Meiosis Division: Definition, Process Stages and Differences with Mitosis

Meiosis Division: Definition, Process Stages and Differences with Mitosis – What is meiotic division? On this occasion, we will discuss it further to understand it better. Let's see together.

Meiosis Division: Definition, Process Stages and Differences with Mitosis

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To better understand what meiosis is, we must first know what is the process of cell division in living things. Cell division is a process that occurs in cells, which divide themselves into two or more. With certain steps and results. Generally aims to duplicate themselves.

There are two types of cell division, direct (amitosis) and indirect division. Direct division occurs in single-celled organisms, for example Amoeba sp. Meanwhile, direct division occurs in multi-celled organisms, such as humans. Indirect division is divided into two, mitosis and meiosis.

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Characteristics of Meiosis

Below are some of the characteristics of meiosis, including:

• Occurs in sex cells
• There are 4 daughter cells
• The chromosomes are 1/2 the size of their parent
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• Cleavage occurs ie 2 times
• With a longer period of time when compared to mitotic division and a more complex process.

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Purpose of Meiosis Division

The goals of cell division by meiosis include:

• In order to produce gametes
• Halve the number of chromosomes
• Increases genetic variability in gametes

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Stages of the Cleavage Process in the Cell Cycle

Mitotic division is division indirectly / through the stages that occur in the multiplication of gamete cells. During one of the phases of meiosis the number of chromosomes will be halved, to be precise Anaphase I. So that the daughter cells will get only half the number of chromosomes as the parent cell.

Meiosis is part of the cell cycle. The cell cycle is the events that take place in a certain sequence starting from cell formation to cell division in the formation of daughter cells. The cell cycle includes two phases namely interphase and meiosis. In some literature, meiosis is said to not have a cell cycle.

Interphase

Interphase is carried out by cells to collect energy to be used in the process of cell division. In this phase, various kinds of synthesis processes of various organic materials occur. The phases contained in the interphase are:

• G1 phase (first growth phase): the cell experiences growth (cell organelles duplicate themselves in preparation for division).
• Synthesis phase (S): the cell replicates/handles the DNA molecule and the synthesis stage of materials such as cell DNA and protein for cell division.
• G2 phase (second growth phase): does not occur in the process of meiosis.

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The Process of Meiosis

Two divisions occur, namely meiosis I and meiosis II.

Meiosis 1 phase

Prophase I

During this phase, the nuclear membrane begins to disappear. Nucleoli also begin to disappear. Chromosomes can be seen. As chromatin threads absorb color and thicken, shorten. The centrosome divides and heads to each pole. Simultaneously, the spindle threads also begin to leave the centrosome towards the opposite pole.

Each chromosome consists of two chromatids which contain identical genetic material. During the process of meiosis (even after this process ends) these two chromatids never separate.

During prophase I, DNA exchange occurs between homologous chromosomes which is called homologous recombination. From this exchange process it is hoped that a new combination of chromosomes will occur. Prophase I consists of several stages, leptotene, Zygotene, Pachytene, Diplotene, Diakinesis, and synchronization processes.

At the time of leptonema, the chromosomes condense to form threads (leptonema means "thin thread"). During the zygonema substage, homologous chromatids form synapses or pair up with each other. Synapses are found in several places on homologous chromosomes. This is an important genetic event because it mediates the exchange of information called crossing over or recombination between homologous chromosomes.

In the pakinema stage, thick and short threads form chromatids. At this stage, homologous chromosomes are composed of 4 chromatids, which are called tetrads.

In the diplonemal stage, chromatids that are in one bivalent separate from their partner chromatids. However, in some places contact called chiasmata (singular: chiasma) occurs. In diakinesis, the chromosomes continue to shorten and condense maximally. Chromosomes are at the equator.

In the synchronization stage, the centrosome moves to each pole of the cell. While producing microtubules whose ends bind the centromere. Later these thread-shaped microtubules will be used to pull each pair of chromosomes to the cell poles.

Stages In Prophase I

The stages that occur during prophase I:

• The nucleus begins to disappear
• Romatin threads shorten and thicken into chromosomes
• Chromoson self-replicating into chromatids.
• Chromosomes duplicate until they are twice the number of the original chromosomes.
• The chromatids thicken, shorten and spread to fill the nucleus
• Homologous chromosomes form synapses.
• Homologs are composed of 4 chromatids/tetra
• The nuclear membrane begins to disappear
• Formation of spindle fibers from microtubules.
• The two pairs of centrioles separate towards the cloudy poles.

In the prophase I stage, what is called crossing over occurs. This happens because each homologous chromatid sticks together, making it possible for the homologous chromatids to exchange genes.

Metaphase I

Half of the tetrad migrates to the area between the poles and the center of the cell. Which each tetrad has a homologue in the opposite part. This section is often referred to as the "metaphase plate". The spindle attaches to the centromere of each chromosome.

Both kinetochores in the chromosome pair will move to the same pole. So that the chromosomes will stick to the spindle from the same pole. This is the big difference between mitosis and meiosis. This results in anaphase I, half of the chromosomes will move to a different part of the cell.

Metaphase I Stages
The stages that occur in Metaphase I:

• Tetrads divide at the equator, so that the halves face opposite poles.
• The divided tetrad hangs from the spindle using the centromere
• The spindle threads are increasingly visible

Anaphase I

In this phase, the cell begins to elongate. The two homologues in each pair of chromosomes separate and migrate to opposite poles, pulled by the microtubules of the spindle apparatus. This is different from mitosis, where the centromere will divide so that the chromosomes will split in two and move to each pole. But in anaphase I, this doesn't happen. So the number of chromosomes is half of the number of parents. Although the number of chromosomes is still the same as the parent.

Anaphase I stages
The stages that occur in anaphase I:

• Spindle threads pull the chromosomes toward each opposite pole.
• Each pole is half the number of chromosomes (no centromere division occurs).

Because the centromere does not divide, each pole will get a pair of chromosomes in the form of a tetrad. This is in contrast to mitotic division in which the centromere divides so that each pole gets the same chromosome.

Telophase I

At each pole, there is half the number of parental chromosomes. But each chromosome is still in the form of a tetrad. A “cleavage furrow” has been formed. And at the end of this phase, the parent cell has divided into two daughter cells. This division of the cytoplasm is called cytokinesis. In some organisms the nuclear membrane appears, this intermediate stage is called interkinesis. But in some other organisms, such as plants, the interkinesis stage is not found, and it goes straight to the next division stage (Meiosis II).

Telophase I stages
The stages that occur in telophase I:

• A nuclear membrane forms around the chromosomes at each pole and the chromosomes elongate.
• Cytokinetic process
• Division of the cytoplasm and other organelles occurs
• Obtained 2 haploid daughter cells (n)

The process of cytokinesis is not a part of the process of meiosis. Cytokinesis is the process of dividing the cytoplasm into two. This process begins with cell elongation at anaphase and ends at telophase.

After telophase, cells will experience a period of interkinesis. This is the resting period to proceed to the process of meiosis II. At this time, the nuclear membrane begins to show again. However, the chromosomes are still in the form of chromatids that are bound at the centromere, not chromatin. In some organisms, this phase does not appear to occur.

The daughter cells produced in meiosis I contain the same number of chromatids as the parents. But still called haploid because the chromosomes are homologous.

Meiosis II phase

Is the second part in the process of meiosis. This stage begins with the two daughter cells resulting from meiosis I. The stages that occur are as follows:

Chromosomes are still in the form of chromatids and paired together, but not yet bound by spindle fibers. If the interkinetic period previously occurred, the nuclear membrane will begin to melt again.

Prophase II

Prophase II stage
The stages that occur in prophase II:

• Chromosomes are still paired with the centromere
• Chromosomes do not duplicate.
• The nuclear membrane melts again.
• Centrioles (in animal cells) divide, separating to opposite poles.
• Spindle threads begin to form.

Metaphase II

The spindle begins to pull the chromosomes toward the poles. Unlike metaphase I, the kinetochores from each centromere are engaged by spindles from opposite poles.

Stages of Metaphase II
The stages that occur in metaphase II:

• Chromatids are located at the equator, attached to the spindle via the centromere.
• The centromere divides so that the chromosomes become two.
• Spindle threads appear more clearly
• Spindle threads begin to bind chromosomes at the centromere.

Anaphase II

During anaphase II the chromatids of each chromosome divide in half and move to opposite poles. When the chromosomes are pulled by the spindle, we can see the V shape of the chromosome where the arms of the chromosome are behind. The poles will be further away so that the cell becomes longer.

Anaphase II stage
The stages that occur in anaphase II:

• Chromatids that have divided into two are brought to each pole of the cell.
• This happens because of the spindle threads that pull each part of the chromosome to the poles of each.
• So that each pole gets the same share as the parent chromosome.

Telophase II

During telophase II, the chromosomes reach opposite poles. Cytokinesis occurs, so that the two cells resulting from meriosis I will become four cells. The nuclear membrane begins to form. Now the process of meiosis has been completed.

Telophase II stage
The stages that occur in telophase II:

• The chromosomal threads are already in their respective polar regions.
• Chromosomes begin to thin out, then turn into thin chromatin threads.
• The nuclear membrane begins to form.
• Nucleoli start to reappear.
• In the equatorial plane there is a thickening of the plasma.
• The cell is formed into four haploid (n) daughter cells.

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Difference between Mitosis and Meiosis

This will happen in multi-cellular organisms when "eukaryotic cells separate chromosomes in the nucleus, cell membrane, cytoplasm and organelles, which then assume the characteristics of the same parent cell (identical)”.

Meiosis is cell division that can produce daughter cells with up to half the chromosomes of the parent cell. This division occurs in the gametocytes (the two sex cells). Chromosomal division is not interspersed with interphase and occurs twice in succession.

Meiosis first occurs through the prophase cycle which then continues through the first metaphase, anaphase, and telophase. The stages that occur in the first prophase are leptone and chromatin. At this stage the chromosomes will be formed. This stage also occurs in the second meiotic cycle. Apart from plants, the difference between mitosis and meiosis also occurs in animals. This difference is commonly called the event of meiosis which is the process that occurs in the gametes.

Mitosis has the goal of multiplying cells that are used for the growth process, while Meiosis has the goal of forming gamete cells. Meristematic tissue is the site of mitosis. This tissue is found in the stems and tips of plant roots while the place where Meiosis occurs is in the sex organs (male and female).

The process of the stages of Mitosis: "Prophase- Metaphase.- Anaphase and Telophase interspersed by Interphase", while stages of Meiosis: “Prophase I- Metaphase I- Anaphase I- Telophase I- Prophase II- Metaphase II- Anaphase II- Telophase II without Interphase. In the first mitosis there is one division by the parent cell, while in the first Meiosis there are two divisions.

In the first mitosis, two daughter cells are produced by the parent cell which has the same number of chromosomes as the parent cell. Whereas in the first Meiosis, the mother cell produces four daughter cells whose chromosomes are only half the number of the parent. The daughter cells in Mitosis are diploid or 2n, while the daughter cells in Meiosis are haploid or n.

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