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:
It is of longest duration and divided further into five phases on the basis of chromosomal and molecular events.
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(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):
Homologous chromosomes pair(synapsis) to form bivalents (two chromosomes with four chromatids) with the help of synaptonemal complex.
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(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):
Nuclear membrane and nucleolus start disappearing. Desynapsis of homologous chromosomes begins and chiasmata start moving towards the ends of bivalents (terminalization).
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(v) Diakinesis:
Complete disappearance of nuclear membrane and nucleolus, maximum terminalization and beginning of spindle formation.
(2) Metaphase I:
Bivalents move towards equator and arrange on metaphase plate with their arms lying on the plate and centromeres pointing towards poles (court-orientation).
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(3) Anaphase I:
Homologous separate (disjunction) and move towards opposite poles by the action of spindle.
(4) Telophase I:
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:
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(1) Prophase II:
It involves longitudinal condensation of chromatic fibres, disappearance of nuclear membrane and nucleolus and formation of bipolar spindle.
(2) Metaphase II:
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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).