Introduction to Cell Division:
Both types of cell division: MITOSIS AND MEIOSIS occur in eukaryotic acellular (unicellular) and multicellular organisms. In sexual reproduction, offspring is produced as a result of fusion of two cells known as gametes. In animals, they would be sperm from male and egg from female. In plants pollen has male gamete while ovule has female equivalent. Of the two kinds of cell division, meiosis takes place only in germ tissues leading to production of gametes. Mitosis and meiosis are cell division processes by which nuclear information of cell is apportioned to daughter cells.
The five stages of mitosis are Interphase, Prophase, Metaphase, anaphase and Telophase.
i) Interphase: This stage is at times explained as resting stage. Such a classification, though, only refers to fact that cell is not dividing. There would be normal metabolic processes going on in cell. Also, in cell preparing to divide duplication of nuclear seems just as jumble of thread-like fibres known as chromatic fibres. It is not possible to trace any particular fibre inside mass. Interphase nucleus also has nucleolus of nucleoli. Duplication of nuclear material is generally completed in interphase stage but it may extend into the next phase. Preparations for mitosis performed during interphase include replication involved in formation of mitotic spindle. No chromosomes are visible in interphase nucleus.
ii) Prophase: All syntheses that had not been finished in interphase are completed during this stage. Chromatic fibres characteristic of previous stage slowly assume another form. They increasingly become shorter as a result of phenomenon called as condensation - coiling of chromatia fibres. In their condensed form they are rod-like structures that take up stain and are thus known as chromosomes (or colored bodies). Chromosomes turn out to be more coiled and can be seen under a light microscope. Nucleolus disappears in prophase. Nuclear envelope disappears at end of prophase. This signals starting of substage known as prometaphase.
iii) Metaphase: With breakdown of nuclear membrane, chromosomes move on to mitotic spindle. They arrange themselves in the single plane at equator of spindle. If one looked at equator form one of the poles of spindle, all chromosomes would be observed, none lying directly on top of other.
iv) Anaphase: At the starting of anaphase, centromere splits and sister chromatids now separate. The significant feature of definition of the chromosome is number of centromeres rather than number of strands. Besides chromatid is fundamentally half of chromosome. Implication of all this thus is that with splitting of centromere we now have single - stranded chromosomes. As anaphase grows daughter chromosomes move to opposite poles with centromere leading and arms of chromosome trailing. As a result of this configuration assumed by a chromosome during anaphase may be inverted V-shape or J-shape or I-shape. The forces liable for movement of chromosome are complex.
v) Telophase: In this stage, two identical sets of chromosomes arrive at opposite poles. Chromosomes are grouped at pole and nuclear membrane starts to form around each cluster of chromosomes. As a result of the tight clustering individual chromosomes are not identifiable. As telophase progresses, furrow get deeper, constricting the cytoplasm, and finally separating it in two daughter cells. In plant cells, cell plate - an aggregation of material - is formed in equatorial region in spindle. Cell plate then grows outwards to periphery of cell, finally separating cell in two daughter cells. Cell plate becomes middle lamella usually found between two plant cells.
Importance of Mitosis:
Multicellular plants and animals begin life as single cells, zygotes or fertilized egg cells; procedure of Mitosis gives rise to several cells that distinguish to create tissues, organs and organ-systems of organism. Mitosis results in increase in size and growth of the organism, cell reproduction is utilized to create new cells to renew certain tissues and to replace worn out cells. Mitosis is also utilized as form of asexual reproduction in some organisms like in unicellular Amoeba and multicellular Hydra and vegetative reproduction in plants.
Introduction to Meiosis:
Meiosis results in reduction in half of chromosome number in a cell form 2n to n (two sets to one set) in daughter cells. It ultimately leads to formation of gametes that are also haploid and will at fertilization restore diploid number of chromosomes. It isn't surprising thus that meiosis in diploid organisms is limited to germplasm i.e. testes and ovaries in animals and anthers and ovaries in plants. Procedure of meiosis comprises of two successive cell divisions, designated meiosis - I and meiosis - II. Meiocyte that undergoes meiosis thus, produces for haploid cells. Phases of meiosis are fundamentally the same as in mitosis and are recognized by suffix I or II.
Meiosis - I:
i) Interphase I: This stage is no different from mitotic interphase. There is duplication of chromosomal material. Thus at the end of interphase-I nucleus of meiocyte has twice as much DNA as diploid non-dividing cell. Only chromatin fibres and nucleolus are visible in nucleus.
ii) Prophase - I: This stage is like mitotic prophase in some respects but it is more complex. One indication of complexity is fact that there are 5 substages namely: Leptotene, Zygotene, Pachytene, Diplotene and Diakinesis. Different stages and sub-stages are just significant sign-posts for consideration of cell division process. Whole procedure is continuous and can't in reality be put in neat little packets.
a) Leptotene: Chromosomes become visible in nucleus but at this time they come out as long as slander threads. Due to the length of these threads, it is generally not possible to trace any chromosome from tip to tip inside entwined lot. One striking characteristic of leptotene is fact that under right microscope threads seem to be single.
b) Zygotene: At this stage, chromosomes still show as single threads, but now, homologous chromosomes attract each other and undertake pairing that proceeds from one end similar to way in which zipper is pulled shut. This pairing procedure is known as synapsis. Synapsis is very particular in those only homologous i.e. corresponding regions of chromosomes pair.
c) Pachytene: At this stage condensation of pronounced, making chromosome appear thicker. Each chromosome is now visibly composed of two chromatids. Thus each synapse unit of pair of chromosome is composed of four chromatids. This unit of four chromatids is known as tetrad, indicating four chromatids or bivalent.
d) Diplotene: This stage is classified by repulsion between homologues. The once strongly paired and entwined homologues pull apart from each other except at few points where they are still joined. These points of continued attachment are known as chiasmata. Chiasmata represent breakage points at which reciprocal exchange in crossing-over occurred. A chiasma can take place at any point along length of chromosome.
e) Diakinesis: This is last sub-stage of prophase I. Condensation reaches its peak at this stage making tetrads emerge as short, thick and heavily-straining bodies. Chiasmata appear to move towards tips of tetrads. Chiasmata that were originally close to tip of tetrad may slip off at this stage. This phenomenon of apparent movement of chiasmata towards tips of paired chromosomes is known as terminalization.
iii) Metaphase - I: In metaphase I, pair of bivalents aligns on metaphase plate. This is different from metaphase in mitosis, where every chromosome align single file on metaphase plate. Position of each chromosome in bivalents is random - either parental homolog can come out on each side. This means that there is 50-50 possibility for daughter cells to get either mother's or father's homolog for each chromosome. A diploid organism having 2n chromosomes will contain 2n probable combinations or ways of arranging chromosomes in metaphase I.
iv) Anaphase I: In anaphase I, homologous chromosomes divide. Homologous chromosomes each are having two chromatids, shift to separate poles. Unlike in mitosis, centromeres don't split and sister chromatids remain paired in anaphase I.
v) Telophase I: In telophase I, homologs of each bivalent enter at opposite poles of cell, and new nuclear membrane forms around each set of chromosomes. Cytokinesis then divides cell in two daughter cells. Each of the two daughter cells is now haploid (n), with half number of chromosomes per nucleus as in meiosis I. In few species, nuclear membrane temporarily forms around chromosomes, while in others it doesn't. Cell now proceeds in meiosis II, with chromosomes remaining condensed.
Meiosis - II:
i) Interphase-II: This stage may or may not take place. If it happens, it is frequently of short duration and there is only limited condensation. More significant, though, is the fact that there is not more DNA synthesis. Chromosomes were duplicated in Interphase -I. This stage is at times stated as interkinesis that is resting stage between divisions.
ii) Prophase-II: This stage, if it happens, is also short. Repulsion between sister chromatide first evident in anaphase-I persists. Spindle for second meiotic division is often oriented at right angles to that of meiosis - I. This is most readily visible in plant cells.
iii) Metaphase-II: Dyads arrange themselves in equatorial plate and are held in spindle by fibres attached to their centromeres.
iv) Anaphase-II: Centromeres divide and daughter chromosomes (monads - single - stranded) move to opposite poles. This event, by reducing number of strands in chromosomes also decreases amount of DNA
v) Telophase - II: On arrival at poles, chromosomes are surrounded in nuclear membrane with concomitant formation of nucleolus. Cytokinesis also takes place. End products in all cases are four haploid cells.
Significance of Meiosis:
1. It maintains same chromosome n umber in sexually reproducing organisms. From the diploid cell, haploid gametes are generated which in turn fuse to form diploid cell.
2. It limits multiplication of chromosome number and maintains stability of species.
3. Maternal and paternal genes get exchanged in crossing over. It results in variations among offspring.
4. All the four chromatids of the homologous pair of chromosomes segregate and go over separately to four different daughter cells. This leads to variation in daughter cells genetically.
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