What are the different stages of meiosis ?

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Meiosis comprises of two divisions, meiosis I and Meiosis II, which occur one after the other with a short or no interphase in between them.

Meiosis I:

It is the reductional or heterotypic cell division comprising of following four phases on the basis of different chromosomal events.

(1) Prophase I:

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It is of longest duration and divided further into five phases on the basis of chromosomal and molecular events.

(i) Leptotene (thin thread stage):

Here nuclear chromatins start condensing longitudinally and the tips of all condensing chromosomes move to a point on nuclear membrane to form a bouquet.

(ii) Zygotene (yolked thread stage):

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Homologous chromosomes pair(synapsis) to form bivalents (two chromosomes with four chromatids) with the help of synaptonemal complex.

(iii) Pachytene (thick thread stage):

Condensation continues and exchange of segments occurs between non-sister chromatids of homologous chromosomes by the process called crossing over with the formation and help of recombination nodules. Points of exchange are called chiasmata.

(iv) Diplotene (double thread stage):

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Nuclear membrane and nucleolus start disappearing. Desynapsis of homologous chromosomes begins and chiasmata start moving towards the ends of bivalents (terminalization).

(v) Diakinesis:

Complete disappearance of nuclear membrane and nucleolus, maximum terminalization and beginning of spindle formation.

(2) Metaphase I:

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Bivalents move towards equator and arrange on metaphase plate with their arms lying on the plate and centromeres pointing towards poles (court-orientation).

(3) Anaphase I:

Homologous separate (disjunction) and move towards opposite poles by the action of spindle.

(4) Telophase I:

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Separating chromosomes (each with two chromatids) each the opposite poles, they elongate to form chromatin network and nuclear membrane and nucleolus reappear. Hence, two haploid nuclei are formed.

Meiosis II:-

It is equational and homotypic cell division, consisting of following four phases:

(1) Prophase II:

It involves longitudinal condensation of chromatic fibres, disappearance of nuclear membrane and nucleolus and formation of bipolar spindle.

(2) Metaphase II:

Chromosomes move and arrange in metaphase plates(perpendicular to metaphase I plate) with their centromeres lying on metaphase plate and arms pointing towards poles(auto-orientation).

(3) Anaphase II:

This involves splitting of centromere and movement of daughter chromosomes towards opposite poles with the help of spindle.

(4) Telophase II:

Four groups of chromosomes reach the respective poles and elongate to form chromatin network. Nuclear membrane and neucleoli reappear, forming four haploid and genetically dissimilar nuclei.

Cytokinesis:

It is the process of division of cytoplasm which may occur once after meiosis I and again after meiosis II (successive type) or both may occur after meiosis II (simultaneous type).

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