Chromatin threads condense to form chromosornes. Chrornosorncs are longest & thinest
Chromosornes become gradually visible under the light microscope.

`->` On Chrornosomes, bead like structures are present i.e. chromomeres. All the chromosomes in nucleus remain directed towards centrioles, so group of chromosomes in nucleus appears like a bouquet in animal cell. (Bouquet stage).

At this stage organism shows a peculiar type of orientation of chromosomes-animals show bouquet type while plants synizesis type.

Zygotene is characterized by pairing of homologous chromosomes (Synapsis). Pairs of homologous chromosomes are called Bivalents. There develops a structure in between homologous chromosomes, Which is termed as synaptonemal complex. Synaptonemal complex is composed of three thick lines of DNA and proteins. According to Mosses (1956) synaptonemal complex helps in pairing and chiasmata formation.

Due to increased attraction, homolo~JOUS chromosomes tightly coil around each other .

`->` Both chromatids of each chromosome become distinct.

`->` Each chromosome in a bivalent at pachytene stage distinctly show two chromatids (Total four chromatids), as a result of which bivalent really consists of four chromatids and is called a tetrad. Both the chromatids of a chromosome are called sister chromatids .

`->` Bivalent is called tetrad and each chromosome is called Dyad .

`->` Recombination nodules between nonsister chromatids of homologous pair develop and these non sister chromatid exchange their parts i.e. crossing over. Crossing over was discovered by Morgan and Castle in Drosophila. Breakage and reunion theory => Stern and Hotta

`->` Breakage and reunion theory is appropriate theory for crossing over which is explained with the help of a most accepted model (Hybrid DNA model) of Holiday and Whitehouse

`->` Crossing over is an enzyme mediated process and the enzyme involved is called recombinase (Endonuclease ligase)

`->` Endonuclease first breaks the nonsister chromatids at the place of recombinalion nodule .

`->` Nonsister chromatids reunite after exchanging their parts by enzyme Ligase As the result of crossing over cross
like structures-chiasmata (discovered by Janssen) form in bivalent .

`->` Number of chiasmata (singular- chiasma) per bivalent depends on the length of chromosome.

The begining of dipotene is recognised by dissolution of synaptonemal complex (desynapsis). Homologous chromosomes start repulsing each other so x-shape structures appeard called chiasmata become visible .

`->` Diplotene (Dictyotene) may last long up to month or years (in oocytes of same vertebrates, 12 to 15 years in human Female)

`->` According to modern scientists chiasmata are not the reason but are only the result of crossing over.

`->` It is final stage of meiotic prophase I.

`->` Marked by terminalization of chiasmata (Chiasmate open in zip like manner) .

`->` Chromosome are fully condensed and meiotic spindle is assembled to prepare the homologous chromosome for separation.

`->` Centrioles move towards the opposite poles .

`->` By the end of diakinesis nuclear membrane and nucleolus disappear

`->` Bivalents arrange on equator (congression) of cell to form metaphase plate. The microtubules from the opposite poles of the spindle attach to the pair of homologous chromosome.

Three types of spindle fibres appear in the cell :-

(i) Chromosomal/ Kinetochore Spindle fibres

(ii) Supporting/ Continuous Spindle fibres

(iii) Interzonal Spindle fibres.

`->` Due to shortening of chromosomal fibre and expansion of interzonal fibre homologous chromosomes segregate from each other and move towards the opposite poles. Sister chromatids remain associated at their centromeres (i.e. chromosomes remain in double chromatid stage)

`->` Anaphase I is characterised by segregation or disjunction of chromosomes. Division of centromere is absent. Anaphase I is responsible for reduction in chromosome number in daughter cells.

The nuclear membrane and nucleolus reappear. Although in many case the chromosomes do undergo some dispersion, but they do not reach the extremely extended state of the interphase nucleus.

`->` Cytokinesis follows telophase-[ and a diploid (2n) cell divides into two haploid (n) daugther cells. This is called as diad of cells .

`->` In animals by constriction & furrow formation (successive) .

`->` In most of the plants cytokinesis does not occur after meiosis I.

`->` Gap between meiosis I and meiosis II is called Interkinesis. Preparations of meiosis II occur during interkinesis. It can't be termed as interphase because replication of DNA is absent in interkinesis.

`->` Interkinesis is generally short lived. Interkinesis is followed by prophase-II, a much simpler prophase than prophase-I