Meiosis is a cellular process in which the first cell of a diploid cell divides into haploid cells, commonly called gametes.
Meiosis, which results in a reduction in the number of chromosomes in the gamete cell, comes from the Greek word Meioun, which means “to make it smaller”. It has nothing to do with mitosis.
Meiosis is necessary for sexual reproduction and therefore occurs in all eukaryotic cells, including sexually reproducing single-celled organisms. Meiosis does not occur in archaea or prokaryotes that reproduce by asexual mitotic division. Increases genetic diversity. It increases the chance of emergence of individuals who are adaptable to changing environmental conditions. Meiosis does not occur in a row. A cell can only undergo meiosis once.
There are four phases in meiosis as in mitosis, namely prophase, metaphase, anaphase and telophase. These stages occur twice in succession without interphase, resulting in 2 types of four daughter cells in terms of genetic characteristics. The biggest difference between meiosis and mitosis is found in prophase. Meiosis takes place in two stages. These stages are called Meiosis-1 and Meiosis-2.
Key features of Meiosis:
1-It is seen only in cells with 2n chromosomes.
From cells with 2-2n chromosomes, 4 cells with n chromosomes are formed.
3-Two karyokinesis and two cytokinesis are seen.
4-It is seen in germ cells and spore mother cells.
5-At the end of the division, gametes and spores are formed.
6-It is the cause of hereditary diversity. The cells formed are genetically different from each other.
7- It is the basic mechanism of sexual reproduction.
8- It is a more advanced feature than mitosis.
9-Separation occurs in both homologous chromosomes and sister chromatids.
10-Synapse, tetrad and crossing-over are seen. 11-The cell that has undergone meiosis cannot undergo meiosis again, but it can undergo mitosis.
Meiosis 1
Metaphase 1
Homologous chromosomes forming tetrads line up mutually on the equatorial plane of the cell.
Homologous chromosome pairs are randomly aligned on the equatorial plane of the cell.
The number of ways homologous chromosome pairs can line up in the equatorial plane of the cell is expressed as 2n. ( n haploid cell count)
Anaphase 1
Homologous chromosomes separate from each other and move to opposite poles. While meiosis is occurring, crossing-over may not occur. Therefore, crossing-over (gene transfer) is not the root cause of diversity. Since homologous chromosomes that adhere to each other in Prophase 1 will be randomly separated in Anaphase 1, gene diversity is ensured in this stage as well.
Telophase 1
Chromosomes at the two poles of the cell begin to elongate and thin. Nuclear membrane is formed around them. Two daughter cells with haploid number of chromosomes are formed by the nodding of the cytoplasm.
In animal cells: After the nuclear division is complete, the cytoplasm is divided into two by intertwining in the middle and two separate cells are formed.
In plant cells: A middle lamella (intermediate lamella) is formed in the middle of the formed two-nucleated cell and reaches up to the cell wall. Therefore, two adjacent cells are formed. Chromosomes at the two poles of the cell begin to elongate and thin. Nuclear membrane is formed around them. Two daughter cells with haploid number of chromosomes are formed by the nodding of the cytoplasm.
Meiosis 2
Prophase 2
The nuclear membrane ruptures without a resting cycle between the telophase of the first division and the second division. New spindle fibers are formed perpendicular to the direction of the first spindle fibers.
Metaphase 2
The haploid (n) chromosome of each daughter cell is aligned on the equatorial plane, there is no crossover.
Anaphase 2
It is the 3rd stage of Meiosis 2, the 2nd part of meiosis. It is also called the intermediate stage. This phase is similar to the anaphase in mitosis. However, in the anaphase of mitosis, sister chromatids are arranged regularly, but in this phase they are irregular. This ensures genetic diversity in meiosis.
Telophase 2
The helices of chromosomes unfold, so they become invisible. Nuclear membranes form and the cytoplasm divides. Thus, 4 haploid cells are formed from one cell.
Cytoplasm division
In animal cells: After the nuclear division is complete, the cytoplasm is divided into two by intertwining in the middle and two separate cells are formed.
In plant cells: A middle lamella (intermediate lamella) is formed in the middle of the formed two-nucleated cell and reaches up to the cell wall. Therefore, two adjacent cells are formed.
As a result of meiosis, 4 cells (gametes) with n chromosomes are formed.