Meiosis-stages

Edward Van Beneden demonstrated reduction division in 1887. The term'meiosis' was introduced by J.B. Farme and Moore in 1905.Meiosis is confined to certain cells and takes place at a particular time. Only the cells of sexually reproducing organisms undergo meiosis, and only special celli in the multicellular organisms switch over from mitosis to meiosis at a specific time in the life cycle. Meiosis produces gametes or-gametic nuclei in animals, some lower plants, and various protists and fungi. Meiosis forms asexual reproduciive bodies, the spores, in higher plants.
 Duration:Meosis takes days to complete instead of hours or minutes needed for mitosis.
Meiocytes. The cells in which meiosis talres place are called meiocytes. 3 types :Oocyte, spermatocytes and sporocytes. The oocytes give rise to female gametes called ova or eggs ; spermatocytes forms gametes termed spetmatozoa or sperms ; produce special cells known as spores. Oocytes and spermatocytes are found in the sex organs or gonads. Sporocytes are found in the organs called sporogonia.
Essential (Special) Features of Meiosis.
These includes-
(i) two successive divisions without DNA replication between them,
(ii) synapsis of homologous chromosomcs,
(iii) crossing over of segments between nonsistcr chromatids of synapsed chromosomes,
(ry) scgregation of homologous chromosomes, and
(v) separation of sister chromatids.
Interphase:Interphase exists before meiosis. It resembles the interphase that precedes mitosis but has relatively long S phase and a short or no G2 phase.
First Meiotic Division or Meiosis-I: In this division, the two homologous chromosomes of each pair separate from each other and go to separate daughter cells. This reduces the number of chromosomes from diploid to haploid condition. Meiosis I is, therefore, known as heterotypic division. The four phases of this division are called prophase I, metaphase -I, anaphase-I and telophase - I.
I. PROPHASE I:The prophase of meiosis  I is more elaborate and lasts longer than the prophase of mitosis, It takes more than 90% of.the time required for meiosis. It may be divided into 5 substages: leptotene, zygotene, pachytene, diplotene and diakinesis. The first foui subphases are also known as leptonema, zygonema, pachynema and diplonema rcspectively.
(a) Leptotene, The centrioles, already cluplicated, move apart in pairs. Astral rays appear uround each pair of centrioles to form an aster. Spindle begins to develop between the centriole pairs. Chromosomes diflferentiate from chronratin fibres. They are very long, fine filaments (G.leptos - fine; tene : thread). They are double, iach consisting of two identical chromatids due to DNA replication during premeiotic interphase. The chromatids are, however, very closely adhered together and are indistinct  Each chromosome is attached by both of its ends to the nuclear matrix that lines the nuclear envelope with the help of a specialized structure termed the attachment plate.

(b) Zygotene. The homologous chromosome come to lie side by side in pairs.This pairing of homologous chromosome is known as synapsis.A pair of homologous chromosome lying together is called a bivalent.The chromatids are still not visible.A fibrillar some what ladder like,organelle,called synaptonemal complex,develops between the synapsed homologous chromosomes.
Pairing may occur n one of the following three ways
a.Proterminal pairing:It starts at the ends and proceeds towards the middle.
b.Procentric pairing:It begins at the centromere and progresses towards the ends.
c.Random pairing:It commences at many points toward the ends,
(c)Pachytene:The synapsed chromosomes  slightly separate and become visible. The two visible chtomatids of a chromosome are referred to as a dyad .A group of four homologous chromatids is called a- tetrad. The two chromatids of the same chromosome are called sister chromatids and those of two homologous chromosomes (bivalent) are termed nonsister chromatids.
Crossing over-(recombination) occurs during pachytene. Recombination involves mutual exchange of the corresponding segments of non sister chromatids of homologous chromosome It takes place by breakage and reunion of chromatid segments. Breakage, called nicking, is assisted by an enzyme endonuclease and reunion, termed annealing, is aided by an enzyme ligase. It has been found that crossing over is a common event. Normally, each tetrad undergoes at least one recombination.
(d) Diplotene:During this stage the synaptic forces keeping the homologous chromosomes together  come to an end. The homologous chromosomes start separating. This is called disjunction. It makes chromatids more distinct and the tetrads very clear. Separation of homologous_chromosomes doesnot take-place at the points called chiasmata (singular' chiasma) The chiasmata mark the sites where crossing over occurred during pachytene.They hold homologous chromosomes together.
(e) Diakinesis:The chromosomes condense again to aquire their characteristic size and  form. The chiasma disappear by sliding towards the tips of the chromosomes due to tight condensation.This process is called terminalisation. The nucleoli disappear,nuclear envelope breaks down into vesicles, setting the tetrads free in the cytoplasm.
II.METAPHASE-I. The spindle shifts to the position formerly occupied by the nucleu.s. The tetrads move to,the.equator of thc spindle and come to lie in two parallel rnetaphase plates. One of these plates is foimed by the n chromosomes, and the other by those .of their homologues. The attach' ment of tetrads td the spindle fibres in metaphase I is different from that of mitotic metaphasechromosomes.Each homologous chromosome has two  kinetochores, one for each of its two chromaticis. Both the kinetochores of a homologous
chromosome connect to the same spindle pole The two kinetochores of its homologue join
the opposite spindle Pole.
III.ANAPHASE-I. From each tetrad, two chromatids of a chromosome move as a unit (dyad) to one pole of a spindle, and the remaining two chromatids of its homologue migrate to the opposite pole. Thus, the homologous chromosomes of each pair, rather than the chromatids of a chromosome, are separated. With the result, half of the chromosomes, which appear in early prophase, go to each pole. It is here in the anaphase - I that the real reduction in the number of chromosomes occurs. Each chromosome at the poles is still double and consists of two chromatids. This is in contrast to the single-stranded chromosomes of mitotic anaphase.
IV.TELOPHASE-I. The chromosomes at each pole of the spindle now uncoil and elongate, but remain straight and often do not assume interphase form. The satellite chromosome develops nucleolus, nucleoplasm appears, and nuclear envelope forms around the chromosomes and nucleoli. The spindle and the astral rays gradually disappear.
The cytoplasm divides at its middle by cleavage (constriction) in an animal cell and by cell plate formation in a plant cell. This produces two daughter cells, each with one nucleus. The nucleus of each daughter cell has received only one chromosome from each homologous pair. Thus, it  has half the number of chromosomes, but double the amount of nuclear DNA as each chromosome is still double.

Interkinesis: Generally there is no interphase between meiosis - I and meiosis - II. A brief interphase, called interkinesis, or intrameiotic interphase, may occur in some cases. There is no replication of chromosomes during this interphase.
2. Second Meiotic Division or Meiosis – II: In this division, the two chromatids of each chromosome separate from each other and go to separate daughter cells. With the result, the number of chromosomes remains the same as produced by meiosis-I.Meiosis- II is, therefore, known as homotypic division. The four stages of  this division are called prophase - II, metaphase - II, anaphase - II, and telophase - II.
1.PROPHASE-II: Prophase-Il is very short. It occurs simultaneously in both the nuclei formed by meiosis-I. Centrioles, already duplicated, move apart in pairs. Each pair develops astral rays round it to form an aster. Spindle is laid down between the 2 pairs of centrioles. The chromosomes, each comprising two chromatids, become visible in the nucleus. They are set free in the cytoplasm by breakdown of the nuclear envelop.. Nucleoli disappcar.
2) METAPHSE-II:The chromosomes take up positions at the equator of the spindle, forming a single mataphase plate. The chromatids of each chromosome are joined at their kinetochore by spindle fibres extending from the  opposite poles of the spindle as in mitosis.
(3) ANAPHASE-II: The two chromatids of each  chromosome start moving away from each other. Finally, they reach the poles of the spindle. Here, they are called the chromosomes. Each pole has haploid number of chromosomes and haploid amount of DNA. This amount is one-fourth of the DNA. present in the original cell that started melosls.
3) TELOPHASE-II: The group of chromosomes at each pole of the spindle gets enclosed by a nuclear envelope. Nucleoli are laiddown. Astral rays and spindle are lost.
Cytokinesis: Cytoplasm divides at its middle by furrowing in an animal cell and by cell plate formation in a plant cell. This produces two daughter cells. The latter have half the number of chromosomes, and half the amount of nuclear DNA. These cells are mature gametes in animals and spores in plants. Cytokinesis may occur after each nuclear division. In such cases, it is said to be of successive type. First the diploid parent cell divides by heterotypic division into two haploid cells, which then produce four haploid cells by homotypic division. The four daughter cells may form a linear or isobilateral tetrad. Often cytokinesis is delayed until both the nuclear divisions are completed, so that four cells are simultaneously formed, each with a haploid nucleus. The cytoplasmic division in such cases is said to be of simultaneous type.