Meiosis. How is it going, what is its meaning?

Meiosis is a type of cell division that occurs when gametes (eggs and sperm) are formed. Its goal is to reduce the number of chromosomes by half. It is necessary for proper fertilization to take place - if it did not happen, the zygote, and then the young organism would have twice as many chromosomes. How does meiosis work? What is its meaning?

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1. What is meiosis?

Meiosis is a division of the cell nucleus (karyokinesis) that changes the number of chromosomes in daughter cells. This is a reductive division as it reduces the number of chromosomes in cells by half. Meiosis is essential in the fertilization process.

The meiotic division leads to the formation of gametes in animals, and in plants - meiotics. Reducing the number of chromos makes it possible to maintain a constant number of them in the next generations. As a result of meiosis, four haploid cells are formed from one diploid cell.

During meiosis, recombination of genes occurs through crossing-over and random segregation of chromosomes. The meiotic division consists of two consecutive divisions, each with four phases, each with names identical to those in the case of mitosis, i.e .:

  • prophase;
  • metaphase;
  • anaphase;
  • telophase.

Before meiosis, there is an S phase (or DNA replication), where the number of DNA molecules doubles and the number of chromosomes in a cell does not change.

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2. How does meiosis work?

Meiosis takes place in two stages:

  • the first meiotic (reduction) division;
  • the second meiotic division.

3. First meiotic division

In the I meiotic division, the chromosomes of each homologous pair separate and go to two different nuclei. This division consists of the above-mentioned four phases.

3.1. Profase I

  • chromatin condenses into chromosomes;
  • nuclear envelope and nucleoli atrophy;
  • a dividing spindle is formed;
  • homologous chromosomes, arranging in pairs, form bivalents (a pair of interconnected homologous chromosomes, otherwise this phenomenon is called the chromatid tetrad);
  • a crossing-over may occur between homologous chromosomes that make up bivalent;
  • finally, homologous chromosomes in bivalents separate;
  • the places where they stay connected are the chiasmas.

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3.2. Metaphase I.

  • bivalents of homologous chromosomes are arranged in the equatorial plane;
  • the spindle fibers join the centromeres of the chromosomes;
  • bivalents connect to the poles of the cell;
  • chiasmas break in bivalents, bivalents break into chromosomes.

3.3. Anaphase I.

  • the spindle fibers of the spindle pull the chromosomes to the opposite poles of the cell;
  • the chromosomes propagate to these poles randomly.

3.4. Telophase I.

  • there is a decondensation of chromosomes into disordered chromatin fibrils;
  • the nucleolus and the nuclear envelope formed around the nuclei are recreated;
  • cytokinesis takes place - the division of the plasma.

3.5. The effect of the first meiotic division

  • it is a reduction division;
  • as a result, one diploid cell produces two different cells with a reduced number of chromosomes.

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4. The second meiotic division

During meiotic II split, the sister chromatids that make up each chromosome behave similarly - they separate and pass into two different nuclei.

4.1. Profase II

  • the indexing spindle is formed;
  • condensation of chromosomes occurs;
  • the nucleolus atrophies and the nuclear envelope breaks down.

4.2. Metaphase II

  • the fibers of the dividing spindle attach to the centromeres;
  • the chromosomes align in the equatorial plane;
  • there is a rupture of centromeres.

4.3. Anaphase II

  • after division of centromeres, the chromatids diverge to opposite poles;
  • the chromatids of one chromosome are not identical as in mitosis because crossing-over took place.

4.4. Telophase II

  • the envelope around the chromatids and the nucleolus is recreating;
  • there is a decondensation of chromatids into chromatin fibrils;
  • cytokinesis takes place.

4.5. The effects of the second meiotic division

  • this phase is very short-lived;
  • four haploid cells are formed;
  • all cells are divided into daughter, haploid; genetically different from the parent cell;
  • the differences are caused by the crossing-over "em (recombination of genetic material) taking place in the first prophase.

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5. What is the meaning of meiosis?

During meiotic division, a cell with a reduced number of chromosomes is created, which restores a diploid cell in the fertilization process. The haploid cells that arise after this division have new combinations of genes.

This is because randomly selected chromosomes from among homologous chromosomes go to the daughter nuclei (this is the case in anaphase I).

In addition, during the meiotic division, there is also a random exchange of chromatids of homologous chromosomes from both parents (crossing-over), which proves genetic variability.

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