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The following points highlight the two main types of meiotic division. The types are: 1. The First Meiotic Division or Heterotypic Division 2. The Second Meiotic Division or Homeoptypic Division.
Type # 1. The First Meiotic Division or Heterotypic Division:
i. First Prophase Stage:
This stage is divided into following stages:
(a) Leptotene sub stage:
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The slide shows the first or leptotene sub-stage belonging to first prophase of first meiotic division and is characterized by following features (Fig. 30):
(1) The nuclear membrane is distinct and so also the nucleolus.
(2) The chromosomes are visible as thread like structures with a somewhat beaded appearance due to presence of chromomeres.
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(3) The centriole has divided into two but is still on the same pole.
(b) Zygotene sub stage:
The slide shows zygotene sub stage belonging to the prophase of first meiotic division and is characterized by following features (Fig. 31):
(1) The nuclear membrane and nucleolus are still present and are quite prominent.
(2) The chromosomes have started condensing and thickening. The homologous chromosome has started pairing known as synapsis.
(3) The pairing begins at a few points only.
(4) The paired chromosomes are called bivalents or diads.
(c) Pachytene sub-stage:
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The slide shows pachytene sub stage of first prophase of first meiotic division (Fig. 32):
(1) The nuclear membrane and nucleolus are still intact and visible.
(2) The synaptic bivalent chromosomes have started showing coiling or twisting around each other. The points of contact are called chiazmata.
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(3) Further, each chromatid of a bivalent homologue starts splitting lengthwise into two Chromatids resulting thereby into a tetravalent or tetrad. There Is no splitting at the centromere.
(d) The Diplotene sub-stage:
The slide shows the fourth or diplotene stage of first prophase of first meiotic division and is characterized by following characters (Fig. 33):
(1) The nuclear membrane and nucleolus have started disintegrating.
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(2) The chromatids from the tetrad have started separating at a number of points, but at certain points they appear to cross each other. These points o< crossing over are known as chiasmata and this phenomena is known as crossing over.
(3) The chromatids have started sliding in opposite directions and thus the chiasmata started moving towards the tip of the chromatids.
(e) The Diakinesis sub-stage:
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The slide shows diakinesis stage of 1st prophase of 1st meiotic division and is characterized by following features (Fig. 34):
(1) The chiasmata have terminated and chromosomes are at this stage very thick and dark bivalents.
(2) It is difficult to differentiate the two chromatids of a bivalent chromosome due to condensation of chromo- meres.
(3) The nucleus and nuclear membranes have disappeared.
ii. The Metaphase Stage of 1st Meiotic Division:
The slide shows metaphase stage belonging to first meiotic division and is characterized by following features (Fig. 35):
(1) It is different from mitotic metaphase.
(2) The nucleolus and nuclear membrane are absent and spindle has formed
(3) The homologous chromosome have started arranging themselves in a line at the equator of the spindle.
(4) The chromosomes get attached to spindle fibres by their centromeres.
(5) The arms of each homologous chromosome of a tetrad are always directed towards the opposite pole.
iii. Anaphase Stage of 1st Meiotic Division:
The slide shows anaphase stage of first prophase of 1st meiotic division and is characterized by following features (Fig 36):
(1) In the beginning of this stage, as soon as spindle fibres starts contracting, the chromosomes of a homologous pair reverse their arrangement from that of metaphase stage and now their arms are facing towards equator and their centromere towards opposite poles.
(2) The most characteristic feature of this division is that each chromosome is having two chromatids, one coming from mother (maternal) and the other from father (paternal).
(3) The spindle fibres in later stage have contracted and the chromosomes have reached mid way to poles.
(4) Hence, the chromatid number has been reduced to half.
iv. Telophase Stage of 1st Prophase of Meiotic Division:
This slide shows telophase stage belonging to first prophase of meiotic division and is characterized by following features (Fig. 37):
(1) In this stage the separation of homologous chromosomes has completed and they have reached the opposite pole.
(2) The nuclear membranes have reappeared around.
(3) The chromosomes have started uncoiting.
(4) The division of nucleus is followed by cytokinesis i.e., the division of cytoplasm.
Type # 2. The Second Meiotic Division or Homeoptypic Division:
The characteristic features of this division are that the spindle now forms at right angle to the previous spindle formed in heterotypic division (first meiotic division). The details are similar to those described for mitosis.
The specific characters are as follows:
i. Prophase II:
(1) The slide shows second prophase stage of meiotic division (Fig. 38):
(2) The nuclear membrane and nucleolus once again disappear.
(3) The spindle starts appearing.
(4) The chromosomes appear as dyads and are thick and coiled.
ii. Metaphase II:
The slide shows metaphase of second meiotic division and is characterized by following features:
(1) The dyads have been arranged on the equator of spindle.
(2) The spindle has formed.
(3) The arms of two chromatids of each dyad are facing towards opposite pole.
iii. Anaphase II:
The slide shows anaphase II stage of second meiosis and is characterized by following features (Fig. 39):
(1) The centromeres of dyad have divided and both have started separating due to contraction of spindle fibres.
(2) The arms of chromatids of a dyad are facing to equator and their centromeres are facing towards poles.
(3) The haploid chromatids are still thick, short, stumpy & coiled.
iv. Telophase II:
The slide shows IInd telophase stage of second meiotic division and is characterized by following features (Fig. 40):
(1) The haploid chromatids have reached the poles.
(2) Around them nuclear membrane has reappeared.
(3) The chromatids have started uncoiling and are now appearing as comparatively long and thin thread like structures.
(4) This stage is followed by cytokinesis in which the two nuclei are separated by cell membrane and they become independent haploid daughter cells.
