorter and thicker and migrate towards the opposite poles of the cell.
4. The migration of the daughter chromosomes towards the opposite poles is achieved by the contraction of chromosomal fibres and the stretching of inter-chromosomal or interzonal fibres. The interzonal fibres push the daughter chromosomes towards the opposite poles.
The telophase is the final stage of mitosis and during this phase following events occur:
1. The chromosomes which reach at the opposite poles of the cells now elongate, the coils of DNA protein fibres loosen and the chromosomes become thread-like.
2. The nucleolus reappears.
3. The endoplasmic reticulum forms the new nuclear envelope around the chromosomes and the nucleolus.
4. The microtubules of the aster and mitotic spindle rearrange and disappear.
Thus, after the telophase, two daughter nuclei are formed due to the karyokinesis. The karyokinesis is followed by the cytokinesis.
In the process of cytokinesis, the cytoplasm splits from the equatorial region and the two daughter halves of the cytoplasm are separated. Soon after a unit membrane of lipoprotein develops in between the two daughter cells.
The cytokinesis of animal cells involves the cyclosis of the cytoplasm, formation of a contractile ring, the expansion of the cell membrane, ATP and interaction of the spindle and asters with the cell surface, while in the plant cells the cytokinesis involves the movement of the endoplasmic reticulum and dicytosomes to the equator where they fuse to form the primary cell wall.
Significance of Mitosis:
The importance of the mitosis for the organisms has been summarised in the following points:
1. In mitotic division, the chromosome number in each daughter cell remains the same like the parent cell.
2. The mitosis helps the cell in maintaining its proper size.
3. Through the process of the mitosis, an equilibrium is maintained in the amount of DNA and RNA contents.
4. The mitosis provides the opportunity for the growth and development of organs and the body of the organisms.
5. Due to the mitosis, the old decaying and dead cells are replaced by the new cells.
6. The mitosis helps the organisms in the asexual reproduction.
7. The gonads and the sex cells also depend on the mitosis for the increase in their number.
Cell Division: Type # 3. Meiosis:
The term meiosis (Gr., meioum = to reduce) was coined by J.S. Farmer in 1905. The meiotic division is of utmost importance for those organisms in which the union of the haploid gametes takes place during the sexual reproduction.
By reducing the number of chromosomes of the diploid germ cells into the haploid gametes the meiosis maintains a constant number of the chromosomes in the species. Thus, meiosis helps in alternation of generation of haploidic generations of plants and animals.
In the process of meiosis, the chromosomes divide once and the nucleus and cytoplasm divide twice. Due to the meiosis, four haploid cells are formed from the single diploid cell.
The process of meiosis is fundamentally same in all the animals and plants but certain biologists recognised following three types of meiotic divisions according to their occurrence at different stages of the life cycle of the organisms:
1. Sporogenetic Meiosis:
The meiosis occurring at the time of spore formation is referred to as the sporogenetic meiosis. This type of meiosis is commonly found in plants.
2. Gametic Meiosis:
In most animals and lower plants, the meiosis occurs at the time of gametogenesis (spermatogenesis and oogenesis) and is known as gametic meiosis.
3. Zygotic Meiosis:
In certain lower plants, sometimes the meiosis occurs immediately after the fertilisation of the egg by a sperm. This type of meiosis is known as zygotic meiosis.
The cells in which the meiosis takes place are known as the meiocytes. The meiocytes of the testes or male gonads are known as the spermatocytes and the meiocytes of the ovaries or female gonads are termed as oocytes. The meiocytes of the plant sporangium are known as sporocytes.
Chemical substances initiating meiosis:
The process of meiosis seems to depend on the balance of nucleic acids in the nucleus or on certain hormones. For instance, insect moulting hormone, the ecdysone is found to initiate the meiosis in certain parasitic flagellate protozoans.
Further, when in the cell the amount of the DNA material is increased in comparison to the RNA, meiosis is found to occur in the cell. However, till now no information is available about the exact chemical substances which may initiate the meiosis.
Significance of Meiosis:
The meiosis has the greatest significance for the biological world because of its following uses:
1. The meiosis maintains a definite and constant number of the chromosomes in the organisms.
2. By crossing over, the meiosis provides an opportunity for the exchange of the genes and, thus, causes the genetical variations among the species. The variations are the raw material of the evolutionary process.
Thus, the meiosis has a peculiar taxonomic, genetical, and evolutionary importance for the sexually reproducing organisms.