MEIOSIS: Phases and Characteristics

Meiosis is a very special type of cell division that allows us to generate eggs and spermatozoa, responsible for the sexual reproduction of mammals, among which we find the humans. This type of cell division is a process that, although it is very fast, is very complex and to study it we divide it into phases or stages. In addition, it is a very special and very important process for the production of gametes, with properties and characteristics that clearly differentiate it from mitosis, the type of cell division suffered by the rest of our cells (cells somatic).

In this lesson from a TEACHER we will explain, step by step, the phases of meiosis and their main characteristics.

Index

1. What is meiosis?
2. Characteristics of meiosis
3. Phases of meiosis
4. Meiosis I: phases
5. Meiosis II: phases

Meiosis is a form of cell reproduction that, in mammals, is carried out solely and exclusively throughout the production or generation of the eggs and sperm. In this process, a diploid cell, that is, with two copies of each of its chromosomes, undergoes two meiotic divisions successive until generating four haploid cells (4 sperm in the case of the male and 1 ovum and 3 polar bodies in the case of the women). This process is different from the normal cycle of division through mitosis and is very complex, which is why it is divided and subdivided into phases or stages. Next we will see first, the

Learn more about this process and its great relevance in this other lesson on What is the importance of meiosis?

The main characteristics of meiosis They are:

• Is a reductional division, that is, the number of chromosomes of the daughter cells is less than that of the mother cell. In meiosis, you start from a diploid cell with two double chromosomes and the resulting cell has only one single chromosome.
• It is a division that only occurs in sex cells, that is, during the formation of gametes (gametogenesis)
• Meiosis allows to generate genetic diversity. During meiosis there is an exchange of genetic material (genetic recombination) that leads to the formation of daughter cells different from each other and from the starting cell (cell mother). In the case of mitosis, the cells produced are copies of both the stem cell and each other. If you are interested in learning more about this topic, we recommend this video lesson on What is genetics.

As we have already mentioned, meiosis is the type of cell division that takes place during the formation of gametes (gametogenesis). During spermatogenesis and oogenesis, two cycles of meiosis complete and followed (consecutive).

These cycles are not completely identical, although they are very similar for what they are called meiosis I and meiosis II, respectively. Both cycles consist of the same stages or phases, but in each one slightly different processes occur that we will see below.

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Prior to meiosis I, a duplication of genetic material of the starting cell. This cell, which was diploid, contained two chromosomes: paternal chromosome and maternal chromosome. With its duplication, this cell happens to have two copies (homologous chromsomes) of each of the parents.

Therefore, the meiosis I It begins with a diploid cell, with two sets of double chromosomes. These chromosomes have just been copied so they are displayed in the nucleus.

• Prophase I: During prophase I, the chromosomes condense and unite with each other so that the exchange of genetic material between homologues takes place (meiotic recombination). In addition, the nuclear membrane disappears and the meiotic spindle appears (the structures that are responsible for the movement of chromosomes) that they move to the ends or poles of cells, allowing homologous chromosomes to join and exchange genetic material.
• Metaphase I: During metaphase I, the chromosomes inside the cell move towards the middle of the cell and are arranged forming a line in the equatorial zone or center of the cell (equatorial or metaphase plate) where the cell will divide in two.
• Anaphase I: The homologous chromosomes, which were still attached to each other, separate and the microtubules drag them to each of the poles of the cell.
• Telophase I: Homologous chromosomes each reach one of the cell's poles and the nuclear membrane re-forms around each set of chromosomes. This is followed by cytokinesis, that is, the division of the cytoplasm, organelles and the cell membrane to form the two complete daughter cells.

At the end of the 4 stages of meiosis I, we have two diploid daughter cells. In the case of spermatogenesis, these two cells are primary spermatocytes while in the case of oogenesis it is a primary oocyte and a polar body.

After the first meiosis is completed, the cells undergo a very short interface, which is followed by the second meiosis. Unlike what happened in meiosis I, DNA duplication does not occur at the beginning of meiosis II, but the stages of division are the same. Thanks to this, we will be able to start from two diploid cells, with a set of homologues each, and generate four haploid cells (two haploid cells from each of them).

The phases of meiosis II They are:

• Prophase IIDuring this phase, the nuclear membrane disappears and the meiotic spindle is formed, which is responsible for the movement of chromosomes in the dividing nucleus.
• Metaphase II: The chromosomes are placed in the center of the cell, forming the equatorial or meiotic plate of the metaphase. These chromosomes are attached to the meiotic spindle.
• Anaphase II: The points that connect each of the arms of the chromosomes, the centromeres, separate and the chromatids separate. When separated, the chromatics form individual chromosomes, which move towards the poles opposites of the cell.
• Telophase II: A nuclear membrane re-forms around each chromosome. Finally, cytokinesis occurs (the cytoplasm divides, organelles separate, and the plasma membrane divides).

As a result of meiosis II we obtain four daughter cells, each with a haploid set of chromosomes. In the case of spermatogenesis, these four cells will be spermatozoa while in oogenesis, they will be an ovum and three polar bodies, which will be discarded.

Now that you have a better understanding of the process, phases, and characteristics of meiosis, you may also be interested in this other lesson on the Types of cell reproduction.

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