Meiosis Cell Division – Science | SLC Notes

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Meiosis Cell DivisionReference | SLC
Notes on “Meiosis Cell Division”
For: Science Class 10

Karyokinesis
Karyokinesis of meiosis cell division complete in 8 phases. They are:

  • Meiotic first division
    -> Prophase I
    -> Metaphase I
    -> Anaphase I
    -> Telophase I
  • Meiotic second division
    -> Prophase II
    -> Metaphase II
    -> Anaphase II
    -> Telophase II

Prophase I
Prophase I of meiosis completes in five sub-phases. The changes that take place in the nucleus of cells in these sub-phases are mentioned below:

  • Leptotene [Thread stage]: Chromatin divides in particular number of chromosomes. The chromosomes are so thin that chromosomes are seen in the form of beads.
  • Zygotene [Paring stage]: Homologous pairs or bivalents are formed. The process of forming the pairs is called synapsis.
  • Pachytene [Thickening stage]: Chromosomes shorten and form tetrads. Crossing over between non-sister chromatids is started.
  • Diplotene [Duplication stage]: Crossing over is completed. The two homologous chromosomes tend to separate from each other but both remain attached at chiasmata.
  • Diakinesis [Moving apart stage]: Overlapped chromo;somes move away from each other.

Prophase - I

Metaphase I

  1. Spindle fibers appears
  2. Chromosomes arrange at the equator centromere of each bivalent face towards opposite pole.
  3. One centromere of each bivalent becomes attached with spindle fibers of one pole and another centromere of each bivalent gets attached with spindle fibers of another pole.

Meiosis - I Cell Division

Anaphase I

  1. Each bivalent separate together
  2. Separated bivalent move towards opposite poles due to contraction of spindle fiber
  3. Two groups of chromosomes are formed at each pole having haploid number of chromosomes

Telophase I

  1. Chromosomes arrange into two groups one at each pole
  2. Nuclear membrane and nucleolus reappears
  3. Spindle fibers disappear
  4. Chromosomes become long elongated to form chromatin

Cytokinesis: It may or may not occurs

Meiosis II
It is called equational division or homotypical division because the cells produced after meiosis I undergo further division to produce four cells where chromosome number remains same as that of parent cells, similar to mitosis.

Karyokinesis
Prophase II

The Chromosome become shorter and thicker, Nuclear membrane and nucleolus disappear. Spindle fibers start to appear

Metaphase II

  1. Chromosomes arranged at the equator
  2. The centromere of each chromosome divides into two centromeres in longitudinal way.
  3. One centromere of each chromosome is connected with spindle fiber of one pole and another centromere is connected with spindle fiber of another pole.

Meiosis II - Cell Division

Anaphase II

  1. A kind of repulsive force is developed between two daughter centromeres
  2. One of the two centromeres having its own chromatids move towards one pole and another centromere move towards another pole
  3. Two groups of chromosomes are formed at each pole due to contraction of spindle fibers.

Telophase II

  1. Chromosomes organize into nucleus at each pole
  2. Chromosomes become long, elongated forming chromatin
  3. Nuclear membrane and nucleolus reappears
  4. Spindle fibers disappears

Cytokinesis
Successive method
The first cytokinesis takes place after meiosis first to form two daughter cells and second cytokinesis takes place after meiosis II to form four daughter cells. The process is called successive Method.

Simultaneous Method
In this method, cytokinesis takes place only after Meiosis II to produce four daughter cells.

Significances of meiosis are:

  • It forms gametes for sexual reproduction.
  • It causes variation, which is the main unit of evolution. Variation is caused by crossing over.
  • It helps to make chromosomes’ number constant.

 

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