In this article, you will learn about Meiosis or reduction division, due to which sexual reproduction of living organisms is possible. In the process of meiosis, the hereditary information of a diploid cell is equally distributed among four haploid cells – gametes (in animals) or spores (in plants).
From the lesson, you will learn more about crossing over or chromosome crossing, and about cell division disorders that can lead to severe genetic diseases. Get acquainted with the syndromes associated with the loss of the X-chromosome germ cell or its acquisition.
What is Meiosis – Definition, Purpose, Function, Stages:
Definition of Meiosis
- Meiosis is the process by which the number of chromosomes is halved during the formation of gametes. In meiosis, cells containing diploid chromosomes are converted into four cells.
- Meiosis or Reduction cell division is a type of nuclear division in which the number of chromosomes is halved.
- Meiosis is a form of nuclear division in which the number of chromosomes decreases from diploid (2n) to haploid (n).
What is Meiosis:
- Somatic cells of multicellular organisms divide by indirect division, or mitosis, resulting in the formation of two identical daughter cells that carry the same genetic makeup as the mother cell.
- At the same time, both the mother cell and the newly formed daughter cells are diploid, i.e. each chromosome has a homologous chromosome with which it is paired.
- This is due to the duplication of DNA in the nucleus of the cell before mitosis.
- The mechanisms of the mitotic process or meiosis, a special method of cell division, in which the number of chromosomes in daughter cells is halved, i.e. the resulting daughter cells become haploid.
- This means that the number of chromosomes characteristic of the mother cell is halved.
- This method of cell division takes place only in reproductive cells during the formation of gametes or germ cells.
- Meiosis is the way of eukaryotic cells (plants, animals, and fungi) reproduce by sexually.
- The term meiosis was coined by J. B. Farmer in 1905. It takes place only in reproductive cells during the formation.
- By this type of division, the number of chromosomes is reduced to half, hence it is also called reductional division.
- The cells in which meiosis takes place are termed meiocytes.
- Meiosis produces four haploid daughter cells from a diploid parent cell.
- Meiosis is completed into two stages: First meiotic division or Heterotypic division – (Meiosis I), and Second meiotic division or Homotypic division (Meiosis II)
- The haploid cells produced by meiosis are germ cells, also known as gametes, sex cells, or spores in plants and fungi.
- Meiosis is important for sexual reproduction: two gamete cells combine to form diploid zygotes, which grow to form other functional adults of the same species.
The Characteristics of Meiosis
- Meiosis Only occurs in gonad cells at the time of gamete formation.
- There are two stages of division, meiosis I and meiosis II.
- There are chromosome pairs that are homologous in meiosis I, then each member of the chromosome pair moves to another pole. In meiosis II, the separation of chromatids only occurs, as in mitosis.
- Cross migration occurs in pairs between homologous chromosomes.
- Occurs in the gonad cells of the body.
- Stages of meiosis: “prophase I – metaphase I – anaphase I – telophase I – prophase II – metaphase II – anaphase II – telophase II without interphase”.
- To maintain a diploma.
- The daughter cell type is haploid or n.
- Cells produced by the process of meiosis have half the number of chromosomes from the original cell.
- Temporal cell division quickly.
- The number of chromosomes is half that of the original nucleus.
- The result of dividing cells is four new cells that have half the number of chromosomes from stem cells.
Purpose of Meiosis
- Both mitotic division and meiosis are found in many animal, plant, and fungal cells.
- The mitotic division is more common than meiosis and is found in many cleavage functions.
- Mitosis affects asexual reproduction in single-cell organisms, cell growth, and cell repair in organisms.
- The only function of meiosis is sexual division. With meiosis, children are slightly different from their parents because daughter cells are the connection of two-parent cells (one egg and one sperm, each carrying half a chromosome).
Stages of Meiosis:
- Meiosis occurs in two main stages: meiosis I and meiosis II.
- During meiosis I, one cell divides into two.
- During meiosis II, the two cells each divide again.
- In each stage of meiosis, there are prophase, metaphase, anaphase, and telophase.
- In meiosis I this is known as prophase I, metaphase I, anaphase I, and telophase I.
- In meiosis II they are known as prophase II, metaphase II, anaphase II, and telophase II.
- Different products are formed by these phases, although the basic principles of each are the same.
At the beginning of meiosis I, human cells contain 46 chromosomes, or 92 chromatids (the same amount as during mitosis). Meiosis I takes place through several stages as follows:
- During prophase I of meiosis, chromosomes consisting of two chromatids are shortened and thickened and connected by a centromere that is visible under a light microscope.
- Homologous chromosomes are tightly connected longitudinally. This process is called conjugation.
- At this point, synapses and crossing over take place during prophase I.
- During prophase, I, two homologous chromosomes come close to each other. Because each homologous chromosome consists of two chromatids, there are four chromatids parallel to each other. This combination of four chromatids is called a tetrad and here synapsis takes place.
- Synapsis is the pairing of two chromosomes that occurs during meiosis.
- After synapsis formation crossing over takes place. In this process, DNA segments from one chromatid in a tetrad pass to another chromatid in the tetrad.
- Chromosome segment exchange takes place in a complex and poorly understood way. They produce new genetic chromatids.
- Crossing over is an important driver of evolution. After a crossing over the four chromatids of the tetrad are genetically different from the original four chromatid
- At the same time, the destruction of the nuclear envelope and the nucleolus occurs, and spindle fibers are formed.
- This phase can be subdivided into five sub-stages as Leptotene, Zygotene, Pachytene, Diplotene, and Diakinesis.
- Leptotene: The volume of the nucleus increases. The chromosomes become distinct, long thread-like, and coiled. They take up a specific orientation- the ‘bouquet stage’ inside the nucleus. This is characterized by the ends of chromosomes converging towards that side of the nucleus where the centrosome lies. The centriole divides into two and migrates to opposite poles.
- Zygotene: Intimate pairing of non-sister chromatids of homologous chromosomes take place by the formation of the synaptonemal complex. This pairing is called synapsis. Each pair consists of a maternal chromosome and a paternal chromosome. Chromosomal pairs are called bivalents or tetrads.
- Pachytene: Each chromosome begins to split longitudinally into two similar chromatids. At this stage, tetrads become more clear in appearance because of the presence of four visible chromatids. The homologous chromosomes of each pair begin to separate from each other. However, they do not completely separate but remain attached at one or more points. These points appear like a cross (X) known as chiasmata. Chromatids break at these points and broken segments are exchanged between non-sister chromatids of homologous chromosomes. This is called a crossing-over or recombination.
- Diplotene: Though chiasmata are formed in pachytene, they become visible in diplotene due to the beginning of repulsion between synapsed homologous chromosomes. This is called desynapsis. It involves the disappearance of the synaptonemal complex.
- Diakinesis: In this phase, the chiasmata beings to move along the length of chromosomes from the centromere toward the ends of chromosomes. The displacement of chiasmata is termed criminalization. The terminal chiasmata exist till the metaphase.
- In metaphase I, the chromosomes line up along the equator, forming the metaphase plate.
- The centromere attaches to the spindle fibers, which extend from the cell pole. One centromere attaches to each spindle fiber.
- In anaphase I, homologous chromosomes separate.
- One homologous chromosome (consisting of two chromatids) moves to one side of the cell, while the other homologous chromosome (which consists of two chromatids) moves to the other side of the cell.
- The result is that 23 chromosomes (each consisting of two chromatids) move to one of the poles, and 23 chromosomes (each consisting of two chromatids) move to the other pole.
- The number of cell chromosomes is halved.
- For this reason, meiosis is also known as reductional cell division.
- In telophase I meiosis, the nucleus reorganizes, chromosomes become chromatin, and the division of the cytoplasm into two cells takes place.
- This process occurs differently in plant and animal cells, as in mitosis.
- During telophase I, two cells with a haploid set of chromosomes are formed.
- The interphase before the second division is very short; DNA is not synthesized in it.
Read : DNA Replication
Meiosis II is the second major subdivision of meiosis. This happens the same way as mitosis. In meiosis II, cells containing 46 chromatids undergo division into two cells, each with 23 chromosomes. Meiosis II takes place through several stages as follows:
- Prophase II is similar to prophase mitosis. the chromatin material solidifies, and each chromosome contains two chromatids attached to the centromere. 23 pairs of chromatids, a total of 46 chromatids, then moved to the Equatorial plate.
- In metaphase II of meiosis, 23 chromatid pairs gather in the middle of the cell before separation. This process is identical to the metaphase in mitosis.
- During anaphase II of meiosis, the centromere divides, and 46 chromatids become known as 46 chromosomes.
- Then 46 chromosomes separate from each other.
- Spindle fibers migrate chromosomes from each pair to one pole of the cell and other members of the pair to the other pole.
- In total, 23 chromosomes move to each pole. The forces and attachments operating in mitosis also operate in anaphase II.
- During telophase II, chromosomes gather at the poles of the cell and become unclear.
- Once again, they form a mass of chromatin.
- The nucleus envelope develops, the nucleoli reappear, and cells undergo cytokinesis as in mitosis.
During meiosis II, each cell containing 46 chromatids produces two cells, each with 23 chromosomes. Initially, two cells undergo meiosis II, so the result of meiosis II is four cells, each with 23 chromosomes. Each of the four cells is haploid, that is, each cell contains a set of chromosomes.
The 23 chromosomes in the four cells of meiosis are not identical because cross-shifting occurred in prophase 1. This cross-over produces variations so that each of the four cells resulting from meiosis is different from the other three. Thus, meiosis provides a mechanism for producing variations in chromosomes. Also, it contributes to the formation of four haploid cells from a single diploid cell.
Examples of Meiosis:
- A typical example of meiotic cell division is the maturation of human germ cells.
- Somatic human cells are diploid and contain 46 chromosomes, or 23 pairs, half of which – 23 chromosomes, got the body from the father’s sperm, the second half – another 23 chromosomes – from the mother’s egg.
- Sex cells carry half of the chromosome set characteristic of the organism, i.e. are haploid.
- The formation of a new organism occurs at the time of fertilization, when the male and female gametes merge, forming the first cell of the future organism – the zygote.
- If, as a result of meiosis, a decrease in the number of chromosomes did not occur, then with the fusion of gametes, the number of chromosomes in the zygote would increase and would not correspond to the karyotype of the given biological species.
- In the process of meiosis, homologous chromosomes fall into different germ cells, and during fertilization, they are again combined into pairs.
- Thus, the complete diploid set of chromosomes characteristic of each biological species is preserved.
- Failures that occur spontaneously in the meiotic cycle can lead to violations of chromosome divergence, as a result of which two homologous chromosomes from a pair can get into one gamete at once, or vice versa, none can get into one.
- If such a pathological gamete is involved in fertilization, then the resulting embryo is threatened with death or severe disturbances in its development.
- This phenomenon is called aneuploidy.
- Usually, a zygote in which the number of autosomes is less than normal does not develop, and zygotes with extra chromosomes sometimes develop, but this leads to the birth of an individual with severe anomalies.
- Thus, nondisjunction in meiosis of 21 pairs of autosomes leads to the birth of a child with Down syndrome, and monosomy on the X chromosome in women leads to the development of Turner syndrome, Klinefelter’s disease is characterized by the presence of at least one extra X chromosome in boys, which leads to impaired puberty in them.
Functions of Meiosis
- The process of meiosis is important in conserving the number of chromosomes in a species.
- If the number of chromosomes is not reduced, and diploid germ cells are produced by each parent, the resulting offspring will have a set of tetraploid chromosomes: that is, it will have four identical sets of chromosomes.
- This number will continue to increase every generation. This is why chromosome reduction is very important for the continuation of each species.
- Meiosis occurs in two different phases: meiosis I and meiosis II.
- There are many similarities and differences between these phases, with each phase producing different products and each phase is equally important for the production of viable seed cells.
- Meiosis occurs during the formation of germ cells in animals and spores in most plants.
- This special type of cell division ensures that the number of chromosomes is maintained in each new generation of organisms during their sexual reproduction.
- If there was no reduction (decrease) in the number of chromosomes during the formation of germ cells, then the number of chromosomes would increase from generation to generation and one of the most important features of each species would be lost – the constancy of the number of chromosomes.
- Due to crossing over, and random segregation of homologous chromosomes in the anaphase of the first division and daughter chromatids in the anaphase of the second division, opportunities are created for the emergence of new combinations of genes in gametes.
- This is one of the reasons for the hereditary variability of organisms.
- That is why sexual reproduction leads to high variability, the emergence of organisms with new hereditary properties as a result of the recombination of various genes from both parents.
Frequently Asked Questions on Meiosis
What is the process of meiosis?
Answer: Meiosis is a process in which a single cell divides twice to produce four cells that contain half the amount of genetic information. a cell during meiosis? Divides twice to form four daughter cells. These four daughter cells have only half the number of chromosomes.
For whom is meiosis done?
Answer: Meiosis, on the other hand, is used in the human body for only one purpose: the gametes – the sex cells, or the production of sperm and eggs. The goal is to make daughter cells with about half the chromosomes as the starting cells.
How many stages of meiosis?
Answer: Eight steps
Meiosis I has Prophase I, Metaphase I, Anaphase I, and Telophase I. Meiosis II includes Prophase II, Metaphase II, Anaphase II, and Telophase II. These 8 sub-stages are often called the eight stages of meiosis.
Where does meiosis occur?
Answer: Meiosis occurs in the reproductive organs of an organism. For females, meiosis occurs in the ovary, where the egg is produced.
Why meiosis is so important?
Answer: Meiosis is important because it ensures that all organisms produced through sexual reproduction have the correct number of chromosomes. Meiosis also leads to genetic variation through the process of recombination.
What is female meiosis?
Answer: The process of meiosis in females is called oogenesis, as it produces oocytes and eventually produces mature ova (eggs). The male counterpart is spermatogenesis, the production of sperm.
What different happens during meiosis?
Answer: Homologous pairs of cells are present in meiosis I and diverge into chromosomes before meiosis II. In meiosis II, these chromosomes are further separated into sister chromatids.
What are the two main functions of meiosis?
Answer: The two major functions of meiosis are to halve the DNA content and to alter the genetic material of the organism to generate genetic diversity between organisms.
How long does meiosis occur in human women?
Answer: About 74 hours: The process of complete meiosis in human males takes about 74 hours. Spermatogenesis usually begins at the age of 12–13 years and continues throughout life. Several hundred million sperm cells are produced daily by healthy young adult males. There are typically 200 and 600 million sperm cells released in each ejaculation.
How does meiosis occur in humans?
Answer: Meiosis in humans is the process by which sperm cells and ovaries are produced. In the male, meiosis occurs after puberty. The diploid cells within the testis undergo meiosis to form haploid sperm cells with 23 chromosomes. A single diploid cell produces four haploid sperm cells through meiosis.
How many chromosomes are there in meiosis?
Answer: During the process of meiosis, the number of chromosomes decreases from 46 (23 pairs) to 23. Because somatic cells have half their total chromosomes, they are called haploid (n). A human egg or sperm has 23 chromosomes, one of which is X or Y.
What is the simple explanation of meiosis?
Answer: Meiosis is a process in which a single cell divides twice to produce four cells that contain half the amount of genetic information. These cells are our sex cells – sperm in men, eggs in women.
Can haploid cells undergo meiosis?
Answer: In short, haploid cells cannot undergo meiosis because you cannot divide a cell with 23 chromosomes and this DNA, while mimicking a haploid cell, translates into a daughter cell, which is the haploid cell. With the same genome preventing genetic variation.
What would happen without meiosis?
Answer: Without meiosis, organisms will not be able to breed effectively. If organisms do not undergo mitosis, they will not be able to grow and replace the damaged cells. They are two of the most important cellular processes in existence.
At what age does meiosis occur?
Answer: In men, meiosis occurs after puberty. The diploid cells within the testis undergo meiosis to form haploid sperm cells with 23 chromosomes. A single diploid cell produces four haploid sperm cells through meiosis. In women, meiosis begins during the embryonic stage when a series of diploid cells enter meiosis.
What are the 3 differences between mitosis and meiosis?
Answer: There are three main differences between meiosis and mitosis:
- The genetic material doubles only once, but two divisions occur, resulting in four nuclei.
- Each of the four nuclei is haploid (containing half the set of chromosomes of the mother cell).
- Haploid nuclei contain new combinations of genetic material.
- Compare mitosis and meiosis. What phases do they consist of? What happens during each of these phases? Answer in the form of a table.
- What is the role of meiosis in the transmission of hereditary traits?
- What is a crossover? When does it happen? What is the importance of the life of organisms?
- Explain the terms diploid, haploid, and triploid. How are these concepts related to human genetic diseases? Give examples.
- Describe the stages of prophase 1 of meiosis. Why is prophase 1 important?