Category: Biology Second Paper

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.

1. 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.
2. 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] 3. 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
4. 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
5. 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.

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.

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.

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.

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 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.

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.

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.

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 h number of chromosomes. Telophase-1 does not occur in many species. Siddique Publications