What is a meiosis: meiosis disivion

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What is meiosis cell division and definition?

What is meiosis cell division
The number of chromosomes in the progeny cells is halved due to two consecutive splits once the chromosomes are replicated. In addition, the coupling of homologous chromosomes in the first prophase and the opposite poles of homologous chromosomes in the first anaphase leads to only one chromosome of each homologous pair in each progeny cells.

You Can say in other words 'What is meiosis'

"In the type of cell division, a cell is divided into four cells, it is called meiosis."

Read about the difference between mitosis and meiosis

> ( Importance of Meiosis )

With mitosis, the number of chromosomes in gametes is half that of somatic chromosomes. As a result, in zygotes derived from fertilization (fusion, the fusion of male and female gametes) - the chromosome number again becomes equal to the somatic chromosome number.

Thus, the chromosome number of all species remains constant by generation.  Therefore, meiosis only enabled sexual reproduction in organisms (otherwise the number of chromosomes would have doubled in every generation).

 The Functions of meiosis are the following.

1. Origin of ( haploid, n) gametes, from which diploid (2n) zygotes are formed after fertilization, And the number of chromosomes in the species is constant.

2. In homologous chromosomes, pairing and subsequent separation lead to segregation of alleles and independent assortment.

3. Crossing over occurs in linked genes.

4. Genetic variation is generated by segregation, independent assortment, and crossing over.

What is meiosis stages?

The meiosis stages are divided into two parts.

1. First meiosis
2. Second Meiosis

1.First meiosis

 It consists of the separation of homologous chromosomes, dividing two cells from one cell, and halving the number of each chromosome.
 The following parts are found in it.

A. First Prophase
i) Leptotene
ii) Zygotene
iii) Pachytene
iv) Diplotene
v) Diakinesis
B. First metaphase
C. First Anaphase
D. First telophase

A. First Prophase

This stage is the longest stage of mitosis.  It is several times longer than the prophase stage of mitosis. In particular, the first prophase stage is very long in female animals.

 At the time of birth in a female human, every oocyte present in the ovary has entered the first prophase. Thus, all the eggs remain in the first prophase for many years, some eggs for many decades.

i) Leptotene

What is Meiosis
In this subphase, the volume of the nucleus increases, and the chromosomes undergo condensation, making them appear like thin threads.  All chromosomes appear to be wrapped in a loose sphere of wool.

It involves the synthesis of RNA, which leads to an increase in the size of the nucleolus.  In addition, protein synthesis also occurs. This protein is required for the condensation of chromosomes.

 RNA and protein synthesis also occurs in the zygotene state, but at significantly reduced rates.  But eggs with lampbrush chromosomes also have very high RNA and protein synthesis in the diplotene stage.

ii) Zygotene

Meiosis

  The coupling of homologous chromosomes in this subphase replicates the remaining 0.3% of DNA in the nucleus (replication does not result in replication of this phase). 

Synthesis of the nuclear protein of a particular nucleus occurs.  The origin of the synaptonemal complex and further condensation of the chromosomes is found.

iii) Pachytene

Meiosis
This phase begins when the chromosome pairing is complete. Its main events are as follows.
 ( 1 ) Chromosome condensation, making each chromosome smaller and more.
( 2 ) The number of chromosomes is reduced to half the somatic chromosome number due to chromosome coupling.
( 3 ) The nucleolus is large and connected to the nucleolus organizer region.
( 4 ) In homologous chromosomes, there is crossing over, and in this stage DNA synthesis occurring in small amounts (0.3%) is probably helpful in exchange.

iv) Diplotene

Meiosis
 At this stage, homologous chromosomes begin to diverge from each other, making two chromosomes visible in each bivalent. Hence this stage was named diplotene.
Segregating homologous chromosomes appear to be interconnected at many points, these points are called chiasma.

In the final phase of diplotene, the cyazma located between the arms of the chromosomes gradually begins to move towards the ends of the chromosomes.  This action is called chiasma terminalization.

v) Diakinesis

Meiosis
The main events of this stage are::
( 1 ) The chiasma is completely terminalized, with cyazma in each bivalent located only at the ends of the chromosomes.
( 2 ) Chromosomes undergo further condensation.
(3) Bivalent go through the whole cell.

B. First Metaphase-

What is meiosis
Just before the onset of this phase, the centripetal disappears, the nucleus is dissected and transformed into the components of the endoplasmic reticulum and becomes the organization of the logic apparatus.

The centromere of both homologous chromosomes of each pair is located on either side of the intermediate plaque.  While their ends are located on the right intermediate plaque.

C. First Anaphase

What is meiosis
At this stage, both homologous chromosomes of each bivalent are completely separated from each other, and they move to opposite poles. 

Anaphase ends after the chromosomes reach the poles.  In anaphase, both chromatids of each chromosome repel each other (Ione).

D. First Telophase

What is meiosis
 At this stage, chromosomes are uncoiled. But mitosis here, the atrophy is only partial. There is a re-organization of the Nucleus to cover the two chromosome groups. In addition, the centreline is re-organized.

( Cytokinesis) In many specimens, such as maize, the cytoplasm of each cell divides into two parts at the end of the first telophase, and each part contains a haploid nucleus.

This type of bicellular structure is called dyad.  In some other species, such as trillium, cytoplasm splits only after the second meiosis.

2. Second Meiosis

What is meiosis
In the second division, there is a simultaneous division of two haploid nuclear in each cell. 
The direction of the second division is perpendicular to the direction of the first division, but in many spaces, such as the neuropore, when the ascospore is formed, both partitions are in the same direction.
 It has a low state.

A. Second Prophase
B. Second Metaphase
C. Second Anaphase
D. Second Telophase

 A. Second Prophase

What is meiosis
This condition is very similar to the prophase of mitosis.  But it does not have relational coiling in the chromatids of each chromosome, making the chromatids distinctly different from each other. 

Chromosomes are quite condensed, small and thick.  In the first telophase, a little uncoiling of chromosomes is eliminated by re-coiling.

B.  Second Metaphase

What is meiosis
This condition resembles mitosis metaphase. In this, the nuclear envelope and nucleolus are absent, A spindle apparatus is present,  The centromere of all chromosomes is located on the equatorial plate.

Both chromatids of each chromosome look different from each other due to repulsion, But they are interconnected at the centromere, and the chromosomes are very condensed, Looks small and thick.  The duration of this phase is relatively short.

C. Second Anaphase

What is meiosis

At this stage, the chromosome of each chromosome is divided vertically into two parts, allowing the centromere of the chromatids to become fully functional.

Therefore, each chromosome has one chromatid on one pole and the other chromatid on the opposite pole, the end of this phase with the chromatid reaching opposite poles.

D. Second Telophase

What is meiosis


After chromatids reach opposite poles, they are uncoiling. Which makes them look like loose circles of a wool. The components of the endoplasmic reticulum regenerate the nuclear envelope, and the nucleolus also regenerates.
What is meiosis

Cytokinesis

 At the end of the second telophase, the cytoplasm of each cell of each dyad is divided into two parts.  As a result, there are four daughter cells, known as Quartet, from the semi circulation in a parent cell.

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