Category: Biology Second Paper

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.

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.

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.

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.

Definition of 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.

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.

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.

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.

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.

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.