The word Meiosis is formed from the Greek words meious meaning to reduce and osis meaning state. In that process the actual cells
The nucleus and cytoplasm divide to form four daughter cells and the chromosome number of the daughter cell is half the number of chromosomes of the mother cell is called meiosis. It is also called reductive or reductional division. The word Miosis is spelled Meiosis based on the Greek root word meioum.
Meiosis requirements and nomenclature
In 1876, the present scientist Gangpadhat Aiwathar Riham was the first to observe the division of myocyte cells in the ovary. In 1883, Bonedeni and Hauser discovered the number of chromosomes in haploids in the worm genome. In 1887, the scientist Dobrangsdahah was the first to describe the reduction process in chromosomes. In 1887, Irawatu was the first to observe meiotic cell division in the roundworm genitalia. In 1888, the scientist Jhathdhangnanthmavat first observed the reductive division of chromosomes in the reproductive mother cell of a flowering plant. In 1905, Vajnyani Ridhatsavat and Gardtava named the first Myosci. In 1911, he was the first to prove crossover and genetic rearrangement in Drosophila.
Where does meiosis occur?
Meiosis occurs in cells with diploid chromosomes. Meiosis occurs in the mother cell of higher organisms. Meiosis occurs in the zygote after fertilization in lower organisms. Meiosis does not occur in bacteria.
Characteristics of meiosis
1. Meiosis Cell division occurs in the mother cell of an organism.
2. It occurs only in diploid and polyploid cells.
3. In this process, four offspring cells (haploid) are formed from each reproductive mother cell (diploid).
4. In this process the nucleus of the cell divides twice and the chromosomes divide once.
5. In this process, the chromosome number of the offspring cells is half of the chromosome number of the mother cell.
6. Distinct arrangement of chromosomes occurs in meiosis.
7. In this process the prophase stage is prolonged and in this stage objects called chromomeres are formed.
8. Prophase-1 is preceded by replication of DNA. Prophase-1 is most significant.
9. Synapsis and bivalents occur between homologous chromosomes in this process.
10. In the process of meiosis, chiasma and crossing over occur between non-sister chromatids. New features appear.
11. Meiosis cell division produces sperms and eggs and plays an important role in sexual reproduction.
12. Meiosis-II is followed by cytokinesis.
13. Due to crossing over and unique arrangement of chromosomes, the resulting cells are never identical to the mother cell.
14. Meiosis in polyploid plants is very complex in nature.
15. Meiosis plays a role in variation and biodiversity.
16. Meiosis Cell division drives evolution in organisms.
Meiosis causes cell division
1. Meiosis Cell division occurs when physiological changes occur in the body of an organism.
2. Meiosis occurs in cell division for reproduction.
3. When organelle growth is completed, meiosis induces division.
4. Meiosis occurs when an organism matures.
5. This division occurs when the balance of nucleic acids and hormones is disturbed.
6. Meiosis occurs when there is more RNA than DNA in the cell.
Why is meiosis called reductive division?
Meiosis occurs in the reproductive mother cell of the organism. Germ cells have a diploid number of chromosomes. Meiosis is cell division
done in two steps. Meiosis-1 and Meiosis-II. In meiosis-1, each cell divides to produce two daughter cells. In this process, both the cell nucleus and the chromosomes divide. The chromosome number of the resulting daughter cell is half the chromosome number of the mother cell. Hence meiosis-1 is called reductive division. The sperm and egg produced in meiosis are haploid in nature. That is, the number of chromosomes in the resulting cell is halved. Hence meiosis is called reductive division.
Types of Meiosis
1. Gametogenic Meiosis: The meiosis that occurs during the formation of gametes is called gametogenic meiosis. It is also called terminal meiosis. It occurs before fertilization. Spermatozoa are formed from mother cells to sperm and ovum from mother cells to ovum.
2. Zygogenic Meiosis: The meiosis that occurs in the zygote of an organism is called zygogenic meiosis. It occurs in thallophyta plant algae and fungi. It occurs after fertilization.
3. Sporogenic Meiosis: The meiosis that occurs during the formation of spores in plants is called sporogenic meiosis. It occurs in mosses and ferns. Microspores are produced inside the anther and megaspores inside the ovary.
Process of Meosis cell division
Meiosis Cell division is accomplished in two stages. Meiosis-1 and Meiosis-II.
Meiosis-1 Cell Division
Meiosis-1 is divided into four stages. Prophase-1, Metaphase-1, Anaphase-1 and Telophase-1.
1. Prophase-1: Prophase-1 is divided into five phases. Leptotene, zygotene, pachytene, diplotene and diakinesis.
(i) Leptotene or Leptotene : Leptos means thin and tene means thread. At this stage, the nucleus of the cell begins to increase in size. Chromosomes look like thin threads. Dehydration begins between the chromosomes. It increases the color or dye capacity of chromosomes. Chromosomes contain granular clusters called chromomeres. DNA duplicates by making its counterpart. Towards the end of this stage, the chromosomes are compressed, short, thick and visible. Nuclear membrane and nucleolus are evident. In animal cells, the ends of chromosomes and the nuclear membrane join towards the aster rays to form the attachment plate. The attachment plate looks a lot like a bouquet of flowers. Scientist Darlington called this state of chromosomes Boke stage or bouquet. When chromosomes in plant cells form bouquet-like structures, it is called synogenesis.
(ii) Zygotene or Zygotene : Zygos means pair and tene means thread. At this stage the homologous chromosomes are arranged in pairs. Attraction begins between homologous chromosomes. Chromosomes come close to each other and pair up (one from the father and the other from the mother). The process of pairing of chromosomes is called synapsis and a pair of chromosomes is called bivalent. Bivalent chromosomes are linked by two synaptonemal complexes. The pairing process may start from one end to the other end, or may begin at the centromere and gradually spread to both sides, or may begin at a site. Bivalents form half the number of chromosomes in each cell. At this stage the chromosomes are more compressed, thicker and shorter. Nuclear membrane and nucleolus are clear. Dr. Siddique Publications
[Chromosomes having the same structure are called homologous chromosomes. One of the homologous chromosomes comes from the father and the other from the mother]
(iii) Pachytene or pachynema : The Greek word pachys means thick or thick and tene means thread. Pachytin is a relatively long-lasting phase. At this stage the chromosomes are more compressed, shorter and thicker. Each chromosome in a bivalent divides longitudinally without the centromere into two chromatids. As a result, four chromatids are formed in each bivalent. This condition is called tetrad. Two chromatids of the same chromosome are called sister chromatids and two chromatids of different chromosomes are called non-sister chromatids. Two non-sister chromatids come close together and form an X-shaped structure. It is called chiasma (chiasma=cross) or kai structure. If bivalent chromatids are shorter in length, zygoma may not form. Again, if the length of the chromatids is longer, the chiasmata may be formed at more than one place. With the help of endonuclease enzymes, non-sister chromatids break into two at the chiasma segment and exchange segments. The exchanged chromatids are then joined with the help of ligase enzyme. In this way, the exchange of parts between two non-sister chromatids is called crossing over or crossover. Qualitative changes occur between chromosomes due to zygomatic crossing over. At this stage, the nuclear membrane and nucleolus are intact. Dr. Siddique Publications
(iv) Diplotene or Diplonema : Diplos means two or two and tene means thread. At this stage the chromosomes become shorter and thicker through continuous contraction. In bivalents, the attraction between the homologous chromosomes decreases and repulsion begins instead. Repulsion is usually observed first and most extensively within the centromere. As a result of repulsion, the chromosomes move away from each other. However, homologous chromosomes may not be completely separate. Chromosomes moving apart is called disjunction. A loop or loop is formed in the bivalent in the presence of two atoms. The clay gradually moves towards the edges. Moving towards the edge of the material is called terminalization. At this time two or more arms are rotated by 90 degree angle. But if there is a kayazma, the arms can rotate through 180 degree angle. Towards the end of this stage, the number of kayazmata decreases. Absence of the nucleolus is indicated although the nuclear membrane is intact.
Oocytes in human oocytes are arrested in the diplotene stage during embryonic stage. This condition is called dicotyledonous condition. Under the influence of luteinizing hormone, the diktyoten phase ends. Dr. Siddique Publications
(v) Diakinesis : Dia means opposite and kinesis means insertion. At this stage the chromosomes are shorter and thicker. The maximum marginalization of kiazmata occurs. Chromatids cannot be identified separately due to the accumulation of matrix on each chromosome in bivalents. Bivalents move from the center of the nucleus to the periphery. At this point the exchanged part is visible through the crossover. Towards the end of this stage, the nucleolus disappears and the nuclear envelope is removed.
2. Metaphase-1
In metaphase-1, the chromosomes become thicker and shorter. Each centromere of a bivalent is poleward, equidistant from the equator. Centromeres are associated with traction fibers. Loops are formed between chromosomes. Towards the end of this stage, the bivalent chromosomes separate under the pull of traction fibers.
3. Anaphase-1
In anaphase-1, homologous chromosomes separate. Chromosomes run in opposite poles. Polar movement of chromosomes occurs due to contraction of chromosome bases and elongation of stem bodies. This is called anaphagic movement. During polar movement, the centromere is the leader and the armature is the follower. When the chromosomes reach the poles, they look like the letters V, L, J or I. As undivided complete chromosomes of bivalents reach the poles, the chromosome number becomes half of the chromosome number of the maternal cell. Siddique Publications
4. Telophase-1
Telophase-1 is the last stage of meiosis-1. Chromosomes are fixed at opposite poles. Chromosome water addition or water absorption (hydration) begins. Chromosomes decrease in capacity to hold dyes. Chromosomes uncoil and become long, narrow and fuzzy. The nuclear membrane and nucleolus appear. As a result, two nuclei are formed at both ends. Cytokinesis occurs at the telophase-1 stage in many species. That is, in the equatorial region of the cell, the cell plate is formed and turns into two daughter cells. A child cell has n number of chromosomes. Telophase-1 does not occur in many species.
The role of meiosis or pachytin in the development of organisms
1. New arrangement of genes: Exchange of segments between two non-sister chromatids occurs at the pachytene stage. As a result, new arrangements of genes occur in the chromosomes.
2. Genetic Variation: At this stage, genetic variation is created in the organism. New varieties can be created in crop plants.
3. Genetic modification: Genetic modification can be done by artificial crossing over. For this reason it plays an important role in reproductive science.
4. Creation of Biodiversity: Biodiversity is the characteristic and characteristic difference between one organism and another organism. Differences between two plants or animals of the same species belong to biodiversity. Biodiversity occurs due to changes in the position and arrangement of genes.
5. Survival in new environments: Characteristic changes occur in organisms during the pachytene phase. In this the organism acquires the ability to survive in the new environment and survive.
6. Genetic research: Genetic research has created a lot of excitement worldwide. This is an interesting topic for theoretical research.
Genetic significance of zygote state
1. At the zygotene stage, attraction between homologous chromosomes begins. Homologous chromosome pairs come close to each other and pair up.
2. The process of pairing of chromosomes is called synapsis and a pair of chromosomes is called bivalent.
3. A structural change occurs as two chromosomes join in bivalents.
4. At a later stage, the bivalent chromosomes divide to form chromatids.
5. Crossing over occurs between non-sister chromatids. Zygotes are required for crossover to occur.
6. Crossing over results in biodiversity. The role of zygotes in creating biodiversity in the living world is undeniable.
Role of meiosis or pachytene in the evolution of organisms
Homologous chromosomes pair up at the pachytene stage. The process of pairing of chromosomes is called synapsis and each pair of chromosomes is bivalent. In bivalents, each chromosome divides longitudinally without the centromere to form two chromatids. As a result, four chromatids are formed in each bivalent. Two chromatids of the same chromosome are called sister chromatids and two chromatids of different chromosomes are called non-sister chromatids. Two non-sister chromatids in bivalents come close together and form an X-shaped zygoma. At the zygoma segment, certain segments of two non-sister chromatids break apart and segments exchange. This is called crossing over. Crossing over causes new arrangements of genes on chromosomes. New arrangement of genes in chromosomes results in various changes in organisms. As a result, evolution takes place in the living world.
Role of meiosis in generation or heredity
The role of meiosis in the heredity of organisms and the generation of new offspring is undeniable. The process of meiosis produces sperm from male reproductive mother cell and egg from female reproductive mother cell. Sperm and egg are haploid germ cells or gametes. The chromosome number of the gamete is half of the chromosome number of the mother cell. A zygote is formed by the union of sperm and egg. Zygote is a diploid cell. The zygote divides repeatedly to form a multicellular embryo. Multicellular embryos develop into full-fledged organisms. So the role of meiosis is important in the generation of new offspring or in heredity.
Homologous chromosomes pair up in the process of meiosis. The process of pairing of chromosomes is called synapsis and each pair of chromosomes is bivalent. Each chromosome in a bivalent divides longitudinally without the centromere to form two chromatids. As a result, four chromatids are formed in each bivalent and this condition is called tetrad. Two chromatids of the same chromosome are called sister chromatids and two chromatids of different chromosomes are called non-sister chromatids. Two non-sister chromatids approach each other and form an X-shaped zygoma. Two non-sister chromatids break at the chiasma segment and exchange segments. This is called crossing over. Crossing over results in structural changes in chromosomes. New arrangements of genes occur. Different types of changes occur in living organisms. New features appear. As a result, new offspring are created. So the role of meiosis in heredity is undeniable.
Interkinasis
Interkinesis is the time between two consecutive cell divisions. The period between meiosis-1 and meiosis-2 is interkinesis. The two nuclei that are formed in meiosis-1 spend the intermediate time to start meiosis-2. During this time some proteins and RNA are synthesized. In animal cells centrioles re-division in pairs.
Meiosis-2 cell division
In the process of meiosis-2, four nuclei are formed from two nuclei. In this process the number of chromosomes is equal. Hence it is called a division analogous to mitosis. It is divided into four states. Prophase-II, Metaphase-II, Anaphase-II and Telophase-II.
1. Prophase-2: Dehydration of water occurs in the chromosomes in Prophase-2. Chromosomes increase in dye capacity. Chromosomes become shorter and thicker. At the end of this stage, the nuclear envelope and nucleolus disappear.
2. Metaphase-2: In metaphase-2, the chromosomes become thicker and shorter. Spindle machine is created. Chromosomes are located at the equator and attached to traction fibers. At the end of this phase, the centromeres of the chromosomes divide.
3. Anaphase-2: In anaphase-2, the centromeres of the chromosomes are completely separated. Polar movement of chromatids occurs due to contraction of traction fibers and elongation of stem bodies. During polar movement, the centromere is the leader and the armature is the follower. When the chromosomes reach the poles, they look like the letters V, L, J or I.
4. Telophase-2: Telophase-2 is the last stage of meiosis-2. At this stage the chromosomes are fixed at opposite poles. Chromosome water hydration occurs. Chromosomes decrease in capacity to hold dyes. Chromosomes are narrow, long and fuzzy. The nuclear membrane and nucleolus appear. As a result, two nuclei are formed at both ends.
Importance of Meiosis Cell Division
1. Germ cell formation: The process of meiosis produces sperm from male reproductive mother cell and ovum from female reproductive mother cell. So meiosis is necessary in sexually fertile organisms.
2. Embryo formation: The process of meiosis produces sperm and egg. A zygote is formed by the union of sperm and egg. The zygote divides repeatedly to form a multicellular embryo. A new organism is created from the embryo.
3. Keeping Chromosome Number Constant: Meiosis produces haploid sperm and egg. A diploid zygote is formed by the union of haploid sperm and egg. As a result, the number of chromosomes remains constant in living organisms.
4. Maintaining the identity of the species: The identity of each species is maintained by keeping the chromosome number correct. Hence all members of a particular species are physically most similar.
5. Continuity of life: Continuity of life is maintained by meiosis cell division.
6. Variation: Crossing over occurs through meiosis. Hence biodiversity is created. Biodiversity is considered as a regulator of evolution.
7. Genetic differentiation: Genetic differentiation occurs in gametes through meiosis. Hence new characteristics are created in the organism and variation can be observed in the biosphere.
8. Determining the relationship between organisms: Through meiosis, the mutual relationship between organisms can be determined.
9. Variation Creation: New arrangements of genes occur during crossing over on homologous chromosomes. As a result, new variations are created in the next generation.
10. Generation: The periodic appearance of gametophytic stage and sporophytic stage in the life cycle is called generation. Meiosis plays a direct role in plant genetics.
11. Mendelism: Gregor Johann Mendel’s theory of heredity is called Mendelism. Mendelism cannot be explained without meiosis.
12. Expression: Meiosis brings about biodiversity. And biodiversity brings the ebb and flow of expression.
Role of Meiosis or Pachytin in Biodiversity
The attraction between homologous chromosomes occurs in the pachytene phase. Homologous chromosomes come close to each other and pair along their length. The process of pairing of chromosomes is called synapsis and each pair of chromosomes is bivalent. In bivalents, each chromosome divides longitudinally without the centromere to form two chromatids. As a result, four chromatids are formed in each bivalent. This condition is called tetrad. Two chromatids of the same chromosome are called sister chromatids and two chromatids of different chromosomes are called non-sister chromatids. Two non-sister chromatids in bivalents come close together and form an X-shaped zygoma. At the chiasma segment, certain parts of the two non-sister chromatids are broken by the action of endonuclease enzymes. A broken chromatid is then joined to another non-sister chromatid with the help of ligase enzyme. In this way, parts are exchanged between two non-sister chromatids and is called crossing over. Crossing over results in new arrangements of genes on chromosomes. The position and arrangement of the genes changes. Genetic variation occurs in organisms. Hereditary changes occur in organisms. New characteristics emerge in the created organisms. As a result, genetic or heredity diversity is created.
The role of meiosis in species identity
Chromosome number remains constant in organisms through meiosis. Male body produces haploid sperm and female body produces haploid egg. A diploid zygote is formed by the union of sperm and egg through sexual reproduction. The zygote divides repeatedly to produce progeny of its own species. As a result, the identity of the species is preserved.
The role of meiosis in the transmission of parental traits to offspring
1. Male genitalia contains reproductive cells. Germ cells develop into primary spermatocytes. Each primary spermatocyte divides by the process of meiosis to form secondary spermatocytes. Secondary spermatocytes divide by the process of meiosis to form spermatids. Spermatids develop into spermatozoa. Human sperm nucleus contains 23 chromosomes. Sperm are haploid cells and carry paternal characteristics.
2. Female genitalia contain reproductive cells. Germ cells develop into primary oocytes. Each primary oocyte divides by the process of meiosis to form secondary oocytes. Secondary oocytes divide by meiosis to form oocytes. Each oocyte develops into an ovule. Human egg nucleus contains 23 chromosomes. Ovum is a haploid cell and carries the characteristics of the mother.
3. Fertilization involves the union of sperm and egg. After fertilization, the nucleus of the sperm and egg swells to form the pronucleus. The pronuclei fuse to form the zygote. A mixture of parental characteristics occurs in the zygote. It is through the zygote that the characteristics of the parents are transferred to the offspring.
4. The zygote divides repeatedly to form an embryo. A new generation is created from the embryo. In the new generation there is an assemblage of the characteristics of the parents. Thus the characteristics of the parents are transferred to the offspring.