Table of Contents
It is the important process which occurs in the living organism, in this process, one cell is divided into two or more than two passing from specific stages. Mainly there are two types of the cell division of it.
- MITOSIS 2.MEIOSIS.
It is the important type of the cell division in which single mother cell is divided into two daughter cell with the same number of the chromosomes as present in the mother cell is called as the mitosis. This process is generally found only in the somatic cells which are the same in the male and female of any species and does not occur in the germ cell. In this process, the first nucleus is broken down into two daughter nuclei and the process is called as KARYOKINESIS, then the cytoplasm is divided into two the phenomenon is called as CYTOKINESIS.
The process of mitosis is completed into four phases.
- Chromatin network breaks up to form long coiled threads called CHROMOSOMES. This then becomes shorten and thicken.
- It begins with the separation of centrioles of centrosome and their migration along the side of the nuclear membrane to the opposite poles.
- The appearance of spindle fibers outside the nucleus.
- Nuclear membrane and Nucleolus are disappeared.
- Mitotic apparatus becomes more distinct.
- The chromosomes become shortest and thickest and arrange themselves at the equatorial plane of the spindle.
- Each chromosome is attached to the spindle fiber by the kinetochore and up to this time, the chromatids remain together.
- Centromere divides and the two chromatids of each chromosome get separated.
- The contraction of spindle fibers takes place at this stage, as a result of which two sets of chromatids migrate towards opposite poles of the spindle.
- During the movement of chromatids, they assume the V, J or I shape because of the different positions of the centromere on the chromosomes.
- The chromosomes reach the opposite poles of the spindle, start uncoiling, elongate and become thin and invisible.
- The nuclear membrane and nucleolus reappear.
- Thus two daughter nuclei are formed, which are then separated into two daughter cells by process of CYTOKINESIS.
The significance of Mitosis:
- The same number of the chromosomes in daughter cells as parent cell contains.
- Through this single cell <zygote> is converted into too many cells, which form the tissues, organs, systems and up to the body.
- Healing of wounds and regeneration in animals is due to the mitosis.
- Formation of the RBCs in the bone marrow.
- Replacement of the outer most layer of the skin by new cells is due to mitosis.
- The growth of hair and nail is due to the mitosis.
- The replacement of dead bark tissue is due to the mitosis.
- Asexual reproduction in lower plants and animals is due to mitosis.
- Vegetative propagation in plants, tissue culture and organ transplant techniques in human and animals are possible due to mitotic division.
It is the important type of cell division, in which diploid <2n> number of chromosomes in parental is reduced to haploid <n> or half in daughter cells. Therefore it is also called as reduction division. The process of meiosis occurs only in the germ cells. In animals, it forms the sperms and ova, while in plants it forms the pollen grain and eggs. Meiosis occurs in two successive divisions that are meiosis-I and meiosis-II.
FIRST MEIOTIC DIVISION
This is an important part of the meiotic division. It is further divided into four stages.
The prophase of the first meiotic division is the longer duration and significantly modified in comparison to the mitotic prophase. The following five sub-stages are recognizable in the prophase-I.
Leptotene OR Leptonema:
In this stage the chromosome number is diploid, and the chromosomes are observed to be thin, long threads and longitudinally single rather than double as in mitosis.
Zygotene OR Zygonema:
Shortening of the chromosomes by further coiling and condensation makes them distinctively visible. The pairing of homologous chromosomes takes place. The pairing is lengthwise, and the process is called as SYNAPSIS. In the result, BIVALENT or TETRAD is formed, which is complex of chromatids.
Pachytene OR Pachynema:
In this stage synapsis of homologous chromosomes is complete. The chromosomes become thicker and clear by further condensation. Nucleoli become more evident. The homologous chromosome forms loops known as CHIASMATA. Exchange of parts occurs in loops; this exchange is called as CROSSING OVER.
Diplotene OR Diplonema:
In this stage, the paired chromosomes repel each other with the result that they begin to separate but are held together by chiasmata. Further shortening of the chromosomes <tetrad>, continue even in this stage. The number and position of chiasmata vary depending on the length of the chromosomes and the species in question.
In the beginning, the nucleoli or nucleolus disappears, but the nuclear membrane still exists. The chiasmata also disappear, and the paired chromosomes are only held at one end. The tetrad distributes evenly in the nucleus and gets condensed to the maximum. At the end of diakinesis, the nuclear membrane is broken into pieces and starts disappearing. The contraction of chromosomes forces the chiasmata to move laterally so that paired chromosomes are joined only at the end. This process is called as TERMINALISATION.
In this stage, the nuclear membrane and the nucleolus completely disappear, and the spindle is formed. The bivalents now line up at the equatorial plane. The tetrads then attach themselves to the half spindle fibers at the centromeres. Each chromosome of the bivalent becomes connected to the half spindle fibers of one pole and the other half with the half spindle fibers of the opposite pole.
In this stage, each chromosome of the bivalent of homologous is pulled towards the opposite pole by the contraction of the half spindle fibers. The anaphase is completed when the two sets of chromosomes reach the opposite poles of the cell.
This stage begins with the arrival of chromosomes at their respective poles. The nucleolus reappears and the nuclear membrane is reformed. The chromosomes start uncoiling and become invisible. Thus a complete nucleus is formed at each pole.
At the end of Telophase-I cytokinesis starts which results in the division of a single parent cell into two daughter cells with a haploid number (n) of chromosomes.
SECOND MEIOTIC DIVISION
The Telophase-I is followed by a short interphase which corresponds with mitotic interphase. Sometimes the interphase may persist for a considerable length of time. At the interphase between the two meiotic divisions, there is no replication of chromosomes. These are now haploid in number although each one consists of two chromatids. It is further is divided into four stages.
The centrioles divide and the spindles are formed which are at right angles to the spindle of the first meiotic division. The nuclear membrane disappears and the split chromosomes arrange themselves at the equatorial plane.
The chromosomes uncoil and form separate groups and around each group, a nuclear membrane is formed. The cytoplasmic division is followed resulting in four daughter cells, each with a haploid number (n) of chromosomes.
The significance of Meiosis:
- Meiosis maintains the number of chromosomes constant from generation to generation.
- During the crossing over between chromatids which produce new variations in nature, and the inheritance of these variations leads to the formation of new species during the course of evolution.
- Hereditary characters are also transferred from parents to offsprings during this process.
DIFFERENCES BETWEEN MITOSIS AND MEIOSIS
|Mitosis is commonly associated with somatic tissues. It provides the mechanism for asexual reproduction as well as vegetative propagation in plants.
|Meiosis is commonly associated with germinal (reproductive) tissues. It provides the mechanism for the formation of male and female gametes.
|Mitosis involves only one nuclear division and one chromosomal division. The nuclear division is equational with respect to the chromosome numbers
|Meiosis involves two nuclear divisions, but the chromosomes divide only once. The first meiotic division (Meiosis I) is reductional, while the second nuclear division (Meiosis II) is equational with respect to the chromosome numbers. The chromosomes divide during the second meiotic division.
|Mitotic prophase is of comparatively short duration.
|Meiotic prophase is of long duration and is the most significant stage genetically. Prophase I is divided into 5 sub-stages: leptotene, zygotene, pachytene, diplotene, and diakinesis.
|In the early prophase, each chromosome shows two chromatids.
|The leptotene chromosomes are structurally duplicated into 2 chromatids, even though their divided nature is not visible under the light microscope.
|There is no pairing or synapsis of the homologous chromosomes, during prophase or afterward.
|There is a pairing or synapsis between the homologous chromosomes during the zygotene stage.
|Each chromosome has only two chromatids. There is no bivalent or tetrad formation.
|Each pair of homologous or homologous chromosomes consists of four chromatids and hence these are referred to as a bivalent or tetrad.
|The phenomenon of chiasmata formation and crossing-over does not occur at any stage.
|The phenomenon of chiasmata formation and crossing-over occurs during, pachytene stage. This brings about genetic recombination.
|The kinetochore of each chromosome is both functionally and structurally double during metaphase and anaphase stages.
|The kinetochore of each chromosome behaves as a single unit during metaphase I and anaphase I stages.
|During anaphase stage, the sister chromatids separate and move toward opposite poles as daughter chromosomes. Therefore, the mitotic distribution of chromosomes is equational.
|During anaphase I, the paternal and maternal chromosomes of bivalent or tetrad separate into dyads and move toward the opposite poles. This separating phenomenon is called disjunction. The first meiotic distribution of chromosomes is reductional. During anaphase II, the sister chromatids separate into daughter chromosomes and move to opposite poles as in mitotic anaphase. The second meiotic division is equational with regard to the distribution of” chromosomes.
|In the mitotic metaphase, (there is no distinction between first and second. metaphase) the two chromatids are in close association throughout their length.
|In the second meiotic metaphase, the two chromatids of each dyad are held together only at the kinetochore region, while their arms diverge widely.
|The final results of the mitotic division are the formation of two daughter cells having the same number of chromosomes as the parent, cell, and with (more or less) same genetic and physiological potential.
|The final result of meiotic divisions is the formation of four daughter cells; each having a haploid number of chromosomes, and genetically, each cell is different from the other.
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