Meiosis : Stages, purpose, function,

The characteristics of Meiosis - understanding, purpose, function, stages


Introduction to Meiosis 

Meiosis is the way of eukaryotic cells to (plants, animals and fungi) reproduce by sexually. Meiosis is a process of reducing chromosomes, which means that diploid cells (this means cells with two sets of complete and identical chromosomes) are reduced to haploid cells (these are cells with only one set of chromosomes).

The term meiosis was coined  by J. B. Farmer in 1905. It takes place only in reproductive cells during the formation
of gametes. 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 take place are termed as meiocytes. Meiosis produces four haploid daughter cells  from a diploid parent cell. Meiosis is of two
subtypes :
  1. First meiotic division or Heterotypic  division – (Meiosis I)
  2. 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 seed cells combine to form diploid zygotes, which grow to form other functional adults of the same species.

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. Most plant and animal cells are diploid. The term diploid comes from the Greek Diplos, which means "double" or "two"; this term implies that plant and animal cells have two sets of chromosomes.

Read : Animal cell Structure

In human cells, for example, 46 chromosomes are arranged in 23 pairs. Therefore, human cells are diploid i.e they have two sets of 23 chromosomes per set. During generative reproduction, the sex cells of the parent organism unite with one another and form a fertilized egg. In this situation, each sex cell is gamete. Gametes from haploid human cells, from Greek haplos, which means "single."

This term implies that each gamete contains a set of 23 chromosomes in humans. When human gametes unite with each other, the original diploid condition of 46 chromosomes is rebuilt. Mitosis then brings the development of diploid cells into organisms. In meiosis, cells containing a diploid number of chromosomes are converted into four cells, each of which has a number of haploid chromosomes. In human cells, for example, reproductive cells containing 46 chromosomes produce four cells, each with 23 chromosomes.

Read : Cell Structure

Conditions Before Meiosis

Before meiosis, chromosomes in the cell nucleus replicate to produce double the amount of chromosomal material. After chromosome replication, the chromosomes separate into sister chromatids. This is known as interphase, and can then be broken down into two phases in the meiotic cycle: Growth (G), and Synthesis (S). During the G phase the proteins and enzymes needed for growth are synthesized, whereas during the S phase the chromosome material multiplies.

Meiosis is then divided into two phases: meiosis I and meiosis II. In each of these phases, there are prophases, metaphases, and anaphases and telophases. In meiosis I this is known as prophase I, metaphase I, anaphase I and telophase I, whereas 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. Also, meiosis I is p>receded by G and S phases, whereas meiosis II is only preceded by S phase: chromosome replication is no longer needed.

The characteristics of Meiosis

  1. Meiosis Only occurs in gonad cells at the time of gamete formation.
  2. There are two stages of division, meiosis I and meiosis II.
  3. 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.
  4. Cross migration occurs in pairs between homologous chromosomes.
  5. Occurs in the gonad cells of the body.
  6. Stages of meiosis: "prophase I - metaphase I - anaphase I - telophase I - prophase II - metaphase II - anaphase II - telophase II without interphase".
  7. With the aim of maintaining a diploma.
  8. The daughter cell type is haploid or n.
  9. Cells produced by the process of meiosis have half the number of chromosomes from the original cell.
  10. Temporal cell division quickly.
  11. The number of chromosomes is half that of the original nucleus.
  12. The end result of dividing cells is four new cells that have half the number of chromosomes from stem cells.

Read : Somatic Cell

Purpose of Meiosis

Both mitotic division and meiosis are found in many animal, plant and fungal cells. 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. Mitosis produces the same cells as their parents. This is the process of children healing adult cells, injured or growing skin cells or growing lizard tails.

Meiosis has more specific subdivisions. This department has half the number of parent chromosomes. Only one 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).

Read : 
  1. Karyokinesis
  2. Cytokinesis

Stages of Meiosis :

Meiosis occurs with a series of steps that resemble the steps of mitosis. Two main phases of meiosis occur: meiosis I and meiosis II. During meiosis I, one cell divides into two. During meiosis II, the two cells each divide again. Stages of demarcation with mitosis occur in meiosis I and meiosis II.

first, the chromosomes of cells as duplicates and proceed into two cells. The chromosomes of the two cells then separate and proceed into four daughter cells. Stem cells have two sets of chromosomes and diploids, while daughter cells have a set of chromosomes each and a haploid. Synapses and cross moves occur in the Professional Phase I.

The members of each pair of chromosomes in a cell are called homologous chromosomes. Homologous chromosomes are similar but not identical. They can carry different versions of the same genetic information. For example, one homologous chromosome can carry information for blond hair while another homologous chromosome might carry information for black hair.

When cells prepare to enter meiosis, each chromosome has been duplicated, as in mitosis. Each chromosome thus consists of two chromatids. Meiosis stages as follows:

Meiosis I

At the beginning of meiosis 1, human cells contain 46 chromosomes, or 92 chromatids (the same amount as during mitosis). Meiosis I takes place through several stages as follows:

Prophase I: Prophase I is similar in several ways to prophase in mitosis. Chromatids shorten and thicken and become visible under a microscope. An important difference, is that a process called synapses occurs. The second process, called crossing over, also takes place during prophase 1.

During prophase 1, two homologous chromosomes come close to each other. Because each homologous chromosome consists of two chromatids, there are actually four chromatids parallel to each other. This combination of four chromatids is called tetrad, and coming together is called the synapses process.

After synapses have taken place, the process of crossing occurs. In this process, DNA segments from one chromatid in a tetrad pass to another chromatite in the tetrad. chromosome segment exchange This takes place in a complex and poorly understood way. They produce new genetic chromatids. Cross-movement is an important driver of evolution. After a cross move has taken place, the four chromatids of the tetrad are genetically different from the original four chromatids.

This phase can be sub-divided into five sub-stages as Leptotene, Zygotene, Pachytene, Diplotene and Diakinesis.

Leptotene : The volume of 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 characterised with the ends of chromosomes converged towards that side
of the nucleus where the centrosome lies.
The centriole divides into two and migrate to opposite poles.

Zygotene : Intimate pairing of non-sister
chromatids of homologous chromosomes
takes place by formation of 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 individual chromosome
begins to split longitudinally into two similar chromatids. At this stage, tetrads become more clear in appearance because of 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 together 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 as crossing-over or recombination.

Diplotene : Though chiasmata are formed
in pachytene, they become clearly visible in diplotene due to the beginning of repulsion
between synapsed homologous chromosomes. This is called desynapsis. It involves disappearence of synaptonemal complex.

Diakinesis : In this phase, the chiasmata beings to move along the length of chromosomes from the centromere towards the ends of chromosomes. The displacement of chiasmata is termed as terminalization. The terminal
chiasmata exist till the metaphase.


Metaphase I: In metaphase I meiosis, tetrad aligns to the Equatorial plate (as in mitosis). The centromere attaches to the spindle fibers, which extend from the cell pole. One centromere attaches to each spindle fiber.

Anaphase I: In anaphase 1, 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. Basically, the number of cell chromosomes is halved. For this reason the process is reduction-division.

Telophase I: 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. Each daughter cell (with 23 chromosomes each consisting of two chromatids) then enters interphase, where there is no DNA duplication. The interphase period may be brief or very long, depending on the organism species.

Read : DNA Replication

Meiosis II

Meiosis II is the second major subdivision of meiosis. This happens basically 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:

Meiosis : Stages, purpose, function,


Prophase II: 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.

Metaphase II: 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.

Anaphase II: 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.

Telophase 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, there are two cells that 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 has 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.


Functions of Meiosis 

  1. The process of meiosis is important in conserving the number of chromosomes in a species. 
  2. 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. 
  3. This number will continue to increase every generation. This is why chromosome reduction is very important for the continuation of each species.
  4. Meiosis occurs in two different phases: meiosis I and meiosis II. 
  5. 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.

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 is different 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 preterm births.


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, you 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 cells. 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 5 differences between mitosis and meiosis?
Answer: Two daughter cells arise after mitosis and cytoplasmic division, while four daughter cells arise after meiosis. Daughter cells resulting from mitosis are diploid, whereas cells arising from meiosis are haploid.