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CELL DIVISION
1.1 INTRODUCTION
Cells are structural and functional unit of life. All the living organisms whether unicellular or multicellular require the cells to divide for their growth, development and reproduction. There are several examples which can be cited to understand this, for instance a single celled zygote divides and re-divides to form large number of cells which ultimately leads to formation of multicellular organism. In plants, cell division in meristematic tissue is responsible for growth as well as formation of new shoots, leaves and roots throughout the life of a plant. Living organism can be classified as eukaryotic or prokaryotic. In eukaryotes, two types of cell division are known i.e., mitosis and meiosis, out of which mitosis occurs in somatic cells and meiosis occurs in gametes. Fig. 4.2: Overvie w of cell cycle
For cell division to occur, cells undergo a process called as Cell Cycle. Cell cycle is a series of events that take place in a cell which involves duplication of its DNA ( DNA replication ) and its division to produce daughter cells. There are four phases of cell cycle; G 1 , S, G 2 and M phase along with a G 0 phase. G 0 phase is also known as resting phase. Cell division is a strictly regulated and controlled process. It means that a cell will undergo the process of cell division only when formation of new cells is required; otherwise no unnecessary cell division occurs in living organism. Now, we can better understand why G 0 phase is called resting phase. When a cell is not dividing, it is said to remain in resting phase and cell cycle is paused or halted, but when, formation of new cells required, cell cycle is resumed. Except M phase, rest of cell cycle is known as Interphase. G 1 is the first phase of interphase, from the end of the M phase till the beginning of DNA synthesis, is called as G 1 phase. It is also called the growth phase where the cell increases its proteins, number of organelles and size. S phase is known as synthetic or replicative phase where replication of DNA occurs. G 2 phase is the period between S and M phase. In this phase of cell cycle, protein synthesis and rapid cell growth occurs which prepares cell for mitosis. M phase is mitotic phase where karyokinesis (division of nucleus) followed by cytokinesis (division of cytoplasm) occurs. Table-1: Differences between Karyokinesis and Cytokinesis S.No. Karyokinesis Cytokinesis (^1) It is the process of splitting of the chromosomal material in the nucleus. It is the process of splitting of the cytoplasm 2 It is followed by cytokinesis. It also occurs without being followed by the cytokinesis. Cytokinesis occurs only when the karyokinesis take place. (^3) It is a sequential process. It is equal distribution of the cell organelles and the nuclei into the daughter cells formed (^4) It is the first step in the M phase of the life cycle. It is the second step of the M phase of the life cycle.
CELL DIVISION
There are two types of cell division known which is mitosis (or indirect cell division) and meiosis (or reductional cell division). Mitosis occurs in somatic cells where as meiosis occurs in reproductive cells or gametes. Beside this there is another type of cell division called as Amitosis ( or direct cell division) which occurs in unicellular organism such as protozoa and also in yeast. Amitosis also occurs in foetal membrane of vertebrates. There is no condensation of chromosomes and spindle formation in this type of cell division.
- Cell division is the basis of repair and regeneration of old and worn out tissues.
1.1.1 Mitosis
The mitosis is a part of somatic cell division which includes the division of the nucleus (called mitosis or karyokinesis) and the division of the cytoplasm (called cytokinesis). Strasburger (1875), a German botanist, was the first to work out the details of mitosis. Later on, W. Flemming (1879) discovered it in animal cells. The term mitosis was coined by Flemming (1882). Mitosis can be studied best in the root tip and shoot tip of several plants. But the most favourable material is the apices of onion roots. Mitosis is a form of eukaryotic cell division that produces two daughter cells with the same genetic component as the parent cell. Chromosomes replicated during the S phase, are divided in such a way as to ensure that, each daughter cell receives a copy of every chromosome. In actively dividing animal cells, the whole process takes about one hour. The replicated chromosomes are attached to a 'mitotic apparatus' that aligns them and then separates the sister chromatids to produce an even partitioning of the genetic material. This separation of the genetic material in a mitotic nuclear division (or karyokinesis ) is followed by a separation of the cell cytoplasm in a cellular division (or cytokinesis ) to produce two daughter cells. In some single-celled organisms mitosis forms the basis of asexual reproduction. In diploid multicellular organisms, sexual reproduction involves the fusion of two haploid gametes to produce a diploid zygote. Mitotic divisions of the zygote and daughter cells are responsible for the subsequent growth and development of the organism. In the adult organism, mitosis plays a role in cell replacement, wound healing and tumour formation. In mitosis, the metabolic nucleus passes through a complicated system of changes in the form of four different stages, viz., prophase, metaphase, anaphase and telophase.
Karyokinesis: Karyokinesis is the name of nuclear division. It has been divided into five
phases; Prophase, Prometaphase, Metaphase, Anaphase and Telophase. Some important aspects of all these stages are discussed below.
1. Prophase
It is the first phase of mitotic karyokinesis in which chromatin fibres condense to form chromosomes. Prophase has three sub phases i.e. early, middle and late. Nucleus becomes spherical and cytoplasm becomes more viscous. The chromatin slowly condenses into well-defined chromosomes. Each chromosome appears as two sister chromatids joined at the centromere. The spindle (microtubules) begins to form outside nucleus. In plants, the spindle apparatus or mitotic spindle is anastral. In animals and brown algae, the mitotic spindle is amphiastral which include two asters in opposite poles of the spindle. Each aster consists of two centrioles surrounded by astral rays.
Poleward movement of daughter chromosomes occurs due to shortening of kinetochore microtubules; appearance and elongation of inter-zonal fibers. Daughter chromosomes appear V-shaped (metacentric), L-shaped (Submetacentric), J-shaped (Acrocentric) and I-shaped (Telocentric). Chromosomal fibres shorten and disappear when chromosomes reach the poles. At the end of anaphase, two groups of single stranded chromosomes are formed. It is the shortest of all stages of mitosis.
5. Telophase
In this stage of karyokinesis, reformation of nuclei occurs. Daughter chromosomes arrive at the poles. Kinetochore microtubules disappear. Nuclear envelope reforms around each chromosome cluster of each pole. Decondensation or unfolding of chromosomes occurs. Chromosomes uncoil into chromatin. Formation of nucleolus and nuclear membrane occur. Nucleolus reappears. Two daughter nuclei are formed at each pole. It is considered as the reverse of prophase. Golgi complex, ER etc. reforms. In animal cells, astral rays and spindle fibres completely disappear in telophase. In plant cells the spindle fibres disappear from near the poles but remain intact towards the equator.
6. Cytokinesis: or C-phase is the division of parent cell having undergone karyokinesis to
produce two daughter cells each with a daughter nucleus. It is the cytoplasmic division that starts during mid-anaphase and completed by the end of telophase. Animal cell cytokinesis significantly differs from plant cell cytokinesis. It takes place by two bare different methods i.e. cell plate method and cleavage or cell furrowing method. (a) Cell plate cytokinesis: It occurs in plant cells. The spindle fibres persist at equatorial plane. The Golgi vesicles fuse at the centre to form barrel-shaped phragmoplast. Further addition of vesicles causes the phragmoplast to grow centrifugally till it meets with plasma membrane of the mother cell. The contents of phragmoplast solidify to become cell plate or future middle lamella which separates the two daughter cells. The daughter protoplast secretes primary wall materials on both sides of the cell plate or middle lamella. (b) Cleavage cytokinesis: It occurs in animal cells and pollen mother cells of some angiosperms. In this process, a cleavage furrow appears at the middle, which gradually deepens and breaks the parent cell into two daughter cells. A special structure called midbody is formed in the centre, and it is a centripetal process.
Duration of mitosis
The duration of mitosis varies from species to species. The time required for mitosis ranges from 6 minutes to several hours. Temperature and nutrition are also known to affect duration of mitosis. If we talk of different phases of mitosis, anaphase is shortest phase, metaphase is intermediate in duration and prophase and telophase are of longest duration.
Mitotic poison
Fig. 4.5: Various stages of mitosis occurring in animal cell There are several chemical substances which are known to inhibit the process of mitosis or prevent the entry of cell into the process of mitosis. Such substances are called as mitotic poison. One of the most commonly known mitotic poisons is colchicines which arrests the cell at metaphase stage. Colchicines is obtained from Colchicum autumnale and other species of Liliaceae family, it inhibits formation of spindle fibres. Another well known mitotic poison is ribonuclease enzyme which acts as prophase poison. Mitotic poison functions to inhibit or arrest the process of mitosis at a particular stage, but if cells are exposed to higher concentration of some of these poison, it may result in immediate cell death. Table-2: Differences between mitosis in animal and plant cell S.No. Animal cell mitosis Plant cell mitosis 1 It is found in bone marrow and epithelial cells It occurs in meristematic cells 2 Animal cell become round before mitosis. There is no change in shape of plant cell before the cell division. 3 Centrosome is required for mitosis No centrosome is present in plant cells.
- If mitosis remains unchecked, it may result in uncontrolled growth of cells leading to cancer or tumour.
1.1.2 Meiosis
Meiosis (from Greek, meiosis, which means lessening) is a specialized type of cell division that reduces the chromosome number by half, creating four haploid cells, each genetically distinct from the parent cell that gave rise to them. This process occurs in all sexually reproducing single-celled and multicellular eukaryotes, including animals, plants, and fungi. It is the form of eukaryotic cell division (reductional division) which occurs during the process of formation of gametes in sexually reproducing organisms and produces haploid sex cells or gametes from diploid cells. The process also occurs during formation of spores in some organisms. In meiosis a diploid cell (2N) undergoes two successive divisions to form four haploid (N) daughter cells. The haploid cells give rise to gametes. Meiosis was discovered and described for the first time in sea urchin eggs in 1876 by the German biologist Oscar Hertwig. It was described again in 1883, at the level of chromosomes, by the Belgian embryologist Edouard Van Beneden , in Ascaris roundworm eggs. While working on the horse threadworm ( Parascaris equorum ), he observed 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, he concluded that the contribution of each of the female and male gametes was half the chromosome number to the zygote. A. Weismann (German biologist) 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. In all organisms the chromosomes remain in pairs. The organisms reproducing asexually multiply by mitosis. Thus, there exists no chance of alteration of chromosome number. On the contrary, sexual reproduction demands contribution from two individuals. Thus there lies a risk of chromosomal imbalance. The process of meiosis helps to avert this probability by reducing the number of chromosomes to half. It may happen after gametic union (as in sporozoa) or before fertilization (in all higher organisms). In higher organisms, therefore, mitosis occurs in both somatic and germ cells but meiosis takes place in the germ cells alone and only during the formation of gametes. The process takes the form of one DNA replication followed by two successive nuclear and cellular divisions (Meiosis I and Meiosis II). As in mitosis, meiosis is preceded by a process of DNA replication that converts each chromosome into two sister chromatids. Meiosis is divided into meiosis I and meiosis II which are further divided into
Karyokinesis I and Cytokinesis I and Karyokinesis II and Cytokinesis II respectively. The
- In each homologous pair of chromosomes one chromosome is paternal and other is maternal.
- In animal cell the chromosomes are found to coverage towards the side of centrosomes like a bouquet. Hence this stage is also called bouquet stage.
- Each chromosome is attached to nuclear envelope by its both ends.
- Centrosome or centrioles pairs, separate and development astral rays occurs. (b) Zygotene:
- Pairing of homologous chromosomes.
- The process of attachment or pairing of homologous chromosomes is called synapsis. Synapsis produces pairs of chromosomes called bivalents.
- The nucleoprotein complex that helps in adherence of sister chromatids and then homologous chromosomes is called synaptonemal complex.
- Asters continue to move away from each other. (c) Pachytene:
- It is of longer duration as compared to Leptotene and Zygotene.
- It is one of the most important stages/ sub-stage of meiosis as chromosome thickening and crossing over takes place in this sub-stage.
- Each bivalent at the end of Zygotene is made up of two homologous chromosomes and each chromosome comprises of two chromatids.
- The two chromatids of the same chromosome are called sister chromatids while chromatids belonging to different chromosomes of the homologous pair are known as non-sister chromatids.
- Crossing over (exchange of chromatid segments or genetic material between the homologous chromosomes) always occurs between non-sister chromatids.
- With the end of Pachytene disintegration of synaptonemal complex begins.
- Asters move further away from each other. (d) Diplotene:
- Separation of homologous chromosomes occurs except in the region of chiasmata.
- Dissolution of synaptonemal complex begins to dissolve bivalent.
- Diplotene is prolonged in many animal oocytes. For example, all the oocytes of human female reach the diplotene stage in the fifth month of foetus and remain so for many years till ovulation is to occur.
- In oocytes of many fishes, amphibians, reptiles and birds the bivalents elongate and become converted into lampbrush chromosomes. (e) Diakinesis:
- Chromosomes condense further during the Diakinesis stage, from Greek words meaning "moving through".
- This is the first point in meiosis where the four parts of the tetrads are actually visible. Reduction in number of chiasmata is observed.
- In this last stage of the first meiotic prophase the chromosomes are shortest and thickest.
- In each pair the chromatids of a chromosome remain attached in the region of centromere. There is also an attachment between non-sister chromatids of the two homologous chromosomes in the region of chiasmata.
- The chromosomes bivalents move towards the periphery of the nucleus and remain connected only at the points of chiasmata.
- Chiasmata slip from their original position and pass outwardly. The phenomenon is called terminalization.
- In case of animal cells, asters reach the opposite sides in position of poles.
- Sites of crossing over entangle together, effectively overlapping, making chiasmata clearly visible. Other than this observation, the rest of the stage closely resembles Prometaphase of mitosis; the nucleoli disappear, the nuclear membrane disintegrates into vesicles, and the meiotic spindle begins to form.
- The chromosomes are finally released into the cytoplasm. Fig. 4.6: Different stages of Prophase I of meiosis I
2. Metaphase I: Two major events of metaphase I include complete disintegration of nuclear membrane and the formation of spindle. All the chromosomes, each along with their two chromatids, move to the equatorial region of the newly formed spindle.
- It is simple and of short duration.
- Shortening of chromosomes occur.
- Nucleolus disappears and nuclear envelope degenerate.
- In animal cells, centrosomes develop astral rays and move to the opposite sides.
2. Metaphase II: The chromosomes get arranged in an equatorial position in the newly-formed spindle. Very soon, the chromosome pair separates, of which each contains its own centromere. This is a very short phase of meiosis division-II.
- Chromosome gets attached to spindle fibres, one from each pole.
- Area of chromosome to which spindle fibres attach is called kinetochore.
- Chromosomal fibres contract and bring the chromosomes get arranged on the equator.
- The limbs of the two chromatids are divergent. 3. Anaphase II: In this phase, the two sister chromosomes of each pair start to move towards the opposite poles of the spindle. They are being drawn towards the opposite poles by their centromeres.
- Separation of centromere of two chromatids occurs.
- The separated chromatids become daughter chromosomes.
- Chromosomes move to opposite poles.
- At the end of anaphase II, each pole has haploid number of chromosomes. 4. Telophase II: Each polar group of chromosomes gets enveloped by a nuclear membrane, and there is the reappearance of nucleolus. Four cells are formed by cytokinesis, and the nucleuses 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.
- De-condensation of chromosomes occurs.
- Chromosomes organize to reform a nucleus, along with formation of nucleolus and nuclear envelope.
- Astral rays and spindle fibres disappear.
In multicellular organisms, like us, mitosis only occurs in somatic cells, which comprise all cells in an organism excluding germ cells. Cells that undergo mitosis duplicate their chromosomes, resulting in cells with two times their normal haploid or diploid numbers (4N chromosomes). Newly synthesized chromosomes remain closely associated with their like- chromosome. These two identical chromosomes are called sister chromatids. Once, the duplicated sisters‘ chromatids are separate such that one copy of each chromosome lines up on opposite ends of the cell. The cell then pinches in the center until it breaks into two different cells. A nucleus then forms around the chromosomes in each cell to yield two cells with the same original number of chromosomes as the preexisting cell. There are two major differences between mitosis and meiosis. First, meiosis involves not one, but two cell divisions. Second, meiosis leads to the production of germ cells, which give rise to gametes. In meiosis, as in mitosis, the maternal and paternal homologues are replicated during DNA replication yielding two pairs of sister chromatids. After the first cell division, each of the resulting cells contains a pair of sister chromatids; one maternal pair and the other paternal. Unlike mitosis, meiosis does not end after one division; it continues with a second cell division. In this division, the sister chromatids are separated yielding four total haploid cells. Table-4: Differences between mitosis and meiosis S.No. Mitosis Meiosis 1 The division occurs in somatic cells and it is a single division. It occurs in reproductive cells and it is a double division. 2 Mitosis takes place throughout the life of a multicellular organism. Meiosis takes place only at the time of sexual reproduction. 3 The daughter cells resemble each other as well as their mother cell. The daughter cells neither resemble one another nor their mother cell. 4 Chromosome number remains the same. Chromosome number is halved. 5 Mitosis is required for growth, repair and healing. It does not introduce variations. Meiosis has no such function. It introduces variations. 6 It occurs in both sexually and asexually reproducing organism. Meiosis is found in only sexually reproducing organism. 7 Prophase: is shorter duration, generally of a few hours. Crossing over does not occur. Prophase-I is longer duration, which may be of several days. Crossing over occurs. 8 Metaphase: Chromosomes are replicated but unpaired. Chromosomes are replicated as well as paired to form bivalents. 9 Anaphase: Chromosomes are single stranded. Chromosomes are double stranded in anaphase I and single stranded in anaphase-II.
10 Telophase: It is an essential component of mitosis Telophase I may be absent. Telophase II always occurs.
1.1.2.1 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 off-springs. If the gametes contained the same number of chromosomes as that of their parents, the off-springs 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 off-springs, 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. Another fascinating aspect of meiosis is that, it begins at the very early life in the individual but remains arrested for a considerably long time in the prophase state. In males the completion depends upon the attainment of sexual maturity. In the female, the completion of the division comes only shortly before or after fertilization. The process of meiosis not only reduces the chromosome number to half for the purpose of reproduction but also by random distribution of paternal and maternal chromosomes and by crossing-over through chiasma, it produces gametes, none of which are exactly alike. Thus, a large number of variations result, which have got great significance in evolution.
- DNA, the sole hereditary material, is distributed equally among the gametes by the process of meiosis.
- It provides stimulus for the formation of gemmates and spores.
- Meiosis causes conversion from sporophytic generation to gametophytic generation in plants. It forms spores (n) from the spore mother cells (2n) and thus maintains the alternation of generations in organisms.
- Meiosis-I reduce the number of chromosomes to one half or single genome where each chromosome is without its homologue.
- The products of meiosis-I posses replicated or dyad chromosomes. Occurrence of dyad chromosome acts as a stimulus for meiosis-II to occur.
- Meiosis causes segregation and random assortment of genes. Random assortment of paternal and maternal chromosomes produces genetic variations.
- Crossing over brings about gene recombination or new combination of genes. It also produces genetic variation within the species. The variations are important raw materials for evolution and also help in improvement of races.
- Non-disjunction and breakage of chromosomes may occur during anaphase-I due to non - dissolution of chiasmata. It produces chromosomal aberrations, aneuploidy and polyploidy.
- Meiosis essentially maintains constancy in chromosomes from generation to generation.
- It leads to the formation of haploid gametes (n) which is an essential process in sexually reproducing organisms. Fertilization restores the normal somatic (2n) chromosome number.
