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.
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.
(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.
(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).
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).
(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.
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.
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).