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Why is meiosis called reduction division? Describe the various stages with relevant diagrams. Add a note on the significance of meiosis.
Meiosis is the division which takes place for the formation of gametes or germ cells in which the number of chromosomes is exactly half the number of chromosomes present in the somatic cell.
As due to the meiotic division the number of chromosomes becomes exactly half of the original number, meiosis is known as reduction division because the number of chromosomes is getting reduced into it.
Meiosis takes place into two broad steps mainly- Meiosis I and Meiosis II.
A. Meiosis I- In meiosis I the reduction of chromosomes takes place. It comprises of four steps- Prophase I, Metaphase I, Anaphase I and Telophase I. Each step of meiosis I am discussed below with its diagram-
I. Prophase I- It is longer than all other phases. During this step unwinding of the chromatin, material takes place and processes like the pairing, crossing over of homologous chromosomes take place. All this take place in five different step which is as follows:
a) Leptotene- During this step condensation of chromosomes takes place.
b) Zygotene- During this step homologous chromosome comes near each other and they start pairing with each other. This process is called synapsis.
c) During pairing, homologous
chromosomes get attached to each other and then those paired chromosomes are called bivalents.
d) Pachytene- In this four chromatids calling tetrad start appearing. During this step crossing over takes place in which there is exchange of segments between non sister chromatids of homologous chromosomes. The point of contact between homologous chromosomes is termed as chiasmata.
e) Diplotene- During this step separation and terminalisation takes place. The homologous chromosomes get separated from each other and they start moving to the terminals.
f) Diakinesis- In this there is dissolution of the nucleolus and the nuclear membrane and spindle apparatus is there.
II. Metaphase I- In this step formation of an equatorial plate takes place which is also known as metaphasic plate. All the bivalents get arranged on that plate and centromere remains attached to the spindle fibers.
III. Anaphase I- In this separation of homologous chromosomes take place and they start moving to the opposite poles. Here reduction of the chromosomes takes place.
IV. Telophase I- In this division of cytoplasm takes place and this process is called cytokinesis. Nuclear membrane and nucleolus start appearing and result in two cells in which chromosome number are exactly half to the original number.
B. Meiosis II- This process is exactly like mitosis in which equational division take place. In this chromosome number remains same and there is no reduction in chromosome number. It consist of four steps- Prophase II, Metaphase II, Anaphase II and Telophase II. Each step is discussed below:
I. Prophase II- In this step nuclear membrane and nucleolus again start disappearing. The chromosomes again get shortened.
II. Metaphase II- In this step formation of an equatorial plate takes place which is also known as metaphasic plate. And the chromatids get arranged on that plate and centromere remains attached to the spindle fibers.
III. Anaphase II- In this step, there is a splitting of the centromere and new chromosomes are formed which move towards the opposite poles.
IV. Telophase II- In this again the formation of nuclear membrane and nucleolus take place. Haploid chromosomes are there at each pole and again cytokinesis take place which divides the cell into two.
And as we have two cells here, so at the end we get four cells having haploid chromosome.
In this way the complete process of meiosis takes place including all the steps mentioned above and at end we get four daughter cells, each having haploid chromosome.
Significance of meiosis-
1. Meiosis helps in maintaining the chromosome number with each generation because if there won’t be reduction division and gametes were also having the same number of chromosomes as somatic cells do, with each fertilisation there would be increment in the ploidy number and there will be nothing like maintenance of chromosome number with generations.
2. It also helps in evolution as it helps in generating variations. During meiosis crossing over take place in which there is exchange of non sister chromatids which ultimately lead to variations. As these variations pass from generation to generation they help in evolution.
3. And as discussed above meiosis helps in formation of gametes.
Figure: Flow chart of Meiosis I and Meiosis II
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