Meiosis:

 This term was introduced by Farmer and Moore. This phase is called Reductional division because number of chromosomes becomes half from parent cells. It results in the formation of four haploid daughter cells with same number of chromosomes. There are two successive divisions of a eukaryotic cell of a sexually reproducing organism, Meiosis-1 and Meiosis-2 explained below:

Meiosis-1 It is longer than meiosis-2 because of longer prophase.

  • Prophase-1:  It is further divided into 5 sub-phases:

1) Leptotene: Centrioles start producing aster rays. There are two similar chromosomes of each type called Homologous chromosomesn (That have diploid number of chromosomes).

2) Zygotene: Here, homologous chromosomes come close to each other and the process of their attachment is called Synapsis. These syanpsed chromosome pair is called Bivalent. Bivalent has 4 chromatids- 2 are sister chromatids,  and other 2 are non-sister chromatids.

3) Pachytene: Here, non-sister chromatids of homologous chromosomes (bivalent) lie close to each other and exchanges positions, that make new combinations, is called Crossing over.

4) Zygotene: Homologous chromosomes in synapsis separate except in the regions of crossing over. And chromatids attain Tetrad (bivalents) stage. A tetrad has 4 chromatids. The point where two homologous non-sister or sister chromatids exchange genetic material during chromosomal crossover during meiosis is called Chiasmata.

5) Diakinesis: Nucleolus and nuclear membrane digenerates.

  • Metaphase-1:

1) Spindles appears.

2) Bivalents of homologous chromosomes aligns on the equatorial plate.

  • Anaphase-1:

 

1) Homologous chromosomes separates and this process is called Disjunction.

2) Now, separated chromosomes are called Dyads(univalents). A dyad has 2 chromatids.

  • Telophase-1:

1) Nucleolus and nuclear membrane start appearing again.

2) Chromosomes have haploid nucleus.

Meiosis-2 → Shorter than typical mitotic division because of shorter prophase.

Prophase-2:

1) The nuclear membrane disappears by the end of prophase II.

2) The chromosomes again become compact.

 

 

Metaphase-2:

1) At this stage the chromosomes align at the equator.

2) The microtubules from opposite poles of the spindle get attached to kinetochores of sister chromatids.

Anaphase-2:

1) Here, sister chromatids separate and start moving towards opposite poles.

Telophase-2:

1) Nuclear envelope reappears.

2) Spindle fibres disappears.

 3) Formation of tetrad of cells i.e., four haploid daughter cells after cytokinesis occurinf after telophase.

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11    PMT    Biology    Cell Cycle and Cell Division    Meiosis